Chapter 5
Genetic Disorders

We could wish that … life-histories were found in every family, showing the health and diseases of its different members. We might thus in time find evidences of pathological connections and morbid liabilities not now suspected.

—William Gull, 1896

It has long been known that heredity affects health. Genetics, the study of single genes and their effects on the body and mind, explains how and why certain traits such as hair color and blood types run in families. Genomics, a discipline that is only about two decades old, is the study of more than single genes; it considers the functions and interactions of all the genes in the genome. In terms of health and disease, genomics has a broader and more promising range than genetics. The science of genomics relies on knowledge of and access to the entire genome and applies to common conditions, such as breast and colorectal cancer, Parkinson's disease, and Alzheimer's disease. It also has a role in infectious diseases once believed to be entirely environmentally caused such as the human immunodeficiency virus (HIV) and tuberculosis. Like most diseases, these frequently occurring disorders result from the interactions of multiple genes and environmental factors. Genetic variations in these disorders may have a protective or a causative role in the expression of diseases.

It is commonly accepted that diseases fall into one of three broad categories: those few that are primarily genetic in origin; those that are largely attributable to environmental causes; and those—most conditions—in which genetics and environmental factors make comparable, though not necessarily equal, contributions. As understanding in genomics advances and scientists identify genes involved in more diseases, the distinctions between these three classes of disorders are diminishing. This chapter considers some of the disorders believed to be predominantly genetic in origin and some that are the result of genes acted on by environmental factors.

There are two types of genes: dominant and recessive. When a dominant gene is passed on to offspring, the feature or trait it determines will appear regardless of the characteristics of the corresponding gene on the chromosome inherited from the other parent. If the gene is recessive, the feature it determines will not show up in the offspring unless both the parents' chromosomes contain the recessive gene for that characteristic. Similarly, among diseases and conditions primarily attributable to a gene or genes, there are autosomal dominant disorders and autosomal recessive disorders.

Another way to characterize genetic disorders is by their pattern of inheritance, as single gene, multifactorial, chromosomal, or mitochondrial. Single-gene disorders (also called Mendelian or monogenic) are caused by mutations in the deoxyribonucleic acid (DNA) sequence of one gene. Because genes code for proteins, when a gene is mutated so that its protein product can no longer carry out its normal function, it may produce a disorder. According to the Human Genome Project Information Web site (December 9, 2003, http://www.ornl.gov/sci/techresources/Human_Genome/medicine/assist.shtml), which is operated by the Department of Energy, there are an estimated 6,000 known single-gene disorders, which occur in about 1 in every 200 births. Examples are cystic fibrosis, sickle-cell anemia, Huntington's disease, and hereditary hemochromatosis (a disorder in which the body absorbs too much iron from food; rather than the excess iron being excreted, it is stored throughout the body, and iron deposits damage the pancreas, liver, skin, and other tissues). Figure 5.1 shows the cystic fibrosis gene and its location on chromosome 7; Figure 5.2 shows the sickle-cell anemia gene found on chromosome 11; and Figure 5.3 shows the hereditary hemochromatosis gene located on chromosome 6. Single-gene disorders are the result of either autosomal dominant, autosomal recessive, or X-linked inheritance.

Multifactorial or polygenic disorders result from a complex combination of environmental factors and mutations in multiple genes. For example, different genes that influence breast cancer susceptibility have been found on seven different chromosomes, rendering it more difficult to analyze than single-gene or chromosomal disorders. Some of the most common chronic diseases are multifactorial in origin. Examples include heart disease, Alzheimer's disease, arthritis, diabetes, and cancer.

Chromosomal disorders are produced by abnormalities in chromosome structure, missing or extra copies of chromosomes, or errors such as translocations (movement of a chromosome section from one chromosome to another). Down syndrome or trisomy 21 is a chromosomal disorder that results when an individual has an extra copy, or a total of three copies, of chromosome 21. Mitochondrial disorders result from mutations in the nonchromosomal DNA of mitochondria, which are organelles involved in cellular respiration. Compared to the three other patterns of inheritance, mitochondrial disorders occur infrequently.

There are significant differences between the nineteenth-century germ theory of disease and the twenty-first-century genomic theory of disease. By the middle of the twentieth century it became possible to improve the quality of life and to save the lives of people with some genetic diseases. Effective treatment included changes in diet to prevent or manage conditions such as phenylketonuria (PKU) and glucose galactose malabsorption (GGM). PKU is an inherited error of metabolism caused by a deficiency in the enzyme phenylalanine hydroxylase. (See Figure 5.4.) It may result in mental retardation, organ damage, and unusual posture. Dietary changes are also used to treat GGM, a rare metabolic disorder caused by lack of the enzyme that converts galactose into glucose. For people with severe cases of GGM, it is vital to avoid lactose (milk sugar), sucrose (table sugar), glucose, and galactose. Other therapeutic measures may involve surgery to correct deformities and avoidance of environmental triggers, as in some types of asthma.

At the dawn of the twenty-first century, the possibility of preventing and changing genetic legacies appears within reach of modern medical science. Genomic medicine predicts the risk of disease in the individual, whether highly probable, as in the case of some of the well-established single-gene disorders, or in terms of an increased suscept-ibility likely to be influenced by environmental factors. The promise of genomic medicine is to make preventive medicine more powerful and treatment more specific to the individual, enabling investigation and treatment that are custom-tailored to an individual's genetic susceptibilities, or to the characteristics of the specific disease or disorder.

COMMON GENETICALLY INHERITED DISEASES

Although many diseases, disorders, and conditions are termed genetic, classifying a disease as genetic simply means that there is an identified genetic component to either its origin or its expression. Many medical geneticists contend that most diseases cannot be classified as strictly genetic or environmental. The phenotype of genetic diseases can sometimes be modified, even to the point of nonexpression, by controlling environmental factors. Similarly, environmental (infectious) diseases may not be expressed because of some genetic predisposition to immunity. Each disease, in each individual, exists along a continuum between a genetic disease and an environmental disease.

A myriad of diseases are believed to have strong genetic contributions, including:

• Heart disease—coronary atherosclerosis, hypertension (high blood pressure), and hyperlipidemia (elevated blood levels of cholesterol and other lipids)
• Diabetes
• Cancer—retinoblastomas, colon, stomach, ovarian, uterine, lung, bladder, breast, skin (melanoma), pancreatic, and prostate
• Neurological disorders—Alzheimer's disease, amyotrophic lateral sclerosis (also known as Lou Gehrig's disease), Gaucher's disease, Huntington's disease, multiple sclerosis, narcolepsy, neurofibromatosis, Parkinson's disease, Tay-Sachs disease, and Tourette's syndrome (a syndrome is a set of symptoms or conditions that taken together suggest the presence of a specific disease or an increased risk of developing the disease)
• Mental illnesses, mental retardation, and behavioral conditions—alcoholism, anxiety disorders, attention deficit hyperactivity disorder, eating disorders, Lesch-Nyhan syndrome, and schizophrenia
• Other genetic disorders—cleft lip and cleft palate, clubfoot, cystic fibrosis, Duchenne muscular dystrophy, glucose galactose malabsorption, hemophilia, Hurler's syndrome, Marfan's syndrome, phenylketonuria, sickle-cell disease, and thalassemia
• Other medical conditions—including alpha-1-antitrypsin, arthritis, asthma, baldness, congenital adrenal hyperplasia, migraine headaches, obesity, periodontal disease, porphyria, and some speech disorders

ALZHEIMER'S DISEASE

Alzheimer's disease (AD) is a progressive, degenerative disease that affects the brain and results in severely impaired memory, thinking, and behavior. The National Center for Biotechnology Information (NCBI) reports in Genes and Disease (2006, http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowSection&rid=gnd.section.193) that it is the fourth leading cause of death in adults, and the incidence of the disease rises with age. AD affects an estimated four million American adults and is the most common form of dementia, or loss of intellectual function. The National Institute of Mental Health (one of the National Institutes of Health), in "The Numbers Count" (December 26, 2006, https://www.nimh.nih.gov/publicat/numbers.cfm#Alzheimer), estimates that 4.5 million Americans suffer from AD. In 2004 it was the seventh-leading cause of death in the United States. (See Table 5.1.) AD also contributes to many more deaths that are attributed to other causes, such as heart and respiratory failure.

AD has become a disease of particular concern in the United States because the nation's older adult population is growing rapidly. The University of North Texas Center for Public Service reports in "Alzheimer's Disease: Statistics" (August 13, 2001, http://www.cps.unt.edu/alzheimers/disease_statistics.htm) that approximately 10% of the population over age sixty-five is afflicted with AD. By 2050 the United States will have approximately 86.7 million people over age sixty-five, according to projections released by the U.S. Census Bureau (March 18, 2004, http://www.census.gov/ipc/www/usinterimproj/). The Alzheimer's Association estimates that between 2000 and 2025 the number of cases of Alzheimer's disease in the United States will increase by 44%, with the largest growth in southern and western states that have high populations of retirees (June 7, 2004, http://www.alz.org/documents/national/FSADState_Growth.pdf). Prevalence (the number of people with a disease at a given time) is partially determined by the length of time people with AD survive. Even though the average survival is eight years after diagnosis, some AD patients have lived longer than twenty years with the disease. Therefore, improvements in AD care, as well as increased length of life of the older adult population in general, will increase the numbers of AD patients.

Genetic Causes of AD

AD is not a normal consequence of growing older, and scientists are continuing to seek its cause. Researchers find some promising genetic clues to the disease. Table 5.2 shows the different patterns of inheritance, ages of onset (when symptoms begin), genes, chromosomes, and proteins linked to the development of AD. Mutations in four genes, situated on chromosomes 1, 14, 19, and 21, are thought to be involved in the disease, and the best described are PS1 (or AD3) on chromosome 14 and PS2 (or AD4) on chromosome 1. (See Figure 5.5 and Figure 5.6.)

The formation of lesions made of fragmented brain cells surrounded by amyloid-family proteins is characteristic of the disease. Interestingly, these lesions and their associated proteins are closely related to similar structures found in Down syndrome. Tangles of filaments largely made up of a protein associated with the cytoskeleton have also been observed in samples taken from AD brain tissue.

The first genetic breakthrough was reported in the February 1991 issue of the British journal Nature. Investigators reported that they had discovered that a mutation in a single gene could cause this progressive neurological illness. Scientists found the defect in the gene that directs cells to produce a substance called amyloid protein. Researchers at the Massachusetts Institute of Technology found that low levels of the brain chemical acetylcholine contribute to the formation of hard deposits of amyloid protein that accumulate in the brain tissue of AD patients. In unaffected people the protein fragments are broken down and excreted by the body. Amyloid protein is found in cells throughout the body. Researchers do not know why it becomes a deadly substance in the brain cells of some people and not others.

In 1995 three more genes linked to AD were identified. One gene appears to be related to the most devastating form of AD, which can strike people in their thirties. When defective, the gene may prevent brain cells from correctly processing a substance called beta amyloid precursor protein. The second gene is linked to another early-onset form of AD that strikes before age sixty-five. This gene also appears to be involved in producing beta amyloid. Researchers believe that the discovery of these two genes will allow them to narrow their search for the proteins responsible for early-onset AD and give them clues to the causes of AD in older people. (See Table 5.2.)

According to the National Center for Biotechnology Information (2007, http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=107741), the third gene, known as apolipoprotein E (apoE), was actually reported as associated with AD in 1993, but its role in the body was not known at that time. Researchers have since found that the gene plays several roles. It regulates lipid metabolism within the organs and helps to redistribute cholesterol. In the brain apoE participates in repairing nerve tissue that has been injured. There are three forms (alleles) of the gene: apoE-2, apoE-3, and apoE-4. Until recently, people with two copies of apoE-4, one from each parent, were thought to have a greatly increased risk of developing AD before age seventy. Between one-half and one-third of all AD patients have at least one apoE-4 gene, whereas only 15.5% of the general population have an apoE-4 gene. In 1998, however, researchers discovered that the apoE-4 gene seems to affect when a person may develop AD, not whether the person will develop the disease.

Another newly discovered gene, A2M-2, appears to affect whether a person will develop AD. The article "Late-Onset Alzheimer's Gene Suggests Interplay" (Neuroscience, August 14, 1998) indicates that nearly one-third (30%) of Americans may carry A2M-2, a genetic variant that more than triples their risk of developing late-onset AD compared to siblings with the normal version of the A2M gene. The discovery of A2M-2 opens up the possibility of developing a drug that mimics the A2M gene's normal function. This can protect susceptible people against brain damage or perhaps even reverse it.

Testing for AD

A complete physical, psychiatric, and neurological evaluation can usually produce a diagnosis of AD that is about 90% accurate. For many years the only sure way to diagnose the disease was to examine brain tissue under a microscope, which was not possible while the AD victim was still alive. An autopsy of someone who has died of AD reveals a characteristic pattern that is the hallmark of the disease: tangles of fibers (neurofibrillary tangles) and clusters of degenerated nerve endings (neuritic plaques) in areas of the brain that are crucial for memory and intellect. Also, the cortex of the brain is shrunken.

Diagnostic tests for AD have included analysis of blood and spinal fluid as well as the use of magnetic resonance imaging (MRI) to measure the volume of brain tissue in areas of the brain used for memory, organizational ability, and planning to accurately identify people with AD and predict who will develop AD in the future.

In "Nanoparticle-Based Detection in Cerebral Spinal Fluid of a Soluble Pathogenic Biomarker for Alzheimer's Disease" (Proceedings of the National Academy of Science, February 15, 2005), Dimitra G. Georganopoulou et al. report development of another diagnostic test that detects small amounts of protein in spinal fluid. The test is called a bio-barcode assay and is as much as a million times more sensitive than other tests. First used to identify a marker for prostate cancer, the test is used to detect a protein in the brain called amyloid beta-derived diffusible ligand (ADDL). ADDLs are small soluble proteins that may be indicative of AD. To detect them, Georga-nopoulou and her colleagues use nanoscale particles that have antibodies specific to ADDL.

Physicians and neuroscientists have been eager for a simple and accurate test that can distinguish people with AD from those with cognitive problems or dementias arising from other causes. An accurate test would allow the detection of AD early enough for the use of experimental medications to slow the progression of the disease, as well as identify those at risk of developing AD. However, the availability of tests raises ethical and practical questions: Do patients really want to know their risks of developing AD? Will health insurers use genetic or other diagnostic test results to deny insurance coverage?

Treatments for AD

There is still no cure or prevention for AD, and treatment focuses on managing symptoms. Medication can lessen some of the symptoms, such as agitation, anxiety, unpredictable behavior, and depression. Physical exercise and good nutrition are important, as is a calm and highly structured environment. The object is to help the AD patient maintain as much comfort, normalcy, and dignity for as long as possible.

By 2005 five prescription drugs were available to treat people who suffer from AD. Four of these—galantamine, rivastigmine, donepezil, and tacrine—are cholinesterase inhibitors and are prescribed for the treatment of mild to moderate AD. These drugs produce some delay in the deterioration of memory and other cognitive skills in some patients. They offer mild benefits at best and may lose their effectiveness over time, but currently they are the only alternatives available to treat mild to moderate AD.

The fifth approved medication, memantine, is prescribed for the treatment of moderate to severe AD. It acts to delay progression of some symptoms of moderate to severe AD and may allow patients to maintain certain daily functions a little longer. It is thought to work by regulating glutamate, a chemical in the brain that, in excessive amounts, may lead to brain cell death.

Other researchers are examining the roles of the hormones estrogen and progesterone on memory and cognitive function. Because AD involves inflammatory processes in the brain, scientists are also studying the use of anti-inflammatory agents such as ibuprofen and prednisone to reduce the risk of developing AD. Researchers are also investigating the relationship between the various gene sites, particularly the mutation on chromosome 21, and environmental influences that may increase susceptibility to AD. Furthermore, researchers are trying to determine whether antioxidants, such as vitamin E, can prevent people with mild memory impairment from progressing to AD.

CANCER

Cancer is a large group of diseases characterized by uncontrolled cell division and the growth and spread of abnormal cells. These cells may grow into masses of tissue called tumors. Tumors composed of cells that are not cancerous are called benign tumors. Tumors consisting of cancer cells are called malignant tumors. The dangerous aspect of cancer is that cancer cells invade and destroy normal tissue.

The mechanisms of action that disrupt the cell cycle are impairment of a DNA repair pathway, transformation of a normal gene into an oncogene (a hyperactive gene that stimulates cell growth), and the malfunction of a tumor-suppressor gene (a gene that inhibits cell division). Figure 5.7 shows the multiple systems that interact to control the cell cycle.

The spread of cancer cells occurs either by local growth of the tumor or by some of the cells becoming detached and traveling through the blood and lymphatic system to seed additional tumors in other parts of the body. Metastasis (the spread of cancer cells) may be confined to a local region of the body, but if left untreated (and often despite treatment), the cancer cells can spread throughout the entire body, eventually causing death. It is perhaps the rapid, invasive, and destructive nature of cancer that makes it, arguably, the most feared of all diseases, even though it is second to heart disease as the leading cause of death in the United States. (Table 5.1 uses the term malignant neoplasms to describe cancer.)

Cancer can be caused by both external environmental influences (chemicals, radiation, and viruses) and internal factors (hormones, immune conditions, and inherited mutations). These factors may act together or in sequence to begin or promote cancer. There is consensus in the scientific community that several cancer-promoting influences accrue and interact before an individual will develop a malignant growth. With only a few exceptions, no single factor or risk alone is sufficient to cause cancer. As with other disorders that arise in response to multiple factors, susceptibility to certain cancers is often attributed to a mutated gene. Figure 5.8 shows how the inheritance of a mutated gene increases susceptibility for retinoblastoma (cancer of the eye that affects approximately 300 children in the United States each year).

Genetic Research

Scientists and physicians have known for some time that predisposition to some forms of breast cancer are inherited and have been searching for the gene or genes responsible so that they can test patients and provide more careful monitoring for those at risk. In 1994 doctors identified the BRCA1 gene, and in late 1995 they also isolated the BRCA2 gene. Since then it has been found that variations of the ATM, BRCA1, BRCA2, CHEK2, and RAD51 genes increase the risk of developing breast cancer, and the AR, DIRAS3, and ERBB2 (also called Her-2/neu) genes are associated with breast cancer.

Viviana Rivera-Varas, in "Breast Cancer Genes and Inheritance" (1998, http://www.cc.ndsu.nodak.edu/instruct/mcclean/plsc431/students98/rivera.htm), notes that if a woman with a family history of breast cancer inherits a defective form of either BRCA1 or BRCA2, she has an estimated 80% to 90% chance of developing breast cancer. Researchers also think that the two genes are linked to ovarian, prostate, and colon cancer and that BRCA2 likely plays some role in breast cancer in men. Scientists suspect that the two genes may also participate in some way in the development of breast cancer in women with no family history of the disease. Sung-Won Kim et al. state in "Prevalence of BRCA2 Mutations in a Hospital Based Series of Unselected Breast Cancer Cases" (Journal of Medical Genetics, 2005) that only about 10% of all cases of breast cancer are attributable to the susceptibility genes BRCA1 and BRCA2. Figure 5.9 shows the location of the BRCA1 gene on the long arm of chromosome 17 at position 21. Figure 5.10 shows the location of the BRCA2 gene on the long arm of chromosome 13 at position 12.3.

According to Els M. Berns et al. in "Oncogene Amplification and Prognosis in Breast Cancer: Relationship with Systemic Treatment" (Gene, June 14, 1995), another form of breast cancer, due to multiple copies of a gene called ERBB2, causes an estimated 25% of the approximately 213,000 new cases of the disease in the United States each year. ERBB2 often triggers an aggressive form of cancer that can cause death more quickly than other breast cancers, often within ten to eighteen months after the cancer spreads. The ERBB2 gene produces a protein on the surface of cells that serves as a receiving point for growth-stimulating hormones. Figure 5.11 shows the location of ERBB2 on the long arm of chromosome 17 between positions 11.2 and 12.

In "The BARD1 Cys557Ser Variant and Breast Cancer Risk in Iceland" (PLOS Medicine, July 2006), Simon N. Stacey et al. report their discovery of another mutation that when present with BRCA1 or BRCA2 significantly increases the risk of developing breast cancer. Stacey and his coauthors studied 1,090 Icelandic women who had breast cancer and compared them to 703 similar women without the disease. They found a specific mutation, BARD1, in 2.8% of women with cancer but only 1.6% of those without cancer. They also found that women with both the BARD1 andBRCA2 mutations were twice as likely to develop breast cancer as women without these mutations.

Another study (Sheila Seal et al., "Truncating Mutations in the Fanconi Anemia J Gene, BRIP1, Are Low Penetrance Breast Cancer Susceptibility Alleles," Nature Genetics, November 2006), identifies a new genetic mutation, BRIP1, that doubles the risk of breast cancer in carriers. Although BRIP1 increases a woman's breast cancer risk twofold, other gene mutations raise it much more. Seal and the other researchers find that mutations in BRCA1, BRCA2, and another gene, TP53, increase the carrier's risk of breast cancer by ten- to twentyfold by age sixty. Mutations in other genes such as CHEK2, ATM, and the newly identified BRIP1 gene are associated with a more moderate risk increase. Like some of the other known breast cancer genes such as BRCA1 and BRCA2, BRIP1 is a DNA-repair gene, so women with a faulty version of this gene cannot repair damaged DNA correctly. Individuals with faulty DNA-repair genes have an increased risk of cancer because their healthy cells are more likely to accumulate genetic damage that can trigger the cell to replicate uncontrollably.

CYSTIC FIBROSIS

Cystic fibrosis (CF) is the most common inherited fatal disease of children and young adults in the United States. According to the National Library of Medicine's Genetics Home Reference (January 5, 2007, http://ghr.nlm.nih.gov/condition%3Dcysticfibrosis), CF occurs in about 1 out of 3,200 whites, 1 out of 15,000 African-Americans, and 1 out of 31,000 Asian-Americans. The National Institutes of Health's National Human Genome Research Institute (NHGRI; March 2006, http://www.genome.gov/10001213) notes that approximately 30,000 young people had the disease in 2006; their median life span was thirty years (meaning half would live longer and half would not live this long). An estimated 10 million Americans, almost all of whom are white, are symptomless carriers of the CF gene. Like sickle-cell disease, it is a recessive genetic disorder—to inherit this disease, a child must receive the CF gene from both parents.

In 1989 the CF gene was identified, and in 1991 it was cloned and sequenced. It is located on the long arm of chromosome 7 at position 31.2. (See Figure 5.12.) The gene was called cystic fibrosis transmembrane conductance regulator (CFTR) because it was discovered to encode a membrane protein that controls the transit of chloride ions across the plasma membrane of cells. Nearly 1,000 mutations of the large gene—250,000 nucleotides—have been identified. Though most are extremely rare, several account for more than two-thirds of all mutations. Figure 5.13 shows three types of defects in CFTR that can cause CF. The most frequently occurring mutation causes faulty processing of the protein such that the protein is degraded before it reaches the cell membrane.

The mutated versions of the gene found in people with CF were seen to cause relatively modest impairment of chloride transport in cells. However, this seemingly minor defect can result in a multisystem disease that affects organs and tissues throughout the body, provoking abnormal, thick secretions from glands and epithelial cells. Ultimately, these secretions fill the lungs and cause affected children to die of respiratory failure.

The progression from the defective gene and protein it encodes to life-threatening illness follows this complex path:

1. The defect in chloride passage across the cell membrane indirectly produces an accumulation of thick mucus secretions in the lungs.
2. The bacteria Pseudomonas aeruginosa grows in the mucus.
3. In a campaign to combat the bacterial invasion, the body's immune system is activated but is unable to access the bacteria because the thick mucus protects it.
4. The immune reaction persists and becomes chronic, resulting in inflammation that harms the lung.
5. Ultimately, it is the affected individual's own immune response, rather than the defective CF gene, protein, or the bacterial infection, that produces the often fatal damage to the lungs.

A simple sweat test is currently the standard diagnostic test for CF. The test measures the amount of salt in the sweat; abnormally high levels are the hallmark of cystic fibrosis.

CF Gene-Screening Falters

In August 1989 researchers isolated the specific gene that causes CF. The mutation of this gene accounts for about 70% of the cases of the disease. In 1990 scientists successfully corrected the biochemical defect by inserting a healthy gene into diseased cells grown in the laboratory, a major step toward developing new therapies for the disease. In 1992 they injected healthy genes into laboratory rats with a deactivated common cold virus as the delivery agent. The rats began to manufacture the missing protein, which regulates the chloride and sodium in the tissues, preventing the deadly buildup of mucus. Scientists were hopeful that within a few years CF would be eliminated as a fatal disease, giving many children the chance for healthy, normal lives.

In 1993, however, optimism faded when the medical community discovered that the CF gene was more com-plicated than expected. Scientists found that the gene can be mutated at more than 950 points, and more points are being recognized at an alarming rate. At the same time, they discovered that many people who have inherited mutated genes from both parents do not have cystic fibrosis. With so many possible mutations, the potential combinations in a person who inherits one gene from each parent are immeasurable.

The combinations of different mutations create different effects. Some may result in crippling and fatal CF, whereas others may cause less serious disorders, such as infertility, asthma, or chronic bronchitis. To further complicate the picture, other genes can alter the way different mutations of the CF gene affect the body.

The Cystic Fibrosis Foundation supports clinical research studies in human gene therapy. One form of gene therapy uses a compacted DNA technology. This gene transfer system compacts single copies of the healthy CFTR gene so that they are small enough to pass through a cell membrane into the nucleus. The goal is for the DNA to produce the CFTR protein that is needed to correct the basic defect in CF cells.

Researchers are also finding that CF mutations may be much more common than previously thought. For example, Stephanie A. Cohen and Lyn Hammond, in "Abstracts from the Nineteenth Annual Education Conference of the National Society of Genetic Counselors (Savannah, Georgia, November 2000)" (Journal of Genetic Counseling, December 2000), cite a study of 5,000 healthy women receiving prenatal care at Kaiser Permanente in northern California who were tested for the CF gene, which is thought to be present in less than 1% of the population. Of those screened, 11% had the mutation. This finding may indicate that many more common diseases, such as asthma, may be caused by mutations of the CF gene. Other scientists speculate that the frequency of CF carriers among people of European descent may have, at some point in time, conferred immunity to some other disorder, in much the same way that the sickle-cell carriers were protected from malaria.

DIABETES

Diabetes is a disease that affects the body's use of food, causing blood glucose (sugar levels in the blood) to become too high. Normally, the body converts sugars, fats, starches, and proteins into a form of sugar called glucose. The blood then carries glucose to all the cells throughout the body. In the cells, with the help of the hormone insulin, which facilitates the entry of glucose into the cells, the glucose is either converted into energy for use immediately or stored for the future. Beta cells of the pancreas, a small organ located behind the stomach, manufacture the insulin. The process of turning food into energy via glucose (blood sugar) is important because the body depends on glucose for its energy source.

In a person with diabetes, food is converted to glucose, but there is a problem with insulin. In one type of diabetes the pancreas does not manufacture enough insulin, and in another type the body has insulin but cannot use the insulin effectively (this latter condition is called insulin resistance). When insulin is either absent or ineffective, glucose cannot get into the cells to be converted into energy. Instead, the unused glucose accumulates in the blood. If a person's blood-glucose level rises high enough, the excess glucose is excreted from the body via urine, causing frequent urination. This, in turn, leads to an increased feeling of thirst as the body tries to compensate for the fluid lost through urination.

Types of Diabetes

There are two distinct types of diabetes. Type 1 diabetes (also called juvenile diabetes) occurs most often in children and young adults. The pancreas stops manufacturing insulin, so the hormone must be injected daily. Type 2 diabetes is most often seen in adults. In this type the pancreas produces insulin, but it is not used effectively and the body resists its effects. Table 5.3 compares the phenotype (presentation) and genotype of Type 1 and Type 2 diabetes.

According to the Centers for Disease Control and Prevention (CDC; October 26, 2005, http://www.cdc.gov/od/oc/media/pressrel/fs051026.htm), in 2005 approximately 21 million Americans (7% of the population) had diabetes. Of these, approximately one-third had not been diagnosed. In 2004 diabetes was the sixth leading cause of death and in 2005, 1.5 million new cases of diabetes were diagnosed. (See Table 5.1 and Figure 5.14.) The individuals most at risk for Type 2 diabetes are usually overweight, over forty years old, and have a family history of diabetes. The CDC (January 31, 2005, http://www.cdc.gov/diabetes/pubs/general.htm) reports that Type 2 patients represent 90% to 95% of diabetes patients, while Type 1 accounts for 5% to 10% of diabetes cases.

Causes of Diabetes

The causes of both Type 1 and Type 2 diabetes are unknown, but a family history of the disease increases the risk for both types, leading researchers to believe there is a genetic component. Some scientists believe that a flaw in the body's immune system may be a factor in Type 1 diabetes. Poor cardiovascular fitness is another risk factor for developing diabetes.

Mutations in several genes probably contribute to the origin and onset of Type 1 diabetes. For example, an insulin-dependent diabetes mellitus (IDDM1) site on chromosome 6 may harbor at least one susceptibility gene for Type 1 diabetes. The role of this mutation in increasing susceptibility is not yet known; however, because chromosome 6 also contains genes for antigens (the molecules that normally tell the immune system not to attack itself), there may be some interaction between immunity and diabetes. In Type 1 diabetes the body's immune system mounts an immunological assault on its own insulin and the pancreatic cells that manufacture it. Some ten sites in the human genome, including a gene at the locus IDDM2 on chromosome 11 and the gene for glucokinase, an enzyme that is crucial for glucose metabolism, on chromosome 7, appear to increase susceptibility to Type 1 diabetes.

In Type 2 diabetes heredity may be a factor, but because the pancreas continues to produce insulin, the disease is considered a problem of insulin resistance, in which the body is not using the hormone efficiently. In people prone to Type 2 diabetes, being overweight can set off the disease because excess fat prevents insulin from working correctly. Maintaining a healthy weight and keeping physically fit can usually prevent noninsulin-dependent diabetes. To date, insulin-dependent diabetes (Type 1) cannot be prevented.

Complications of Diabetes

Because diabetes deprives body cells of the glucose needed to function properly, complications can develop that threaten the lives of diabetics. Complications of diabetes include higher risk and rates of heart disease; circulatory problems, especially in the legs, often severe enough to require surgery or even amputation; diabetic retinopathy, a condition that can cause blindness; kidney disease that may require dialysis; dental problems; impaired healing and increased risk of infection; and problems of pregnancy. People who pay close attention to the roles of diet, exercise, weight management, and pharmacological control (proper use of insulin and other medication) to manage their disease suffer the fewest complications.

HUNTINGTON'S DISEASE

Huntington's disease (HD) is an inherited, progressive brain disorder. It causes the degeneration of cells in the basal ganglia, a pair of nerve clusters deep in the brain that affect both the body and the mind. HD is caused by a single dominant gene that affects men and women of all races and ethnic groups. Figure 5.15 shows the inheritance pattern of HD.

Gene Responsible for HD Found

The gene mutation that produces HD was mapped to chromosome 4 in 1983 and cloned in 1993. (See Figure 5.16.) In the HD gene the mutation involves a triplet of nucleotides, cytosine (C), adenine (A), and guanine (G), known as CAG. The mutation is an expansion of a nucleotide triplet repeat in the DNA that codes for the protein huntingtin. In unaffected people the gene has thirty or fewer of these triplets, but HD patients have forty or more. These increased multiples either destroy the gene's ability to make the necessary protein or cause it to produce a misshapen and malfunctioning protein. Either way, the defect results in the death of brain cells.

The number of repeated triplets is inversely related to the age when the individual first experiences symptoms—the more repeated triplets, the younger the age of onset of the disease. Like myotonic dystrophy, in which the symptoms of the disease often increase in severity from one generation to the next, the unstable triplet repeat sequence can lengthen from one generation to the next, with a resultant decrease in the age when symptoms first appear.

HD does not usually strike until mid-adulthood, between the ages thirty and fifty, although there is a juvenile form that can affect children and adolescents. Early symptoms, such as forgetfulness, a lack of muscle coordination, or a loss of balance, are often ignored, delaying the diagnosis. The disease gradually takes its toll over a ten- to twenty-five-year period.

Within a few years characteristic involuntary movement (chorea) of the body, limbs, and facial muscles appears. As HD progresses, speech becomes slurred and swallowing becomes difficult. The patients' cognitive abilities decline, and there are distinct personality changes—depression and withdrawal, sometimes countered with euphoria. Eventually, nearly all patients must be institutionalized, and they usually die as a result of choking or infections.

Prevalence of HD

HD, once considered rare, is now recognized as one of the more common hereditary diseases. According to the Genetics Home Reference (January 5, 2007, http://ghr.nlm.nih.gov/condition=huntingtondisease), HD affects 3 to 7 per 100,000 people of European ancestry. HD appears to be less common in other populations, including people of Japanese, Chinese, and African descent. The NHGRI (October 2006, http://www.genome.gov/10001215) reports that in the United States about 30,000 people have HD, an additional 35,000 people exhibit some symptoms, and 75,000 people carry the abnormal gene that will cause them to develop the disease.

Prediction Test

In 1983 researchers identified a DNA marker that made it possible to offer a test to determine whether an individual has inherited the HD gene before symptoms appear. In some cases it is even possible to make a prenatal diagnosis on an unborn child. Many people, however, prefer not to know whether or not they carry the defective gene. Currently, researchers are trying to determine if the exact number of excess triplets indicates when in life a person will be affected by the disease. Some scientists fear that the ability to tell people that they are going to develop an incurable disease and pinpoint when they will develop it will make genetic testing, already a difficult decision, even more complicated.

MUSCULAR DYSTROPHY

Muscular dystrophy (MD) is a term that applies to a group of more than thirty types of hereditary muscle-destroying disorders. More than a million Americans are affected by one of the forms of MD. Each variant of the disease is caused by defects in the genes that play important roles in the growth and development of muscles. Duchenne muscular dystrophy (DMD) is one of the most frequently occurring types of MD and is characterized by rapid progression of muscle degeneration that occurs early in life. In all forms of MD the proteins produced by the defective genes are abnormal, causing the muscles to waste away. Unable to function properly, the muscle cells die and are replaced by fat and connective tissue. The symptoms of MD may not be noticed until as much as 50% of the muscle tissue has been affected.

DMD is X-linked, affects mostly males, and, according to the NCBI (2007, http://www.ncbi.nlm.nih.gov/books/bv.fcgi?call=bv.View..ShowSection&rid=gnd.section.161), strikes 1 out of 3,500 boys worldwide. The gene for DMD is located on the X chromosome and encodes a large protein called dystrophin. (See Figure 5.17.) Dystrophin provides structural support for muscle cells and without it the cell membrane becomes penetrable, allowing extracellular components into the cell. (See Figure 5.18.) These additional components increase the intracellular pressure, causing the muscle cell to die.

With myotonic dystrophy the muscles contract but have diminishing ability to relax, and there is muscle weakening and wasting. Typically, the initial complaints are the loss of hand strength or tripping while walking or climbing stairs. Along with decreased muscle strength, myotonic dystrophy may cause mental deficiency, hair loss, and cataracts. According to HealthAtoZ.com (2006, https://www.healthatoz.com/healthatoz/Atoz/common/standard/transform.jsp?requestURI=/healthatoz/Atoz/ency/myotonic_dystrophy.jsp), it is an autosomal dominant disorder that occurs in 1 out of 20,000 people; it usually begins in young adulthood but can start at any age and varies in terms of severity.

The myotonic dystrophy gene is a protein kinase gene found on the long arm of chromosome 19. (See Figure 5.19.) The defect is a repeated set of three nucleotides—cytosine (C), thymine (T), and guanine (G)—in the gene. The symptoms of myotonic dystrophy frequently become more severe with each generation because mistakes in copying the gene from one generation to the next result in amplification of a genomic AGC/CTG triplet repeat, similar to the process observed in Huntington's disease. Unaffected individuals have CTG repeats with between three and thirty-seven iterations (repetitions) of the triplet. In contrast, people with the mild phenotype of myotonic dystrophy have between 40 and 170, and those with more serious forms of the disease have between 100 and 1,000 iterations.

All the various disorders labeled MD cause progressive weakening and wasting of muscle tissues. They vary, however, in terms of the usual age at the onset of symptoms, rate of progression, and initial group of muscles affected. The most common type, DMD, affects young boys, who show symptoms in early childhood and usually die from respiratory weakness or damage to the heart before adulthood. The gene is passed from the mother to her children. Females who inherit the defective gene generally do not manifest symptoms—they become carriers of the defective genes, and their children have a 50% chance of inheriting the disease. Other forms of MD appear later in life and are usually not fatal.

In 1992 scientists discovered the defect in the gene that causes myotonic dystrophy. In people with this disorder, a segment of the gene is enlarged and unstable. This finding helps physicians more accurately diagnose myotonic dystrophy. Researchers have since identified genes linked to other types of MD, including DMD, Becker MD, limb-girdle MD, and Emery-Dreifuss MD.

In 2005 Duygu Selcen and Andrew G. Engel of the Mayo Clinic identified a new form of MD that involves mutations in a protein called ZASP, which binds to cardiac (heart) and skeletal muscles, and reported their finding in "Mutations in ZASP Define a Novel Form of Muscular Dystrophy in Humans" (Annals of Neurology, February 2005). Selcen and Engel detected ZASP mutations in eleven patients; in seven of these, they observed a dominant pattern of inheritance.

Treatment and Hope

There is no known cure for MD, but patients can be made more comfortable and functional by a combination of physical therapy, exercise programs, and orthopedic devices (special shoes, braces, or powered wheelchairs) that help them to maintain mobility and independence as long as possible.

Genetic research offers hope of finding effective treatments, and even cures, for these diseases. Gene therapy experiments designed to find a cure or a treatment for one or more of these types of MD are ongoing. Research teams have identified the crucial proteins produced by these genes, such as dystrophin, beta sarcoglycan, gamma sarcoglycan, and adhalin. One experimental treatment approach involves substituting a protein of comparable size, such as utrophin for dystrophin, to compensate for the loss of dystrophin. (See Figure 5.18.)

Because defective or absent proteins cause MD, researchers hope that experimental treatments to transplant normal muscle cells into wasting muscles will replace the diseased cells. Muscle cells, unlike other cells in the body, fuse together to become giant cells. Scientists hope that if cells with healthy genes can be introduced into the muscles and accepted by the body's immune system, the muscle cells will then begin to produce the missing proteins.

New delivery methods called vectors are also being tested, such as implanting a healthy gene into a virus that has been stripped of all of its harmful properties and then injecting the modified virus into a patient. Researchers hope this will reduce the amount of rejection by the patient's immune system, allowing the healthy gene to restore the missing muscle protein.

PHENYLKETONURIA

Phenylketonuria (PKU) is an example of a disorder caused by a gene-environment interaction. As a result of the defect, the affected individual is unable to convert phenylalanine into tyrosine. Phenylalanine in the body accumulates in the blood and can reach toxic levels. (See Figure 5.4.) This toxicity may impair brain and nerve development and result in mental retardation, organ damage, and unusual posture. When it occurs during pregnancy, it may jeopardize the health and viability of the unborn child.

Originally, PKU was considered simply an autosomal recessive inherited error of metabolism that occurred when an individual received two defective copies, caused by mutations in both alleles of the phenylalanine hydrox-ylase gene found on chromosome 12. (See Figure 5.20.) The environmental trigger—dietary phenylalanine—was not identified at first because phenylalanine is so prevalent in the diet, occurring in common foods such as milk and eggs and in the artificial sweetener aspartame. Recognition of dietary phenylalanine as a critical environmental trigger has enabled children born with PKU to lead normal lives when they are placed on low-phenylalanine diets, and mothers with the disease can bear healthy children.

The March of Dimes reports that to identify people at risk of PKU, all newborns in the United States are screened at birth for high levels of phenylalanine in the blood (http://search.marchofdimes.com/cgi-bin/MsmGo.exe?grab_id=0 &page_id=162&query=PKU&hiword=PKU%20). After a second screening is done for those with elevated blood levels, approximately 1 out of 10,000 infants is diagnosed with PKU and, with proper diet, is likely to lead a healthy, normal life.

SICKLE-CELL DISEASE

Sickle-cell disease (SCD) is a group of hereditary diseases, including sickle-cell anemia (SCA) and sickle B-thalassemia, in which the red blood cells contain an abnormal hemoglobin, called hemoglobin S (HbS). HbS is responsible for the premature destruction of red blood cells, or hemolysis. In addition, it causes the red cells to become deformed, actually taking on a sickle shape, particularly in parts of the body where the amount of oxygen is relatively low. These abnormally shaped cells cannot travel smoothly through the smaller blood vessels and capillaries. They tend to clog the vessels and prevent blood from reaching vital tissues. This blockage produces anoxia (lack of oxygen), which in turn causes more sick-ling and more damage.

SCA is an autosomal recessive disease caused by a point mutation in the hemoglobin beta gene (HBB) found on chromosome 11p15.5. (Figure 5.2.) A mutation in HBB results in the production of hemoglobin with an abnormal structure. Figure 5.21 shows how a point mutation in SCA causes the amino acid glutamine to be replaced by valine to produce abnormal hemoglobin called HbS. It also shows how when red blood cells with HbS are oxygen-deprived they become sickle shaped and may cause blockages that result in tissue death.

Symptoms of SCA

People with SCA have symptoms of anemia, including fatigue, weakness, fainting, and palpitations or an increased awareness of their heartbeat. These palpitations result from the heart's attempts to compensate for the anemia by pumping blood faster than normal.

In addition, patients experience occasional sickle-cell crises—attacks of pain in the bones and abdomen. Blood clots may also develop in the lungs, kidneys, brain, and other organs. A severe crisis or several acute crises can permanently damage various organs of the body. This damage can lead to death from heart failure, kidney failure, or stroke. The frequency of these crises varies from patient to patient. A sickle-cell crisis, however, occurs more often during infections and after an accident or an injury.

Who Contracts SCD?

Both the sickle-cell trait and the disease exist almost exclusively in people of African, Native American, and Hispanic descent and in those from parts of Italy, Greece, Middle Eastern countries, and India. If one parent has the sickle-cell gene, then the couple's offspring will carry the trait; if both the mother and the father have the trait, then their children may be born with SCA. This trait is relatively common among African-Americans. People of African descent are advised to seek genetic counseling and testing for the trait before starting a family. According to the World Health Organization (March 10, 2006, http://www.who.int/genomics/public/geneticdiseases/en/index2.html#SCA), the sickle-cell trait is present in one out of twelve African-Americans, or about 2 million people. SCD is the most common inherited blood disorder in the United States, affecting 72,000 Americans, most of whom have African ancestry. SCA occurs in approximately 1 out of 500 African-American births and in 1 out of 1,000 to 1,400 Hispanic births. The occurrence of SCA in other groups is much lower.

Treatment of SCD

There is no universal cure for SCD, but the symptoms can be treated. Crises accompanied by extreme pain are the most common problems and can usually be treated with painkillers. Maintaining healthy eating and behavior and prompt treatment for any type of infection or injury is important. Special precautions are often necessary before any type of surgery, and for major surgery some patients receive transfusions to boost their levels of hemoglobin (the oxygen-bearing, iron-containing protein in red blood cells). In early 1995 a medication that prevented the cells from clogging vessels and cutting off oxygen was approved by the Food and Drug Administration.

BIOMEDICAL ADVANCES

Many adults with SCD now take hydroxyurea, an anticancer drug that causes the body to produce red blood cells that resist sickling. In 1995 a multicenter study showed that among adults with three or more painful crises per year, hydroxyurea lowered the median number of crises requiring hospitalization by 58%. In 2003 Martin H. Steinberg et al. reported in "Effect of Hydroxyurea on Mortality and Morbidity in Adult Sickle Cell Anemia: Risks and Benefits Up to 9 Years of Treatment" (Journal of the American Medical Association, April 2, 2003) that not only do patients on hydroxyurea have fewer crises but also they have a significant survival advantage when compared to SCD patients who do not take the medication. Subjects treated with it showed 40% lower mortality than others.

TAY-SACHS DISEASE

Tay-Sachs disease (TSD) is caused by mutations in the HEXA gene, located on the long arm of chromosome 15. (See Figure 5.22.) It is a fatal genetic disorder in children that causes the progressive destruction of the central nervous system. It is caused by the absence of an important enzyme called hexosaminidase A (hex-A). Without hex-A, a fatty substance called GM2 ganglioside builds up abnormally in the cells, particularly the brain's nerve cells. (Figure 5.23 shows how the absence of, or defect in, the hex-A protein prevents complete processing of GM2 ganglioside.) Eventually, these cells degenerate and die. This destructive process begins early in the development of a fetus, but the disease is not usually diagnosed until the baby is several months old. By the time a child with TSD is four or five years old, the nervous system is so badly damaged that the child dies.

How Is TSD Inherited?

TSD is an autosomal recessive genetic disorder caused by mutations in both alleles of the HEXA gene on chromosome 15. Both the mother and the father must be carriers of the defective TSD gene to produce a child with the disease.

People who carry the gene for TSD are entirely unaffected and usually unaware that they have the potential to pass this disease to their offspring. A blood test distinguishes Tay-Sachs carriers from noncarriers. Blood samples may be analyzed by enzyme assay or DNA studies. Enzyme assay measures the level of hex-A in blood. Carriers have less hex-A than noncarriers. When only one parent is a carrier, the couple will not have a child with TSD. When both parents carry the recessive TSD gene, they have a one in four chance in every pregnancy of having a child with the disease. They also have a 50% chance of bearing a child who is also a carrier. Prenatal diagnosis early in pregnancy can predict if the unborn child has TSD. If the fetus has the disease, the couple may choose to terminate the pregnancy.

Who Is at Risk?

Some genetic diseases, such as TSD, occur most frequently in a specific population. As the American neurologist Bernard Sachs observed, individuals of East European (Ashkenazi) Jewish descent have the highest risk of being carriers of TSD. According to the National Tay-Sachs and Allied Diseases Association (2007, http://www.tay-sachs.org/taysachs.php), approximately one out of twenty-seven Jews in the United States is a carrier of the TSD gene. French-Canadians and Cajuns also have the same carrier rate as Ashkenazi Jews. In the general population the carrier rate is 1 out of 250.

CHAPTER 5
GENETIC DISORDERS

We could wish that … life-histories were found in every family, showing the health and diseases of its different members. We might thus in time find evidences of pathological connections and morbid liabilities not now suspected.

—William Gull, 1896

It has long been known that heredity affects health. Genetics, the study of single genes and their effects on the body and mind, explains how and why certain traits such as hair color and blood types run in families. Genomics, a discipline that is only about two decades old, is the study of more than single genes; it considers the functions and interactions of all the genes in the genome. In terms of health and disease, genomics has a broader and more promising range than genetics. The science of genomics relies on knowledge of and access to the entire genome and applies to common conditions, such as breast and colorectal cancer, Parkinson's disease, and Alzheimer's disease. It also has a role in infectious diseases once believed to be entirely environmentally caused such as human immunodeficiency virus (HIV, which is the virus that causes acquired immune deficiency syndrome [AIDS]) infection and tuberculosis. Like most diseases, these frequently occurring disorders are due to the interactions of multiple genes and environmental factors. Genetic variations in these disorders may have a protective or a causative role in the expression of diseases.

It is commonly accepted that diseases fall into one of three broad categories: those few that are primarily genetic in origin; those that are largely attributable to environmental causes; and those—the majority of conditions—in which genetics and environmental factors make comparable, though not necessarily equal, contributions. As understanding in genomics advances and scientists identify genes involved in more diseases, the distinctions between these three classes of disorders is diminishing. This chapter considers some of the disorders believed to be predominantly genetic in origin and some that are the result of genes acted on by environmental factors.

There are two types of genes: dominant and recessive. When a dominant gene is passed on to offspring, the feature or trait it determines will appear regardless of the characteristics of the corresponding gene on the chromosome inherited from the other parent. If the gene is recessive, the feature it determines will not show up in the offspring unless both the parents' chromosomes contain the recessive gene for that characteristic. Similarly, among diseases and conditions primarily attributable to a gene or genes, there are autosomal dominant disorders and autosomal recessive disorders.

Another way to characterize genetic disorders is by their pattern of inheritance, as single gene, multifactorial, chromosomal, or mitochondrial. Single-gene disorders (also called Mendelian or monogenic) are caused by mutations in the deoxyribonucleic acid (DNA) sequence of one gene. Since genes code for proteins, when a gene is mutated so that its protein product can no longer carry out its normal function, it may produce a disorder. There are more than 6,000 known single-gene disorders, which occur in about one in every 200 births. Examples are cystic fibrosis, sickle-cell anemia, Huntington's disease, and hereditary hemochromatosis (a disorder in which the body absorbs too much iron from food; rather than the excess iron being excreted, it is stored throughout the body, and iron deposits damage the pancreas, liver, skin, and other tissues). Figure 5.1 shows the cystic fibrosis gene and its location on chromosome 7; Figure 5.2 shows the sickle-cell anemia gene found on chromosome 11; and Figure 5.3 shows the hereditary hemochromatosis gene located on chromosome 6. Single-gene disorders are the result of either autosomal dominant, autosomal recessive, or X-linked inheritance.

Multifactorial or polygenic disorders result from a complex combination of environmental factors and mutations in multiple genes. For example, different genes that influence breast cancer susceptibility have been found on seven different chromosomes, rendering it more difficult to analyze than single-gene or chromosomal disorders. Some of the most common chronic diseases are multifactorial in origin. Examples include heart disease, Alzheimer's disease, arthritis, diabetes, and cancer.

Chromosomal disorders are produced by abnormalities in chromosome structure, missing or extra copies of chromosomes, or errors such as translocations (movement of a chromosome section from one chromosome to another). Down's syndrome or trisomy 21 is a chromosomal disorder that results when an individual has an extra copy, or a total of three copies, of chromosome 21. Mitochondrial disorders result from mutations in the nonchromosomal DNA of mitochondria, which are organelles involved in cellular respiration. Compared with the three other patterns of inheritance, mitochondrial disorders occur infrequently.

There are significant differences between the nineteenth-century germ theory of disease and the twenty-first-century genomic theory of disease. By the middle of the twentieth century it became possible to improve the quality of life and to save the lives of people with some genetic diseases. Effective treatment included changes in diet to prevent or manage conditions such as phenylketonuria (PKU) and glucose galactose malabsorption (GGM). PKU is an inherited error of metabolism caused by a deficiency in the enzyme phenylalanine hydroxylase. (See Figure 5.4.) It may result in mental retardation, organ damage, and unusual posture. Dietary changes are also used to treat GGM, a rare metabolic disorder caused by lack of the enzyme that converts galactose into glucose. For people with severe cases of GGM, it is vital to avoid lactose (milk sugar), sucrose (table sugar), glucose, and galactose. Other therapeutic measures may involve surgery to correct deformities and avoidance of environmental triggers, as in some types of asthma.

At the dawn of the twenty-first century, the possibility of preventing and changing genetic legacies appears within reach of modern medical science. Genomic medicine predicts the risk of disease in the individual, whether highly probable, as in the case of some of the well-established single-gene disorders, or in terms of an increased susceptibility likely to be influenced by environmental factors. The promise of genomic medicine is to make preventive medicine more powerful and treatment more specific to the individual, enabling investigation and treatment that are custom-tailored to an individual's genetic susceptibilities, or to the characteristics of the specific disease or disorder.

COMMON GENETICALLY INHERITED
DISEASES

Although many diseases, disorders, and conditions are termed "genetic," classifying a disease as genetic simply means that there is an identified genetic component to either its origin or its expression. Many medical geneticists contend that the majority of diseases cannot be classified as strictly genetic or environmental. The phenotype of genetic diseases can sometimes be modified, even to the point of nonexpression, by controlling environmental factors. Similarly, environmental (infectious) diseases may not be expressed because of some genetic predisposition to immunity. Each disease, in each individual, exists along a continuum between a genetic disease and an environmental disease.

Myriad diseases are believed to have strong genetic contributions, including:

• Heart disease—coronary atherosclerosis, hypertension (high blood pressure), and hyperlipidemia (elevated blood levels of cholesterol and other lipids)
• Diabetes
• Cancer—retinoblastomas, colon, stomach, ovarian, uterine, lung, bladder, breast, skin (melanoma), pancreatic, and prostate
• Neurological disorders—Alzheimer's disease, amyotrophic lateral sclerosis (also known as Lou Gehrig's disease), Gaucher's disease, Huntington's disease, multiple sclerosis, narcolepsy, neurofibromatosis, Parkinson's disease, Tay-Sachs disease, and Tourette's syndrome
• Mental illnesses, mental retardation, and behavioral conditions—alcoholism, anxiety disorders, attention deficit hyperactivity disorder, eating disorders, Lesch-Nyhan syndrome, manic depression, and schizophrenia
• Other genetic disorders—cleft lip and cleft palate, clubfoot, cystic fibrosis, Duchenne muscular dystrophy, glucose galactose malabsorption, hemophilia, Hurler's syndrome, Marfan's syndrome, phenylketonuria, sickle-cell disease, and thalassemia
• Other medical conditions—including alpha-1-antitrypsin, arthritis, asthma, baldness, congenital adrenal hyperplasia, migraine headaches, obesity, periodontal disease, porphyria, and some speech disorders

ALZHEIMER'S DISEASE

Alzheimer's disease (AD) is a progressive, degenerative disease that affects the brain and results in severely impaired memory, thinking, and behavior. It is the fourth leading cause of death in adults, and the incidence of the disease rises with age. AD affects an estimated four million American adults and is the most common form of dementia, or loss of intellectual function. The U.S. Department of Health and Human Services (DHHS) estimates that 4.5 million Americans suffered from AD in 2004, and in 2002 it was the eighth-leading cause of death in the United States. AD also contributes to many more deaths that are attributed to other causes, such as heart and respiratory failure. (See Table 5.1.)

The disease was first described by the German physician Alois Alzheimer (1864–1915) in 1907 after he had cared for a patient with an unusual mental illness. Alzheimer observed anatomical changes in his patient's brain and described them as abnormal clumps and tangled bundles of fibers. Nearly a century later, these abnormal findings, now described as amyloid plaques and neurofibrillary tangles, along with abnormal clusters of proteins in the brain, are the characteristic markers of AD.

The disease knows no social or economic boundaries and affects men and women almost equally. While 90% of AD victims are over age sixty-five, AD can strike as early as the thirties, forties, and fifties. Most patients are cared for at home as long as possible, a situation that can be emotionally and physically devastating for the affected individuals and their families.

AD has become a disease of particular concern in the United States because the nation's older adult population is growing rapidly. Approximately 10% of the population over age sixty-five is afflicted with AD. This percentage rises to more than 47% in those over age eighty-five, the fastest-growing segment of our society. By 2050 the United States will have approximately 86.7 million people over age sixty-five, according to projections released by the U.S. Census Bureau in March 2004 (http://www.census.gov/ipc/www/usinterimproj/). According to the Census Bureau, the numbers of cases of AD are expected to increase by more than 12% every five years through 2015. Prevalence (the number of people with a disease at a given time) is partially determined by the length of time people with AD survive. While average survival is eight years after diagnosis, some AD patients have lived longer than twenty years with the disease. Therefore, improvements in AD care, as well as increased length of life of the older adult population in general, will increase the numbers of AD patients.

In November 1994 former president Ronald Reagan, at age eighty-three, announced that he had been diagnosed with AD and wanted to make Americans more aware of the disease. Some observers believe that it is theoretically possible that AD had affected Reagan before he left the White House in January 1989. In 2004 former president Reagan died as a result of the disease. In August 2002 another American legend, actor and activist Charlton Heston, announced that he, too, had been diagnosed with AD.

Genetic Causes of AD

AD is not a normal consequence of growing older, and scientists are continuing to seek its cause. Researchers have found some promising genetic clues to the disease. Table 5.2 shows the different patterns of inheritance, ages of onset (when symptoms begin), genes, chromosomes, and proteins linked to the development of AD. Mutations in four genes, situated on chromosomes 1, 14, 19, and 21, are thought to be involved in the disease, and the best described are PS1 (or AD3) on chromosome 14 and PS2 (or AD4) on chromosome 1. (See Figure 5.5 and Figure 5.6.)

The formation of lesions made of fragmented brain cells surrounded by amyloid-family proteins is characteristic of the disease. Interestingly, these lesions and their associated proteins are closely related to similar structures found in Down's syndrome. Tangles of filaments largely made up of a protein associated with the cytoskeleton have also been observed in samples taken from AD brain tissue.

The first genetic breakthrough was reported in the February 1991 issue of the British journal Nature. Investigators reported that they had discovered that a mutation in a single gene could cause this progressive neurological illness. Scientists found the defect in the gene that directs cells to produce a substance called amyloid protein. Researchers at the Massachusetts Institute of Technology found that low levels of the brain chemical acetylcholine contribute to the formation of hard deposits of amyloid protein that accumulate in the brain tissue of AD patients. In normal people the protein fragments are broken down and excreted by the body. Amyloid protein is found in cells throughout the body. Researchers do not know how it becomes a deadly substance in the brain cells of some people and not others.

In 1995 three more genes linked to AD were identified. One gene appears to be related to the most devastating form of AD, which can strike people in their thirties. When defective, the gene may prevent brain cells from correctly processing a substance called beta amyloid precursor protein. The second gene is linked to another early-onset form of AD that strikes before age sixty-five. This gene also appears to be involved in producing beta amyloid. Researchers believe that the discovery of these two genes will allow them to narrow their search for the proteins responsible for early-onset AD and give them clues to the causes of AD in older people. (See Table 5.2.)

The third gene, known as apolipoprotein E (apoE), was actually reported as associated with AD in 1993, but its role in the body was not known at that time. Researchers have since found that the gene plays several roles. Within the body, it regulates lipid metabolism within the organs and helps to redistribute cholesterol. In the brain apoE participates in repairing nerve tissue that has been injured. There are three forms (alleles) of the gene: apoE-2, apoE-3, and apoE-4. Until recently, people with two copies of apoE-4, one from each parent, were thought to have a greatly increased risk of developing AD before age seventy. Between one-half and one-third of all AD patients have at least one apoE-4 gene, while only 15% of the general population have an apoE-4 gene. In 1998, however, researchers discovered that the apoE-4 gene seems to affect when a person may develop AD, not whether the person will develop the disease.

Another newly discovered gene, A2M-2, appears to affect whether a person will develop AD. Nearly one-third (30%) of Americans may carry A2M-2, a genetic variant that more than triples their risk of developing late-onset AD compared to siblings with the normal version of the A2M gene. The discovery of A2M-2 opens up the possibility of developing a drug that mimics the A2M gene's normal function. This has the potential to

protect susceptible persons against brain damage or perhaps even reverse it.

Symptoms of AD

AD begins slowly. The symptoms include difficulty with memory and a loss of cognitive function (intellectual abilities). The AD patient may also experience confusion; language problems, such as trouble finding words; impaired judgment; disorientation in place and time; and changes in mood, behavior, and personality. How quickly these changes occur varies from person to person, but eventually the disease leaves its victims unable to care for themselves. In their terminal stages AD victims require care twenty-four hours a day. They no longer recognize family members or themselves, and they need help with such daily activities as eating, dressing, bathing, and using the toilet. Eventually, they may become incontinent, blind, and unable to communicate. Finally, their bodies may "forget" how to breathe or make the heart beat. Many patients die from pneumonia.

Testing for AD

A complete physical, psychiatric, and neurological evaluation can usually produce a diagnosis of AD that is about 90% accurate. For many years, the only sure way to diagnose the disease was to examine brain tissue under a microscope, which was not possible while the AD victim was still alive. An autopsy of someone who has died of AD reveals a characteristic pattern that is the hallmark of the disease—tangles of fibers (neurofibrillary tangles) and clusters of degenerated nerve endings (neuritic plaques) in areas of the brain that are crucial for memory and intellect. Also, the cortex of the brain is shrunken.

In 1996 a San Francisco biotechnology firm developed a diagnostic test for AD. The test, which involves analysis of blood and spinal fluid, produced conclusive results in 60% of older patients with dementia attributed to AD.

In 2000 researchers at Brigham and Women's Hospital in Boston found that by using magnetic resonance imaging (MRI) they could measure the volume of brain tissue in areas of the brain used for memory, organizational ability, and planning, and using these measurements could accurately identify people with AD and predict who would develop AD in the future. The same year other scientists at New York University Medical Center, the Mayo Clinic, and the National Hospital for Neurology and Neurosurgery in London reported using MRI to identify parts of the brain affected by AD before symptoms appear and to measure brain atrophy to monitor the progression of AD.

In early 2005 researchers Chad Mirkin and William Klein at Northwestern University announced development of yet another diagnostic test that detects small amounts of protein in spinal fluid. The test is called a bio-barcode assay and is as much as a million times more sensitive than other tests. First used to identify a marker for prostate cancer, the test is used to detect a protein in the brain called amyloid-beta-derived diffusible ligand (ADDL). ADDLs are small soluble proteins that may be indicative of Alzheimer's disease. To detect them the researchers used nanoscale particles that had antibodies specific to ADDL. The researchers' findings were reported in the February 9, 2005, issue of the Proceedings of the National Academy of Science.

Investigators continue to look at other biological markers, such as blood tests, for AD and at neuropsychological tests, which measure memory, orientation, judgment, and problem solving, to see if they can accurately predict whether healthy, unaffected older adults will develop AD or whether those with mild cognitive impairment will go on to develop AD.

Physicians and neuroscientists have long been eager for a simple and accurate test that can distinguish people with AD from those with cognitive problems or dementias arising from other causes. An accurate test would allow the detection of AD early enough for the use of experimental medications to slow the progression of the disease, as well as identify those at risk of developing AD. But the availability of tests raises ethical and practical questions: Do patients really want to know their risks of developing AD? Will health insurers use genetic or other diagnostic test results to deny insurance coverage?

Treatments for AD

There is still no cure or prevention for AD, and treatment focuses on managing symptoms. Medication can lessen some of the symptoms, such as agitation, anxiety, unpredictable behavior, and depression. Physical exercise and good nutrition are important, as is a calm and highly structured environment. The object is to help the AD patient maintain as much comfort, normalcy, and dignity for as long as possible.

Until 1997 tacrine (marketed as Cognex®) was the nation's only Alzheimer's medication. But in early 1997 the Food and Drug Administration approved a new drug, donepezil, to be marketed under the trade name Aricept. Both drugs are cholinesterase inhibitors, which produce some delay in the deterioration of memory and other cognitive skills. They offer only mild benefits at best, but currently they are the only alternatives available to AD patients.

By 2002 the National Institute on Aging (NIA) reported that National Institutes of Health (NIH) affiliates and pharmaceutical companies were involved in clinical trials of fifty to sixty new drugs to treat AD. All of the drugs being tested are intended to improve the symptoms of AD and slow its progression, but none is expected to "cure" AD. The investigational drugs aim to address three aspects of AD: to improve cognitive function in persons with early AD, slow or postpone the progression of the disease, and control behavioral problems such as wandering, aggression, and agitation of patients with AD.

By 2005 five prescription drugs were available to treat people who suffer from AD. Four of these—Reminyl® (galantamine), Exelon® (rivastigmine), Aricept® (donepezil), and Cognex® (tacrine)—are cholinesterase inhibitors and are prescribed for the treatment of mild to moderate AD. These drugs produce some delay in the deterioration of memory and other cognitive skills in some patients. They offer mild benefits at best, and may lose their effectiveness over time, but currently they are the only alternatives available to treat mild to moderate AD.

The fifth approved medication, Namenda® (memantine), is prescribed for the treatment of moderate to severe AD. It acts to delay progression of some symptoms of moderate to severe AD and may allow patients to maintain certain daily functions a little longer. It is thought to act by regulating glutamate, a chemical in the brain that, in excessive amounts, may lead to brain cell death.

Other researchers are examining the roles of the hormones estrogen and progesterone on memory and cognitive function. Since AD involves inflammatory processes in the brain, scientists are also studying the use of anti-inflammatory agents such as ibuprofen or prednisone to reduce the risk of developing AD. Researchers are also investigating the relationship between the various gene sites, particularly the mutation on chromosome 21, and environmental influences that may increase susceptibility to AD. Another NIA-funded study is trying to find out whether antioxidants, such as vitamin E, can prevent people with mild memory impairment from progressing to AD.

In 2005 the NIA and Alzheimer's Association launched the AD Genetics Study, recruiting subjects from families in which two siblings developed AD after age sixty. They hope to compare the genes of the afflicted siblings with those from another family member who does not have AD in an effort to find the genes that contribute to AD.

CANCER

Cancer is a large group of diseases characterized by uncontrolled cell division and the growth and spread of abnormal cells. These cells may grow into masses of tissue called tumors. Tumors composed of cells that are not cancerous are called benign tumors. Tumors consisting of cancer cells are called malignant tumors. The dangerous aspect of cancer is that cancer cells invade and destroy normal tissue.

The mechanisms of action that disrupt the cell cycle are: impairment of a DNA repair pathway, transformation of a normal gene into an oncogene (a hyperactive gene that stimulates cell growth), and the malfunction of a tumor suppressor gene (a gene that inhibits cell division). Figure 5.7 shows the multiple systems that interact to control the cell cycle.

The spread of cancer cells occurs either by local growth of the tumor or by some of the cells becoming detached and traveling through the blood and lymphatic system to develop additional tumors in other parts of the body. Metastasis (the spread of cancer cells) may be confined to a local region of the body, but if left untreated (and often despite treatment), the cancer cells can spread throughout the entire body, causing death. It is perhaps the rapid, invasive, and destructive nature of cancer that makes it, arguably, the most feared of all diseases, even though it is second to heart disease as the leading cause of death in the United States.

Cancer can be caused by both external environmental influences (chemicals, radiation, and viruses) and internal factors (hormones, immune conditions, and inherited mutations). These factors may act together or in sequence to begin or promote cancer. There is consensus in the scientific community that several cancer-promoting influences accrue and interact before an individual will develop a malignant growth. With only a few exceptions, no single factor or risk alone is sufficient to cause cancer. As with other disorders that arise in response to multiple factors, susceptibility to certain cancers is often attributed to a mutated gene. Figure 5.8 shows how the inheritance of a mutated gene increases susceptibility for retinoblastoma (cancer of the eye that affects approximately 300 children in the United States each year).

Genetic Research

Scientists and physicians have known for some time that predisposition to some forms of breast cancer are inherited and have been searching for the gene or genes responsible so that they can test patients and provide more careful monitoring for those at risk. In 1994 doctors identified the BRCA1 gene, and in late 1995 they also isolated the BRCA2 gene. Since then it has been found that variations of the ATM, BRCA1, BRCA2, CHEK2, and RAD51 genes increase the risk of developing breast cancer, and the AR, DIRAS3, and ERBB2 (also called Her-2/neu) genes are associated with breast cancer.

If a woman with a family history of breast cancer inherits a defective form of either BRCA1 or BRCA2, she has an estimated 80% to 90% chance of developing breast cancer. Researchers also think that the two genes are linked to ovarian, prostate, and colon cancer, and BRCA2 likely plays some role in breast cancer in men. Scientists suspect that the two genes may also participate in some way in the development of breast cancer in women with no family history of the disease. Only about 5% of all cases of breast cancer are attributable to the susceptibility genes BRCA1 and BRCA2. Figure 5.9 shows the locations of the BRCA1 gene on the long arm of chromosome 17 at position 2. Figure 5.10 shows the location of the BRCA2 gene on the long arm of chromosome 13 at position 12.3.

A November 1996 study headed by Stephen C. Rubin, a professor and chief of gynecological oncology (cancers of the female reproductive organs) at the University of Pennsylvania, reported a result that was totally unexpected. The study found that women with defective BRCA1 genes who developed ovarian cancer survived longer than those without the mutated gene who developed ovarian cancer. Women with the defective gene lived an average of seventy-seven months after diagnosis, while those without the mutated gene averaged only a twenty-nine-month survival. Researchers do not know if those with the defective gene had less deadly types of cancers or if their cancers responded better to treatment.

Another form of breast cancer, due to multiple copies of a gene called ERBB2, causes an estimated 25% of the approximately 180,000 new cases of the disease in the United States each year. ERBB2 often triggers an aggressive form of cancer that can cause death more quickly than other breast cancers, often within ten to eighteen months after the cancer spreads. The ERBB2 gene produces a protein on the surface of cells that serves as a receiving point for growth-stimulating hormones. Figure 5.11 shows the location of ERBB2 on the long arm of chromosome 17 between positions 11.2 and 12.

Herceptin, a genetically engineered antibody, increases the benefits of chemotherapy by shrinking tumors and slowing the progression of ERBB2. Herceptin became available in late 1998.

CYSTIC FIBROSIS

Cystic fibrosis (CF) is the most common inherited fatal disease of children and young adults in the United States. It occurs in about one in every 3,000 Caucasian births, one in 15,000 African-Americans and one in 31,000 Asian-Americans. According to the NIH (http://www.genome.gov/10001213), approximately 30,000 young people had the disease in 2005; their median (half above and half below) life span is thirty years. An estimated twelve million Americans (one in twenty-five), almost all of whom are white, are symptomless carriers of the CF gene. Like sickle-cell disease, it is a recessive genetic disorder—in order to inherit this disease, a child must receive the CF gene from both parents.

In 1989 the CF gene was identified, and in 1991 it was cloned and sequenced. It is located on the long arm of chromosome 7 at position 31.2 (See Figure 5.12.) The gene was called cystic fibrosis transmembrane conductance regulator (CFTR) because it was discovered to encode a membrane protein that controls the transit of chloride ions across the plasma membrane of cells. Nearly 1,000 mutations of the large gene—250,000 nucleotides—have been identified. Though most are extremely rare, several account for more than two-thirds of all mutations. Figure 5.13 shows three types of defects in cystic fibrosis transmembrane conductance regulator that can cause CF. The most frequently occurring mutation causes faulty processing of the protein such that the protein is degraded before it reaches the cell membrane.

The mutated versions of the gene found in people with CF were seen to cause relatively modest impairment of chloride transport in cells. But this seemingly minor defect can result in a multisystem disease that affects organs and tissues throughout the body, provoking abnormal, thick secretions from glands and epithelial cells. Ultimately, these secretions fill the lungs and cause affected children to die of respiratory failure.

The progression from the defective gene and protein it encodes to life-threatening illness follows this complex path:

1. The defect in chloride passage across the cell membrane indirectly produces an accumulation of thick mucus secretions in the lungs.
2. The bacteria Pseudomonas aeruginosa grows in the mucus.
3. In a campaign to combat the bacterial invasion, the body's immune system is activated but is unable to access the bacteria because the thick mucus protects it.
4. The immune reaction persists and becomes chronic, resulting in inflammation that harms the lung.
5. Ultimately, it is the affected individual's own immune response, rather than the defective CF gene, protein, or the bacterial infection, that produces the often fatal damage to the lungs.

At first, a child with CF does not appear to be suffering from a serious illness, but the diagnosis is usually made by the age of three. Often, the only signs are a persistent cough, a large appetite but poor weight gain, an extremely salty taste to the skin, and large, foul-smelling bowel movements. A simple "sweat test" is currently the standard diagnostic test for CF. The test measures the amount of salt in the sweat; abnormally high levels are the hallmark of cystic fibrosis. Over time children with CF develop great difficulty in breathing. The CF gene causes the body to produce thick, sticky mucus in the lungs and pancreas, causing difficulty in breathing and interference with digestion.

CF Gene-Screening Falters

In August 1989 researchers isolated the specific gene that causes CF. The mutation of this gene accounts for about 70% of the cases of the disease. In 1990 scientists successfully corrected the biochemical defect by inserting a healthy gene into diseased cells grown in the laboratory, a major step toward developing new therapies for the disease. In 1992 they injected healthy genes into laboratory rats by using a deactivated common cold virus as the delivery agent. The rats began to manufacture the missing protein, which regulates the chloride and sodium in the tissues, preventing the deadly buildup of mucus. Scientists were hopeful that within a few years CF would be eliminated as a fatal disease, giving many children the chance for healthy, normal lives.

In 1993, however, optimism faded when the medical community discovered that the CF gene was more complicated than expected. Scientists found that the gene can be mutated at more than 950 points, and more points are appearing at an alarming rate. At the same time, they discovered that many people who have inherited mutated genes from both parents do not have cystic fibrosis. With so many possible mutations, the potential combinations in a person who inherits one gene from each parent are immeasurable.

The combinations of different mutations create different effects. Some may result in crippling and fatal CF, while others may cause less serious disorders, such as infertility, asthma, or chronic bronchitis. To further complicate the picture, other genes can alter the way different mutations of the CF gene affect the body.

In 2005 the Cystic Fibrosis Foundation continued to support clinical research studies in human gene therapy. Several studies used the adenovirus rather than the common cold virus as the vehicle for delivering healthy genes to lung or nasal tissue. Another study is using liposomes (fat cells) as a delivery vehicle. Still another form of gene therapy uses a compacted DNA technology. The nonviral gene transfer system compacts single copies of the healthy CFTR gene so that they are small enough to pass through a cell membrane into the nucleus. The goal is for the DNA to produce the CFTR protein that is needed to correct the basic defect in CF cells.

Researchers are also finding that CF mutations may be much more common than previously thought. For example, 5,000 healthy women receiving prenatal care at Kaiser Permanente in northern California were tested for the CF gene, thought to be present in less than 1% of the population. Of those screened, 11% had the mutation. This finding may indicate that many more common diseases, such as asthma, may be caused by mutations of the CF gene. Other scientists have speculated that the frequency of CF carriers among people of European descent may have, at some point in time, conferred immunity to some other disorder, in much the same way that the sickle-cell carriers were protected from malaria.

CF Carriers Do Not Always Inform Family Members

Since there is a relatively high frequency of carriers of the defective gene in the general population, in 2000 the NIH, the American College of Medical Genetics, and the American College of Obstetricians and Gynecologists issued a recommendation that CF screening be offered to every white woman who is pregnant or considering having a baby. However, the same year the results of a research study conducted at Northwestern University Medical School in Chicago found that many people who carry the CF gene fail to inform family members about their risk.

The investigators and other health educators believe that if carriers were better informed about their risks they might be more likely to disclose them. Pretest education and counseling were seen as key to increasing carriers' understanding of the significance of findings and their family planning options. For example, when both parents are carriers, the risk of their child having CF is one in four, and the risk of their child being a carrier is one in two. In this case parents may choose to have prenatal diagnosis using chorionic villus sampling (CVS) or amniocentesis to find out whether their unborn child will have the disease. Alternatively, they may choose to use assisted reproductive technology such as in vitro fertilization (in which the egg and sperm are united outside of the body) because it offers the option of preimplantation diagnosis. This is a genetic test that enables parents undergoing in vitro fertilization to screen an embryo for CF genetic mutations before it is implanted in the uterus to grow and develop.

DIABETES

Diabetes is a disease that affects the body's use of food, causing blood glucose (sugar levels in the blood) to become too high. Normally, the body converts sugars, fats, starches, and proteins into a form of sugar called glucose. The blood then carries glucose to all the cells throughout the body. In the cells, with the help of the hormone insulin, which facilitates the entry of glucose into the cells, the glucose is either converted into energy for use immediately or stored for the future. Beta cells of the pancreas, a small organ located behind the stomach, manufacture the insulin. The process of turning food into energy via glucose (blood sugar) is important because the body depends on glucose for its energy source.

In a body with diabetes, food is converted to glucose, but there is a problem with insulin. In one type of diabetes the pancreas does not manufacture enough insulin, and in another type the body has insulin but cannot use the insulin effectively (this latter condition is called insulin resistance). When insulin is either absent or ineffective, glucose cannot get into the cells to be converted into energy. Instead, the unused glucose accumulates in the blood. If a person's blood-glucose level rises high enough, the excess glucose "spills over" into the urine, causing frequent urination. This, in turn, leads to an increased feeling of thirst as the body tries to compensate for the fluid lost through urination.

Types of Diabetes

There are two distinct types of diabetes. Insulin-dependent diabetes (also called Type 1 or juvenile diabetes), the most severe form of the disease, occurs most often in children and young adults. The pancreas stops manufacturing insulin, and the hormone must be injected daily. Non-insulin-dependent diabetes (also known as Type 2) is most often seen in adults. In this type the pancreas produces insulin, but it is not used effectively and the body resists its effects. Table 5.3 compares the phenotype (presentation) and genotype of Type 1 and Type 2 diabetes.

According to the American Diabetes Association (ADA; http://www.diabetes.org/) in early 2005, approximately 18.2 million Americans (6.3% of the population) had diabetes. Of these, approximately one-third had not been diagnosed. The ADA reported that more than thirteen million Americans have Type 2 diabetes. The individuals most at risk for Type 2 diabetes are usually overweight, over forty years old, and have a family history of diabetes. Type 2 patients represent about 90% of diabetes patients. Type 1 accounts for only about 5 to 10% of diabetes cases.

Causes of Diabetes

The causes of both Type 1 and Type 2 diabetes are unknown, but a family history of the disease increases the risk for both types, leading researchers to believe there is a genetic component. Some scientists believe that a flaw in the body's immune system may be a factor in Type 1 diabetes. Poor cardiovascular fitness is another risk factor for developing diabetes.

Mutations in several genes probably contribute to the origin and onset of Type 1 diabetes. For example, an insulin-dependent diabetes mellitus (IDDM1) site on chromosome 6 may harbor at least one susceptibility gene for Type 1 diabetes. The role of this mutation in increasing susceptibility is not yet known; however, since chromosome 6 also contains genes for antigens (the molecules that normally tell the immune system not to attack itself), there may be some interaction between immunity and diabetes. In Type 1 diabetes the body's immune system mounts an immunological assault on its own insulin and the pancreatic cells that manufacture it. Some ten sites in the human genome, including a gene at the locus IDDM2 on chromosome 11 and the gene for glucokinase, an enzyme that is crucial for glucose metabolism, on chromosome 7, appear to increase susceptibility to Type 1 diabetes.

In noninsulin-dependent diabetes, heredity may be a factor, but since the pancreas continues to produce insulin, the disease is considered a problem of insulin resistance, in which the body is not using the hormone efficiently. In people prone to Type 2 diabetes, being overweight can set off the disease because excess fat prevents insulin from working correctly. Maintaining a healthy weight and keeping physically fit can usually prevent noninsulin-dependent diabetes. To date, insulin-dependent diabetes (Type 1) cannot be prevented.

Blacks are somewhat more likely to develop Type 2 diabetes than whites, while Hispanic Americans are much more likely to develop this type of the disease than the general population. Diabetes is also prevalent among many Native Americans. The Pima Indians of southern Arizona, for example, have the highest incidence of Type 2 diabetes in the world (50% of those over age thirty-five).

Complications of Diabetes

Because diabetes deprives body cells of the glucose needed to function properly, several complications can develop to further threaten the lives of victims. The healing process of the body is slowed or impaired and there is increased risk of infection. Complications of diabetes include higher risk and rates of heart disease; circulatory problems, especially in the legs, often severe enough to require surgery or even amputation; diabetic retinopathy, a condition that can cause blindness; kidney disease that may require dialysis; dental problems; and problems of pregnancy.

Failure to follow the proper diet can be particularly dangerous to those who suffer from Type 1 diabetes. People who pay close attention to the roles of diet, exercise, weight management, and pharmacological control (proper use of insulin and other medication) to manage their disease suffer the fewest complications.

HUNTINGTON'S DISEASE

Named for an American physician, George Sumner Huntington (1850–1916), Huntington's disease (HD), or Huntington's chorea, is an inherited, progressive brain disorder. It causes the degeneration of cells in the basal ganglia, a pair of nerve clusters deep in the brain that affect both the body and the mind. HD is caused by a single dominant gene that affects men and women of all races and ethnic groups. Figure 5.14 shows the inheritance pattern of HD.

Gene Responsible for HD Found

The gene mutation that produces HD was mapped to chromosome 4 in 1983 and cloned in 1993. In the HD gene the mutation involves a triplet of nucleotides, cytosine (C), adenine (A), and guanine (G), known as CAG. The mutation is an expansion of a nucleotide triplet repeat in the DNA that codes for the protein huntingtin. In unaffected persons the gene has thirty or fewer of these triplets, but HD patients have forty or more. These increased multiples either destroy the gene's ability to make the necessary protein or cause it to produce a misshapen and malfunctioning protein. Either way, the defect results in the death of brain cells.

The international team of scientists from the United States and the United Kingdom that discovered the gene responsible for HD examined seventy-five families with a history of HD and found the abnormal expansion in each case of an afflicted patient. The number of repeated triplets is inversely related to the age when the individual first experiences symptoms—the more repeated triplets, the younger the age of onset of the disease. Like myotonic dystrophy, in which the symptoms of the disease often increase in severity from one generation to the next, the unstable triplet repeat sequence can lengthen from one generation to the next, with a resultant decrease in the age when symptoms first appear.

HD does not usually strike until mid-adulthood, between ages thirty and fifty, although there is a juvenile form that can affect children and adolescents. Early symptoms, such as forgetfulness, a lack of muscle coordination, or a loss of balance, are often ignored, delaying the diagnosis. The disease gradually takes its toll over a ten- to twenty-five-year period.

Within a few years, characteristic involuntary movement (chorea) of the body, limbs, and facial muscles appears. As HD progresses, speech becomes slurred and swallowing becomes difficult. The patients' cognitive abilities decline, and there are distinct personality changes—depression and withdrawal, sometimes countered with euphoria. Eventually, nearly all patients must be institutionalized, and they usually die as a result of choking or infections.

Prevalence of HD

HD, once considered rare, is now recognized as one of the more common hereditary diseases. It is known to affect about 30,000 Americans, according to the National Institute of Neurological Disorders and Stroke (NINDS) in Huntington's Disease: Hope through Research (NIH Publication No. 98-49). Another 150,000 are at a 50% risk of inheriting it from an affected parent. NINDS estimates its prevalence at about one in 10,000 persons.

Prediction Test

In 1983 researchers identified a DNA marker that made it possible to offer a test to determine whether an individual has inherited the HD gene before symptoms appear. In some cases it is even possible to make a prenatal diagnosis on an unborn child. Many people, however, prefer not to know whether or not they carry the defective gene. Currently, they are trying to determine if the exact number of excess triplets indicates when in life a person will be affected by the disease. Some scientists fear that the ability to tell people that they are going to develop an incurable disease and pinpoint when they will develop it will make genetic testing, already a difficult decision, even more complicated.

A testing center at Johns Hopkins Hospital in Maryland reports that a high proportion of the people who come in for testing find out that they are not carrying the gene. Of the people who do not choose to be tested, the center's physicians believe that many may already have very mild symptoms and suspect that they have the disease.

Promising Research Findings

In December 2001 researchers at the University of South Florida reported that fetal tissue—neurons obtained from fetuses—transplanted into the damaged areas of the brains of patients with HD remained disease-free. Earlier in the same year French researchers had reported similar results, confirming that HD does not appear to enter or affect the implanted fetal tissue.

The HD patients who received the transplants showed measurable improvement in motor and cognitive functions and did not appear to reject the transplanted tissue, leading investigators to speculate that fetal tissue grafts might become the first effective treatment for HD.

MUSCULAR DYSTROPHY

Muscular dystrophy (MD) is a term that applies to a group of more than thirty types of hereditary muscle-destroying disorders. Approximately one million Americans are affected by one of the forms of MD. Each variant of the disease is caused by defects in the genes that play important roles in the growth and development of muscles. Duchenne muscular dystrophy (DMD) is one of the most frequently occurring types of MD and is characterized by rapid progression of muscle degeneration that occurs early in life. The disorder was named for the French neurologist Guillaume Benjamin Amand Duchenne (1806–75), who first described it. In all forms of MD the proteins produced by the defective genes are abnormal, causing the muscles to waste away. Unable to function properly, the muscle cells die and are replaced by fat and connective tissue. The symptoms of MD may not be noticed until as much as 50% of the muscle tissue has been affected.

DMD is X-linked, affects mostly males, and, according to the National Center for Biotechnology Information, strikes one in 3,500 boys worldwide. The gene for DMD is located on the X chromosome and encodes a large protein called dystrophin. (See Figure 5.15.) Dystrophin provides structural support for muscle cells and without it, the cell membrane becomes penetrable, allowing extracellular components into the cell. (See Figure 5.16.) These additional components increase the intracellular pressure, causing the muscle cell to die.

With myotonic dystrophy the muscles contract but have diminishing ability to relax, and there is muscle weakening and wasting. Typically, the initial complaints are the loss of hand strength or tripping while walking or climbing stairs. Along with decreased muscle strength, myotonic dystrophy may cause mental deficiency, hair loss, and cataracts. It is an autosomal dominant disorder that occurs in one in 20,000 people; it usually begins in young adulthood but can start at any age and varies in terms of severity.

The myotonic dystrophy gene is a protein kinase gene found on the long arm of chromosome 19. (See Figure 5.17.) The defect is a repeated set of three nucleotides—cytosine (C), thymine (T), and guanine (G)—in the gene. The symptoms of myotonic dystrophy frequently become more severe with each generation because mistakes in copying the gene from one generation to the next result in amplification of a genomic AGC/CTG triplet repeat, similar to the process observed in Huntington's disease. Unaffected individuals have CTG repeats with between three and thirty-seven iterations (repetitions) of the triplet. In contrast, people with the mild phenotype of myotonic dystrophy have between forty and 170, and those with more serious forms of the disease have between 100 and 1,000 iterations.

The symptoms of this disorder generally become more severe with each successive generation. This is because mistakes in the reliable copying of the gene from one generation to the next result in the amplification of a genomic "AGC/CTG triplet repeat," similar to that found in Huntington's disease.

All of the various disorders labeled MD cause progressive weakening and wasting of muscle tissues. They vary, however, in terms of the usual age at the onset of symptoms, rate of progression, and initial group of muscles affected. The most common type, DMD, affects young boys, who show symptoms in early childhood and usually die from respiratory weakness or damage to the heart before adulthood. The gene is passed from the mother to her children. Females who inherit the defective gene generally do not manifest symptoms—they become carriers of the defective genes, and their children have a 50% chance of inheriting the disease. Other forms of MD appear later in life and are usually not fatal.

In 1992 scientists discovered the defect in the gene that causes myotonic dystrophy. In people with this disorder, a segment of the gene is enlarged and unstable. This finding helps physicians more accurately diagnose myotonic dystrophy, which affects at least one in 8,000 people. Researchers have since identified genes linked to other types of MD, including DMD, Becker MD, limb-girdle MD, and Emery-Dreifuss MD.

In January 2005 researchers from the Mayo Clinic in Rochester, Minnesota, identified a new form of muscular dystrophy that involves mutations in a protein called ZASP, which binds to cardiac (heart) and skeletal muscles. The researchers detected ZASP mutations in eleven patients; in seven of these, they observed a dominant pattern of inheritance (Duygu Selcen and Andrew G. Engel, "Mutations in ZASP Define a Novel Form of Muscular Dystrophy in Humans," Annals of Neurology, February 2005).

In addition to its commitment to MD research, the Muscular Dystrophy Association supports research into the causes and treatment of related neuromuscular illnesses, such as amyotrophic lateral sclerosis (also known as Lou Gehrig's disease), Charcot-Marie-Tooth disease, and myasthenia gravis. All of these are progressive, wasting, and weakening conditions that rob individuals of their ability to live, work, and function normally. Genes linked to these diseases were identified during the 1990s.

Treatment and Hope

There is no known cure for MD, but patients can be made more comfortable and functional by a combination of physical therapy, exercise programs, and orthopedic devices (special shoes, braces, or powered wheelchairs) that help them to maintain mobility and independence as long as possible.

Genetic research offers hope of finding effective treatments, and even cures, for these diseases. Gene therapy experiments designed to find a cure or a treatment for one or more of these types of MD are ongoing. Research teams have identified the crucial proteins produced by these genes, such as dystrophin, beta sarcoglyan, gamma sarcoglyan, and adhalin. One experimental treatment approach involves substituting a protein of comparable size, such as utrophin for dystrophin, to compensate for the loss of dystrophin. (See Figure 5.16.)

Since defective or absent proteins cause MD, researchers hope that experimental treatments to transplant normal muscle cells into wasting muscles will replace the diseased cells. Muscle cells, unlike other cells in the body, fuse together to become giant cells. Scientists hope that if cells with healthy genes can be introduced into the muscles and accepted by the body's immune system, the muscle cells will then begin to produce the missing proteins.

New delivery methods called vectors are also being tested, such as implanting a healthy gene into a virus that has been stripped of all of its harmful properties, and then injecting the modified virus into a patient. Researchers hope this will reduce the amount of rejection by the patient's immune system, allowing the healthy gene to restore the missing muscle protein.

PHENYLKETONURIA

Phenylketonuria (PKU) is an example of a disorder caused by a gene-environment interaction. As a result of the defect, the affected individual is unable to convert phenylalanine into tyrosine. Phenylalanine in the body accumulates in the blood and can reach toxic levels. (See Figure 5.4.) This toxicity may impair brain and nerve development and result in mental retardation, organ damage, unusual posture, and, when it occurs during pregnancy, may jeopardize the health and viability of the unborn child.

Originally, PKU was considered simply an autosomal recessive inherited error of metabolism that occurred when an individual received two defective copies, caused by mutations in both alleles of the phenylalanine hydroxylase gene found on chromosome 12. (See Figure 5.18.) The environmental trigger—dietary phenylalanine—was not identified at first because phenylalanine is so prevalent in the diet, occurring in common foods such as milk and eggs and in the artificial sweetener aspartame. Recognition of dietary phenylalanine as a critical environmental trigger has enabled children born with PKU to lead normal lives when they are placed on low-phenylalanine diets, and mothers with the disease can bear healthy children.

To identify people at risk of PKU, all newborns in the United States and many other countries are screened at birth for high levels of phenylalanine in the blood. After additional screening of those with elevated blood levels, approximately one in 10,000 infants is diagnosed with PKU and, with proper diet, is likely to lead a healthy, normal life.

SICKLE-CELL DISEASE

Sickle-cell disease (SCD) is a group of hereditary diseases, including sickle-cell anemia (SCA) and sickle B-thalassemia, in which the red blood cells contain an abnormal hemoglobin, termed hemoglobin S (HbS). HbS is responsible for the premature destruction of red blood cells, or hemolysis. In addition, it causes the red cells to become deformed, actually taking on a sickle shape, particularly in parts of the body where the amount of oxygen is relatively low. These abnormally shaped cells cannot travel smoothly through the smaller blood vessels and capillaries. They tend to clog the vessels and prevent blood from reaching vital tissues. This blockage produces anoxia (lack of oxygen), which in turn causes more sickling and more damage. (See Figure 5.19.)

Sickle-cell anemia is an autosomal recessive disease caused by a point mutation in the hemoglobin beta gene (HBB) found on chromosome 11p15.4. (See Figure 5.20.) A mutation in HBB results in the production of hemoglobin with an abnormal structure. Figure 5.19 shows how in SCA a point mutationcauses the amino acid glutamine to be replaced by valine to produce abnormal hemoglobin called HbS. It also shows how when red blood cells with HbS are oxygen-deprived they become sickle shaped and may cause blockages that result in tissue death.

Symptoms of SCA

People with SCA have the symptoms of anemia, including fatigue, weakness, fainting, and palpitations or an increased awareness of their heartbeat. These palpitations result from the heart's attempts to compensate for the anemia by pumping blood faster than normal.

In addition, patients experience occasional sickle-cell crises—attacks of pain in the bones and abdomen. Blood clots may also develop in the lungs, kidneys, brain, and other organs. A severe crisis or several acute crises can permanently damage various organs of the body. This damage can lead to death from heart failure, kidney failure, or stroke. The frequency of these crises varies from patient to patient. A sickle-cell crisis, however, occurs more often during infections and after an accident or an injury.

Who Contracts SCD?

Both the sickle-cell trait and the disease exist almost exclusively in people of African, Native American, and Hispanic descent and in those from parts of Italy, Greece, Middle Eastern countries, and India. When one parent has the sickle-cell gene, a couple's offspring will carry the trait but only if both the mother and the father have the trait can they produce a child with sickle-cell anemia. This trait is relatively common among African-Americans, and about six in 1,000 black couples carry the risk of having a child with the disease. People of African descent are advised to seek genetic counseling and testing for the trait before starting a family. The sickle-cell trait is present in one in twelve African-Americans, or about two million people. According to the National Heart Lung and Blood Institute (NHLBI), in the United States the disease afflicts approximately 72,000 people, most of whose ancestors come from Africa. SCA occurs in approximately one in 500 African-American births and in one in 1,000 to 1,400 Hispanic-American births. Numbers are much lower for other groups.

A CALL FOR UNIVERSAL SCREENING.

In 1993 a federal panel of researchers, clinicians, and policy makers called for sickle-cell screening of all newborns because early diagnosis and treatment significantly improve future health. Because of intermarriage, it is becoming more difficult to be certain of a person's racial or ethnic background based on physical appearance, surname, or self-reporting. Many sickle-cell sufferers could possibly be missed by exclusively screening a target population such as African-Americans.

By 1993 thirty-four states and jurisdictions had already instituted the universal screening of infants recommended by an earlier study group of the National Institutes of Health in 1988. Another ten states had targeted screening aimed at groups traditionally considered at higher risk, and eight states and jurisdictions had no sickle-cell screening program. As of 2005, according to the NHLBI, forty-four states, the District of Columbia, Puerto Rico, and the Virgin Islands screened all newborns for sickle-cell disease, and the remaining six states provided screening at the request of parents.

Early diagnosis (soon after birth) could save the lives of children born with SCD. Studies have found wide differences in the mortality rates of children with SCD. To improve survival rates for children with SCD living in high mortality areas, public health advocates recommend further study of the accessibility and quality of available screening and medical care, and the duplication of successful treatment programs. In addition, they emphasize the importance of educating parents about the disease and its treatments ("Mortality among Children with Sickle Cell Disease Identified by Newborn Screening during 1990–1994—California, Illinois, and New York," Mortality and Morbidity Weekly Report, vol. 46, no. 9, March 13, 1998; "Geographic Differences in Mortality of Young Children with Sickle Cell Disease in the United States," Public Health Reports, vol. 112, no. 1, January–February 1997).

Treatment of SCD

There is no universal cure for SCD, but the symptoms can be treated. Crises accompanied by extreme pain are the most common problems and can usually be treated with painkillers. Maintaining healthy eating and behavior and prompt treatment for any type of infection or injury is important. Special precautions are often necessary before any type of surgery, and for major surgery some patients receive transfusions to boost their levels of hemoglobin (the oxygen-bearing, iron-containing protein in red blood cells). In early 1995 a medication that prevented the cells from clogging vessels and cutting off oxygen was approved.

The 1993 federal panel of experts on SCD recommended that all infants diagnosed with the disease receive daily doses of penicillin to prevent infections. Parents are urged to make sure that these children receive the scheduled childhood immunizations and are vaccinated against influenza, meningitis, pneumonia, and hepatitis B. In the mid-1980s, 20% of children with SCD died before their first birthday; by 1993, primarily because of preventive antibiotics, that proportion had dropped to less than 3%. Although there is neither uniform SCD reporting nor national reports of incidence or prevalence, public health professionals believe that the antibiotic prophylaxis (prevention) has further reduced SCD mortality.

BIOMEDICAL ADVANCES.

Many adults with SCD now take hydroxyurea, an anticancer drug that causes the body to produce red blood cells that resist sickling. In 1995 a multicenter study showed that among adults with three or more painful crises per year, hydroxyurea lowered the median number of crises requiring hospitalization by 58%. In 2003 an extension of that study showed that not only do patients on hydroxyurea have fewer crises but they also have a significant survival advantage when compared to SCD patients who do not take the medication. Subjects treated with it overall showed 40% lower mortality than others. (Martin H. Steinberg et al., "Effect of Hydroxyurea on Mortality and Morbidity in Adult Sickle Cell Anemia: Risks and Benefits Up to 9 Years of Treatment," Journal of the American Medical Association, April 2, 2003).

An international study conducted in 1996 found that bone marrow transplants were successful in curing SCD in sixteen out of twenty-two patients, or 72.7% of the patients in the five-year study. All of the participants in the study were under age fourteen, had advanced symptoms, and had siblings who were compatible bone marrow donors. Of the remaining six patients, four (18%) rejected the donor marrow, and their sickle-cell symptoms returned, while two (9%) died.

Because of the risks associated with bone marrow transplants and the difficulties of finding matching donors, transplants are not appropriate for most patients. Further studies are needed to test the procedure for older patients and to reduce the proportion of transplant rejects. Blood harvested from umbilical cords and placentas has been found to be less likely to trigger rejection or graft-versus-host disease, in which the transplanted cells attack the cells of the bone marrow recipient, causing organ damage.

TAY-SACHS DISEASE

Tay-Sachs disease (TSD) is caused by mutations in the HEXA gene, located on the long arm of chromosome 15. (See Figure 5.21.) It is a fatal genetic disorder in children that causes the progressive destruction of the central nervous system. It is caused by the absence of an important enzyme called hexosaminidase A (hex-A). Without hex-A, a fatty substance called GM2 ganglioside builds up abnormally in the cells, particularly the brain's nerve cells. (Figure 5.22 shows how the absence of, or defect in, the hex-A protein prevents complete processing of GM2 ganglioside.) Eventually, these cells degenerate and die. This destructive process begins early in the development of a fetus, but the disease is not usually diagnosed until the baby is several months old. By the time a child with TSD is four or five years old, the nervous system is so badly damaged that the child dies.

TSD was named for Warren Tay (1843–1927), a British ophthalmologist who first described a patient with a cherry-red spot on the retina of the eye that has become a hallmark of TSD, and Bernard Sachs (1858–1944), a New York neurologist who provided the first description of the cellular changes in TSD. Dr. Sachs also recognized the inheritance pattern of the disorder and observed that most cases of Tay-Sachs disease occurred in families of eastern European Jewish origin.

Symptoms of TSD

A baby with TSD seems normal at birth and usually develops normally for about the first six months of life, but then development slows. The child begins to regress and loses skills one by one—the ability to crawl, to sit, to reach out, and to turn over. The victim gradually becomes blind, deaf, and unable to swallow. The muscles begin to atrophy and paralysis sets in. Mental retardation occurs, and the child is unable to relate to the outside world. Death usually occurs between ages three and five. There is no cure or treatment for this disease.

How Is TSD Inherited?

TSD is an autosomal recessive genetic disorder caused by mutations in both alleles of the HEXA gene on chromosome 15. Both the mother and the father must be carriers of the defective TSD gene in order to produce a child with the disease.

People who carry the gene for TSD are entirely unaffected and usually unaware that they have the potential to pass this disease to their offspring. A blood test distinguishes Tay-Sachs carriers from noncarriers. Blood samples may be analyzed by enzyme assay or DNA studies. Enzyme assay measures the level of hex-A in blood. Carriers have less hex-A than noncarriers. When only one parent is a carrier, the couple will not have a child with TSD. When both parents carry the recessive TSD gene, they have a one in four chance in every pregnancy of having a child with the disease. They also have a 50% chance of bearing a child who is also a carrier. Prenatal diagnosis early in pregnancy can predict if the unborn child has TSD. If the fetus has the disease, the couple may choose to terminate the pregnancy.

Who Is at Risk?

Some genetic diseases, such as TSD, occur most frequently in a specific population. As Dr. Sachs observed, individuals of east European (Ashkenazi) Jewish descent have the highest risk of being carriers of TSD. According to the National Tay-Sachs and Allied Diseases Association, approximately one in twenty-seven Jews in the United States are carriers of the TSD gene, and 85% of the children who are victims of this disease are Jewish. Italians also have a higher than average risk of being carriers. In the general population, the carrier rate is one in 250.

Genetic Disorders

Principles of genetic inheritance patterns

Types of genetic inheritance

Dominant and recessive

Genetic analysis

Dominant genetic disorders

Recessive genetic disorders

X-Linked genetic disorders

Multi-factorial genetic disorders

Aneuploidy

Genetic screening

Resources

Genetic disorders refer to medical conditions that develop as the result of abnormalities in an individuals genetic material, usually that is inherited. Inheriting or developing a genetic disorder leads to a collection of clinical manifestations known as a syndrome. These clinical manifestations can vary from person to person with the same genetic defect or have similar presentations.

Principles of genetic inheritance patterns

Genetic information is packaged into chromosomes that are found in thecells nucleus, or DNA (deoxyribonucleic acid) containing organelle. Virtually every human cell has 46 chromosomes, except for the sperm and egg (reproductive cells) that each have 23 chromosomes. Fertilization of the egg by the sperm results in a newly formed cell called the zygote. Each zygote receives 23 chromosomes from the egg and 23 chromosomes from the sperm. All but one of the 23 chromosomes are called autosomes. The remaining chromosome is the sex chromosome and it is either an X or a Y. After these sex cells unite, gender is determined. Females have two X’s (XX), and males have one of each (XY). Females can only pass an X to their offspring, and males can pass either an X or a Y. Therefore, the male sperm is responsible for gender selection.

The duplicated set of 22 autosomes, numbered 1 through 22, are called homologous pairs in that there are two chromosome number 1 with similar genes and genetic material. Upon the near-completion of the Human Genome Project (HGP) in 2003, there are approximately 30,000 genes on all the chromosomes. Individual genes are made of deoxyribonucleic acid (DNA), which makes up the genetic code or alphabet that produces specific proteins. Proteins can play important structural and functional roles in the body. Each gene has a set locus, or position, on a particular chromosome. Identical genes that are

located on the same locus on corresponding chromosomes are called alleles.

A persons genotype represents the genes that they inherited. If is an autosomally inherited disease, the genotypes are written as a lower or an upper case letter A (such as AA, aa or Aa to represent both alleles known as the genotype), where capital letters define dominant genes and lower case letters define recessive genes. Genotypes are either homozygous or heterozygous. Having two identical alleles, such as AA or aa, makes the genotype homozygous for that locus. Having different alleles (for example, Aa) at a locus represents a heterozygous genotype. The observable features that characterize an individual are collectively called the phenotype. A phenotype can also extend to observable characteristics that can only be visualized with the help of a microscope or other equipment.

Types of genetic inheritance

There are many types of genetic disorders. Mendelian disorders are a group of disorders that are inherited either as an autosomal (through one of the 44 chromosomes, excluding the X or Y chromosome) or X-linked (through the X chromosome) defect in a dominant or recessive pattern. Although most of them are commonly associated with inherited defects, many of them can involve spontaneous or de novo alterations in an individuals genetic material. There are also genetic disorders that have an unknown

etiology and the mechanisms for how a disease develops have not been clearly elucidated. Some genetic disorders can be induced by environmental factors and are collectively called multifactorial disorders. Mitochondrial disorders involve disorders in the mito-chondrial genome and are inherited only from the mother. The mitochondria are tiny compartments found in almost every cell and has a distinct genome from the DNA found in the nucleus and is involved primarily in producing energy for all the tissues of the body. Finally, polygenic disorders mean that many genes act together to produce a genetic disease.

Dominant and recessive

A dominant gene means that a single allele can control whether the disease develops. If only one parent (usually affected) passes on an autosomal, defective gene, which results in the child having a genetic disorder, then the disorder is called autosomal dominant. A recessive gene means that there is enough normal protein product to function properly from the normal gene and, therefore, two copies of the defective gene are necessary for the disease to develop. If both parents are unaffected and they each pass on a defective gene causing their child to be affected, then the genetic disorder is autosomal recessive. The parents are called carriers. For example, sickle-cell anemia is a recessive disorder characterized by abnormal hemoglobin production. The genetic defect involves a gene that produces hemoglobin. Although sickle-cell carriers produce, in part, abnormal hemoglobin, although they usually do not experience clinical manifestations since the normal hemoglobin produced from the normal gene is enough to function normally.

However, Many other genetic disorders are caused by defects related to the sex chromosomes, or the X and Y chromosomes. If a defective gene on the X-chromo-some is inherited, it is called X-linked. Like autosomal disorders, X-linked genetic diseases also can be inherited by dominant and recessive mechanisms. X-linked dominant means that if the father passes on the defective gene on his only X chromosome, all his offspring (which will be females) will be affected. If he passes on his Y chromosome, none of these males will be affected. Therefore, there is no male-to-male transmission. If it is X-linked recessive, all daughters will be carriers. If the mother passes on a recessive X-linked gene, then all her sons will be affected and all her daughters will be carriers. Understanding the mechanisms by which genetic disorders are inherited are very important for interpreting recurrence risks.

A variation of Mendelian patterns of inheritance is called incomplete dominance. Incomplete dominance occurs when both alleles are expressed. An example of this is observed in Four-O-Clock flower-color. A white and a red phenotype are neither dominant nor recessive. If a flower is heterozygous and carries genes that produce both the white color and the red color, the flower color results in pink. Another variation is codominance. In incomplete dominance, the phenotype is a blending of the two different gene effects. In codominance, both gene variants are expressed at the same time, representing a third phenotype. For example, a flower with alleles that can produce either white or red in the homozygote form will produce both colors in the heterozygote form expressed as spotted flowers.

Genetic analysis

Chromosomal analysis can be performed on cell samples from an individual, a technique called karyo-typing. A karyotype involves visualizing the chromosomes using a specific dye and a high-resolution microscope. The chromosomes can be distinguished from one another in size and staining pattern. Corresponding chromosomes 1 through 22 and the sex chromosomes can be lined up and visually inspected for abnormalities. Any obvious defect can indicate a diseased state. Sometimes it is apparent that a part of one chromosome was incorrectly combined with a different chromosome during cellular division. This is called a translocation and represents a structural abnormality. Numerical abnormalities occur when an extra chromosome is present. When more than 46 chromosomes are observed in chromosome, it is called aneuploidy. Most aneuploidies that occur are called trisomies which involve three homologous chromosomes, or the presence of extra chromosome. Numerical abnormalities are typically incompatible with life, with the exception of Trisomy 21 (Down Syndrome) and a few other rare genetic disorders.

Dominant genetic disorders

If one parent has an autosomal dominant disease, then offspring have a 50% chance of inheriting that disease. There are roughly 2,000 autosomal dominant disorders (ADDs) with effects that range from mild clinical manifestations to death. These diseases may develop early or late in life. ADDs include Huntington disease, Marfan syndrome (extra long limbs), achondroplasia (a type of dwarfism), some forms of glaucoma, most forms of porphyrias, and hypercho-lesterolemia (high bloodcholesterol).

Huntington Disease is a late onset neurodegen-erative disease. It is characterized by progressive chorea (involuntary, rapid, jerky motions) and mental deterioration that often develop after the fourth decade of life, eventually leading to death approximately 15 years later. The Huntington disease gene locus is on chromosome 4, and can be identified.

Marfan syndrome is an ADD characterized by long arms, legs, and fingers. People with Marfan syndrome also have a blue sclera that represents a hallmark clinical feature that can be detected when observing the eyes. In addition, these individuals have a high incidence of eye and aortic heart problems. The elasticity of the vessels in the aorta are susceptible to rupture, which can cause death. Not all people with Marfan syndrome inherit it from a parent, about 15% of Marfan syndrome cases are caused by a spontaneous mutation.

Recessive genetic disorders

Recessive genetic disorders (RGD) result from inheriting two defective recessive alleles of a gene, one from each parent. RGD often require careful molecular or biochemical genetic analyses to determine carrier status. Hence, the birth of a child with a recessive disorder may surprise unknowing parents. The probability of two carrier or heterozygous parents having an affected child is 25% each time they conceive. The chance that they will have a heterozygous (carrier) child is 50% for each conception. And the chance of having an unaffected homozygous child is also 25% for each pregnancy. More than 1,000 RGDs have been identified and include: cystic fibrosis, phe-nylketonuria (PKU), galactosemia, retinoblastoma (Rb), albinism, sickle-cell anemia, thalassemia, Tay-Sachs disease, autism, growth hormone deficiency, adenosine deaminase deficiency, and Werner’s syndrome (juvenile muscular dystrophy).

A number of eye disorders are RGDs and are often associated with a mutant gene on chromosome 13. The Retinoblastoma (Rb) gene was the first human gene found to cause cancer and the first human cancer gene in which its location on a chromosome was determined. Rb is a gene that can cause a tumor in the retina, called a retinoblastoma. Most retinoblasto-mas are inherited; however, in some cases, inheriting a mutation on one allele combined with a spontaneous mutation on the other allele can result in retinoblastoma. Environmental carcinogens (cancer causing agents) can induce a spontaneous mutation. Other recessive eye disorders include myopia (nearsighted-ness), albinism, day blindness, displaced pupils, and dry eyes.

Some RGDs affect people having a particular ethnic background. For example, cystic fibrosis (CF), sickle-cell anemia (SCA), and Tay-Sachs disease (TSD) all have specific mutations in the gene that causes each disorder that are preferentially found in individuals of a certain ethnic background. CF is a common autosomal recessive disease in individuals of Northern European decent and one in every 25 people in this population is a carrier. SCA usually is most common in black and Hispanic populations; however, some mutations in the gene that causes SCA are also found in Italian, Greek, Arabian, Maltese, Southern Asian, and Turkish populations. About 1 in 12 blacks are carriers for SCA gene mutations in one of the two hemoglobin genes. Hemoglobin carries oxygen in red blood cells to tissues and organs throughout the body. SCA patients have red blood cells that live only a fraction of the normal life span of 120 days. The abnormal blood cells have a sickled appearance and cannot transport oxygen efficiently. TSD gene mutation carriers are commonly found in Ashkenazi Jewish populations and approximately 1 in 30 are carriers of a mutation.

Galactosemia and PKU are examples of metabolic RGDs and are commonly called biochemical disorders. Both these disorders result from mutations in two different genes, both of which produce dysfunctional enzymes that are important in metabolism. Enzymes speed up chemical reactions and are essential for many cellular processes. People with galactosemia are enzyme deficient for Galactose-1-phosphate uridyl transferase, without which they can not metabolize galactose, a sugar found in milk. If milk and other galactose-containing food items are consumed, these individual cannot digest this compound properly and the result is severe developmental delay. Individuals that inherit PKU are deficient in the enzyme Phenylalanine hydroxylase, which is responsible for converting phenylalanine (an amino acid, a building block of protein) to tyrosine. The build-up of phenylalanine leads to severe developmental delay, and preventing clinical manifestations can be achieved by dietary modification. A phenylalanine-free diet containing sufficient amino acids is available for people diagnosed with PKU. Both of these metabolic disorders result from mutations in two different genes. Carriers are detected through a nationwide newborn screening programs, which differ in testing services from state to state.

Adenoside deaminase deficiency another autosomal recessive genetic disorder that is also called severe combined immunodeficiency. It is caused by a single mutation on chromosome 20 in a gene that encodes an enzyme important for proper functioning of the immune system. Adenosine deaminase converts, or breaks down, a small molecule called adenosine. Gene therapy has been successful in delivering a normal gene and prevents the damaging effects of adenosine on specific cells important for immune system function.

X-Linked genetic disorders

X-linked genetic disorders (XLGDs) can be either dominant or recessive. Dominant XLGDs affect females and males. Dominant XLGD’s include: Albright’s hereditary osteodystrophy (seizures, mental retardation, stunted growth), Goltz’s syndrome (mental retardation), cylindromatosis (deafness and upper body tumors), oral-facial-digital syndrome (no teeth, cleft tongue, some mental retardation), and incontinentia pigmenti (abnormal swirled skin pigmentation).

Recessive XLGDs are passed to sons through their mothers, who are carriers. Often, a carrier mother will have an affected male relative. Major XLGDs include: severe combined immune deficiency syndrome (SCID), color blindness, hemophilia, Duchenne’s muscular dystrophy (DMD), some spinal ataxias, and Lesch-Nyhan syndrome. Of these XLGDs, approximately one-third results from a spontaneous mutation. Of these disorders, color blindness is the most benign.

Hemophilia is a more serious XLGD caused by failure of one of the clotting proteins, which serves to prevent an injured person from bleeding to death. Hemophilia A is the most severe form of this disease, and is characterized by extreme bleeding. It primarily affects males, although a few females can develop symptoms. This disease has been associated with royalty, as England’s queen Victoria was a carrier and her descendants became rulers in several European countries.

Multi-factorial genetic disorders

Statistics and twin studies are often used to determine the genetic basis for multi-factorial genetic disorders (MFGDs). Because environment can play an important role in the development of these diseases, identical and fraternal twins who have been raised in different homes are ideally studied. MFGD include some disorders associated with diet and metabolism, such as obesity, diabetes, alcoholism, rickets, and high blood pressure. And, the tendency of contracting certain infections such as measles, scarlet fever, and tuberculosis can be considered MFGDs. In addition, schizophrenia and some other psychological illnesses represent as well as congenital hip, club foot, and cleftlip are also inherited in this manner. Cancer, where the risk is associated with the environmental exposure, also falls into this class of disorders. Certain breast, colon, skin, and small-cell lung cancers have been shown to have a genetic link. Certain genes predisposed people to a certain type of cancer and this risk is enhanced when there is a specific environmental exposure. This susceptibility is influenced by inherited variations in genes, which encode proteins that may be more or less functional. For example, if the protein is involved in the metabolism of a cancer-causing substance and an individual inherits a variation in the gene sequence, this might affect the function of the protein it encodes. If it reduced its function, this might lead to more damaging effects of the cancer causing substance.

Aneuploidy

The two most common aneuploidies, trisomies and extra sex chromosomes, can be due to maternal or paternal factors including advanced age. A number of aneuploidies can be attributed to dispermy, or when two sperm fertilizes one egg. The resulting genetic disorders can result in a spontaneous mutation. Live-born children with autosomal aneuploidies can have trisomy 13, 18, or 21, and all have some form of developmental delay, while trisomy of any other chromosome is usually always fatal. Trisomy 13 (Patau’s syndrome) is characterized by retarded growth, cleft lip, small head and chin, and often polydactyly. Trisomy 18 (Edward’s syndrome) is marked by severe, variable abnormalities of the head, thumbs, ears, mouth, and feet. Trisomy 21 (Down’s syndrome) occurs equally in all ethnic groups, and is closely related to increased maternal age.

Aneuploidy of the sex chromosomes can cause abnormal genital development, sterility, and other growth problems. The most common aberrations involve multiple X chromosome syndromes. Males with an XXY aneuploidy have Klinefelter’s syndrome, and have small testes and typically no sperm. Hermaphrodites are individuals that have both male and female genitals, are extremely rare, and result from cell lines that have two different chromosome patterns with both XX and XY cells.

Genetic screening

Genetic tests are available and can reveal varying degrees of genetic information depending on the test. Most of these tests are performed by isolating chromosomes or by measuring a protein product that is excreted in the urine to test for biochemical defects. These tests can be used prior to conception to

KEY TERMS

Allele —Any of two or more alternative forms of a gene that occupy the same location on a chromosome.

Chromosomes —The structures that carry genetic information in the form of DNA. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body.

Dominant trait —A trait which can manifest when inherited from one parent.

Gene —A discrete unit of inheritance, represented by a portion of DNA located on a chromosome. The gene is a code for the production of a specific kind of protein or RNA molecule, and therefore for a specific inherited characteristic.

Heterozygote —A person possessing two non-identical alleles.

Homozygote —A person possessing two identical alleles.

Multifactorial trait —A trait that results from both genetic and environmental influences.

Recessive trait —A trait that is not expressed in heterozygotes but is expressed if two defective genes are inherited by carrier parents.

Sclera —White of the eye.

X-Linked trait —A trait that is inherited due to mutations in genes on the X-chromosome.

determine a couple’s risk for having an affected child, during pregnancy, at birth or later in life.

The most successful wide-spread test for genetic disorders is the newborn program that tests for diseases such as PKU. Newborn screening for hypothyr-oidism and galactosemia are also performed in several states. Prenatal testing in embryos and fetuses include chorionic villus sampling (CVS), amniocentesis, and ultrasound. CVS can detect Down syndrome, hemophilia, DMD, CF, SCA, and sex chromosomal aberrations. Amniocentesis can detect Tay-Sachs disease, Down syndrome, hemophilia, spina bifida, and other abnormalities. Ultrasound is used to visualize the developing baby; it can detect spina bifida, anence-phaly (no brain), and limb deformities.

Genetic counseling and testing can help people find out if they carry the gene for some disorders, or whether they will develop a late-onset genetic disorder themselves. Genetic probes can identify the genes for Huntington’s disease, cystic fibrosis, Tay-Sachs, sickle-cell, thalassemia, and abnormalities associated with growth hormone. Genetic testing capabilities increase each year as additional genetic disorders are better characterized and the gene localization and protein function is determined.

See also ADA (adenosine deaminase) deficiency; Albinism; Birth defects; Embryo and embryonic development.

Resources

BOOKS

Carlson, Elof Axel. Mendel’s Legacy: The Origin of Classical Genetics. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press, 2004.

Hartl, Daniel, and Elizabeth Jones. Genetics: Analysis of Genes and Genomes. Sudbury, MA: Jones and Bartlett Publishers, 2005.

King, Robert C., William D. Stansfield, and Pamela K. Mulligan. A Dictionary of Genetics. New York: Oxford University Press, 2006.

Lewin, Benjamin. Essential Genes. Upper Saddle River, NJ: Pearson Education, 2006.

Lewis, Ricki. Human Genetics: Concepts and Applications. Boston, MA: McGraw-Hill, 2005.

Lodish, Harvey, et al. Molecular Cell Biology New York: W.H. Freeman and Company, 2004.

Robert L. Nussbaum, Roderick R. McInnes, Huntington F. Willard Genetics in Medicine. Philadelphia, PA: Saunders, 2001.

David L. Rimoin Emery and Rimoin’s principles and practice of medical genetics. London, UK; New York: Churchill Livingstone, 2002.

Louise Dickerson
Bryan Cobb, PhD

GENETIC DISORDERS

The traditional method used to study an inherited disease is to observe the pattern of its distribution in families through examination of a pedigree, the construction of which begins with the individual first known to have the disease. The pedigree pattern allows one to judge whether or not the distribution conforms to Mendelian principles of segregation and assortment, and thus represents single-factor inheritance. Patterns that do not conform to Mendelian principles may represent polygenic traits, which represent the cumulative effects of a number of different genes. These complex patterns underlie the vast majority of human diseases.

Disorders caused by single mutant genes show one of four simple (Mendelian) patterns of inheritance: (1) autosomal dominant, (2) autosomal recessive, (3) X-linked dominant, or (4) X-linked recessive. A dominant trait is one that is expressed in the heterozygote (as well as in the homozygote or hemizygote). A recessive trait is one that is expressed in a homozygote (or a hemizygote), but silent in the heterozygote. The terms "dominant" and "recessive" refer to the phenotypic expression of a trait, not to the expression of the gene. Thus it is incorrect to speak of a dominant or recessive gene. A gene is either expressed or not expressed. Whether the trait is considered dominant or recessive often depends upon the level of observation. Sickle cell anemia is a recessive trait—it requires a double dose of the abnormal gene for expression at the clinical level. Nevertheless, the sickle gene can be expressed in single dose as well, giving rise to carriers with SA hemoglobin. In a state of reduced oxygen tension, red cells in SA carriers may sickle. Recessive traits may thus be codominant when viewed biochemically at the level of the gene product, or dominant in an altered environment.

AUTOSOMAL DOMINANT TRAITS

By definition, genes that are situated on chromosomes other than the X or Y sex chromosomes are autosomal. Dominant traits are fully evident when only one abnormal gene (mutant allele) is present and the corresponding partner allele on the homologous chromosome is normal (a heterozygous state). The representative initial for the dominant gene is typically capitalized, and the recessive gene is placed in lower case. Thus, if there are two alleles of a given gene that are referred to as "A" and "a," three possible genotypes exist: AA, Aa, and aa. Genotypes AA and aa are homozygotes; Aa is a heterozygote.

Autosomal dominant traits bear the following characteristic features: (1) an affected individual usually bears an equal number of affected and unaffected offspring; (2) unless the condition arose by a new mutation in a germ cell that formed the individual, each affected individual has an affected parent; (3) males and females are affected in equal numbers; (4) each gender can transmit the trait to male and female; (5) normal children of an affected individual have only normal offspring; and (6) when the trait does not impair viability or reproductive capacity, vertical transmission of the trait occurs through successive generations. The best evidence of a dominant trait is three or more generations of male-to-male transmission.

Autosomal dominant disorders often show two additional characteristics that are rarely seen in recessive disorders: (1) marked variability in the severity, or expressivity, of the disorder and (2) delayed age of onset. In heterozygotes the expression of the abnormal gene can be so weak that a generation appears to be skipped because the carrier of the abnormal gene is clinically normal. In such fortunate individuals, the trait is said to be "nonpenetrant." In some diseases, such as Huntington's disease and adult polycystic kidney disease, the disorder may not become manifest clinically until adult life, even though the mutant gene has been present since conception.

In every autosomal dominant disease, some affected persons owe their disorder to a new mutation rather than to an inherited allele. A reasonable estimate of the frequency of mutation is on the order of 5 × 10^-6 mutations per allele per generation. Because a dominant trait requires a mutation in only one of the parental gametes, the expected frequency for a new autosomal dominant disease in any given gene is one in 100,000 newborns.

A classic example of a dominant trait in humans is familial hypercholesterolemia, an autosomal dominant disorder characterized by elevation of serum cholesterol bound to low-density lipoprotein (LDL). Mutations in the LDL receptor (LDLR) gene on chromosome 19 cause the disorder. Heterozygotes develop fatty collections on their tendons, a corneal arc, and, of greatest concern, coronary artery disease, which typically presents in the fourth or fifth decade of life. Homozygotes develop these features at an accelerated rate. In the United States, the frequency of homozygotes is approximately one in a million, and the frequency of heterozygotes is approximately one in five hundred. However, among patients with a history of myocardial infarction (heart attacks), the heterozygote frequency is about one in twenty.

AUTOSOMAL RECESSIVE DISORDERS

Autosomal recessive conditions are clinically apparent only in the homozygous state—when both alleles at a particular genetic locus are deleterious. In most autosomal recessive disorders the clinical presentation tends to be more uniform than in dominant diseases, and the onset is often early in life. The following features are characteristic: (1) on average, male and female siblings are affected in equal proportions; (2) the parents are clinically normal; (3) all of the children of the union between an affected individual and a homozygous normal individual are heterozygous carriers, but none will be affected; (4) on average, half of the children are affected when an affected individual mates with a heterozygous carrier (a pseudo-dominant pedigree); (5) all of the children of a union between two individuals homozygous for the same mutant gene will be affected; (6) on average, if both parents are heterozygous at the same genetic locus, one-fourth of their children are homozygous affected, one-fourth are homozygous normal, and half are heterozygous carriers of the same mutant gene; and (7) the less frequent the mutant gene is in the population, the greater the likelihood that the affected individual is the product of consanguineous parents.

Consanguinity increases the likelihood of a child presenting with a recessive disease because the likelihood of inheriting the same rare mutation from a distant common ancestor, or "founder" increases. First cousins share, on the average, one-eighth of their genes. When two first cousins marry, an offspring has, on average, one-sixteenth of the loci homozygous for a gene derived from a common ancestor. In general, offspring of first cousins are slightly more likely to have congenital malformations, as well as mental defects and metabolic diseases, than are children born to unrelated parents.

Increased frequency of consanguinity is not observed if the recessive disease is common. Cystic fibrosis exemplifies an autosomal recessive disorder that is common among individuals of Northern European descent. In the United States, the frequency of individuals heterozygous for a mutation in the cystic fibrosis conductance regulator gene (CFTR) is quoted as one in twenty-five. Inheritance of two malfunctioning genes leads to the disruption of pancreatic exocrine function and chronic bronchitis with emphysema, as well as biliary cirrhosis, meconium ileus, and an enhanced loss of salt through the skin, which is the basis of the "sweat test" used for screening purposes. The frequency of individuals affected with cystic fibrosis is one in 2,500, and typically the parents are unrelated.

X-LINKED INHERITANCE

Diseases or traits that result from genes located on the X chromosome are termed "X-LINKED." Because the female has two X chromosomes, she may be either heterozygous or homozygous for the mutant gene, and the trait may exhibit recessive or dominant expression. The terms "X-LINKED dominant" and "X-LINKED recessive" refer only to expression of the trait in females. The male has only one X chromosome and therefore is hemizygous for X-linked traits. Males can be expected to express X-linked traits regardless of their recessive or dominant behavior in the female. This accounts for the large numbers of X-linked diseases. Affected males do not transmit an X chromosome to their sons; thus an important feature of X-linked inheritance is the absence of male-to-male transmission. In contrast, since all females inherit their fathers' single X chromosomes, their daughters are all obligate carriers.

Although genotypically females have two X chromosomes, functionally they behave as though they only have one X chromosome, like their brothers. This is due to the process of X-inactivation, which was first proposed by Mary Lyon and is termed "lyonization" in her honor. During ontogeny, one of the X chromosomes becomes inactive, condensing to form a "Barr body." Inactivation is random, so each cell has an equal probability that the paternally or maternally derived X chromosome will be inactivated. Once one of the two X chromosomes is inactivated, the same X chromosome remains inactive throughout all subsequent cell divisions. Thus, on average, half of the cells of a female express the X chromosome of her father and half of her mother.

For the vast majority of genes on the X chromosome, the normal female is a mosaic, with her cells expressing one or the other X chromosome, but not necessarily a 50–50 mosaic. Inactivation of one of the X chromosomes occurs early in development and is random; hence many females may, by chance, have many more cells that carry an active X chromosome derived from one parent than from the other. Similarly, if one of the X chromosomes carries a mutant gene that confers a metabolic disadvantage upon cells with that mutation, these cells may survive less frequently during development, and the female offspring may have cells that carry predominantly or exclusively the active X chromosome without the mutation.

X-linked dominant traits are uncommon. The characteristic features are as follows: (1) females are affected about twice as often as males; (2) heterozygous females transmit the trait to both genders with a frequency of 50 percent; (3) hemizygous affected males transmit the trait to all of their daughters and none of their sons; and (4) the expression is more variable and generally less severe in heterozygous females than in hemizygous affected males. Some rare X-linked dominant disorders occur only in the heterozygous female, because the condition is lethal in the hemizygous affected male. Additional characteristics of this form of inheritance are as follows: (1) an affected mother transmits the trait to half of her daughters (heterozygotes), and (2) an increased frequency of abortions occurs in affected women, the abortions representing affected male fetuses.

X-linked recessive traits are relatively common. The characteristic features are as follows:(1) the disorder is fully expressed only in the hemizygous affected male; (2) heterozygous females are usually normal, although occasionally they may exhibit mild features of the disorder, and in females who have unfortunately inactivated the wrong X chromosome may be almost as severely affected as the hemizygous affected male; (3) on average, a heterozygous female transmits the trait to half of her sons (hemizygous affected), but the other half are normal; (4) on average, half of the daughters of a heterozygous female are carriers and half are normal; (5) all daughters of an affected male and a normal female are carriers, and no sons of such a union are affected (no father-toson transmission); (6) in the rare event of the union of an affected male and a heterozygous female, half of the daughters are homozygous affected and half are heterozygous carriers; while half of the sons are hemizygous affected (maternal inheritance) and half are normal; (7) if the trait is rare, parents and relatives are normal except for male relatives in the female line (e.g., on average, half of maternal uncles are affected). This "uncle and nephew" pattern gives rise to an oblique pedigree pattern, in contrast to the horizontal pattern of autosomal recessive conditions and the vertical pattern of autosomal dominant conditions.

Duchenne muscular dystrophy (DMD) and the milder Becker muscular dystrophy (BMD) are the product of mutations in the dystrophin gene on the X chromosome. The most distinctive feature of Duchenne muscular dystrophy is a progressive proximal muscular dystrophy with characteristic pseudohypertrophy of the calves. On average, a typical patient with DMD is diagnosed around the age of five, is wheelchair dependent at twelve years of age, and is dead before the age of twenty. One-third of cases represent new mutations.

POLYGENIC TRAITS AND MULTIFACTORIAL GENETIC DISEASES

Most phenotypic traits are determined by many genes collaborating at different loci (polygenic) rather than by single gene effects. Parents and offspring, and usually siblings, have 50 percent of their genes in common. Second-degree relatives share, on average, one-fourth of all genes, and third-degree relatives (cousins) share one-eighth. As the degree of relation becomes more distant, the probability of inheriting the same combination of genes is reduced, and the degree of resemblance is likely to be less.

Many common chronic diseases (e.g., essential hypertension, coronary artery disease, and schizophrenia) and the common birth defects of children (e.g., cleft palate, cleft lip, and neural tube defects) that tend to run in families fit best into the category of multifactorial genetic diseases. Multifactorial genetic diseases have both a polygenic component and an environmental component of causative factors. Susceptibility, or risk, genes are present in low frequency in the population at large. However, if any one individual has a particularly large number of such genes, the disease may manifest. When an individual is unfortunate enough to have inherited just the right (or wrong) combination of risk genes, he or she passes beyond a "risk threshold" at which environmental factors may determine the expression and severity of disease. In order for another family member to develop the same disease, that individual would have to inherit the same, or a very similar, combination of genes. The likelihood of such an occurrence is clearly greater in first-degree than in more distant relatives. The chances of another relative inheriting the right combination of risk genes decreases as the number of genes required to express a given trait increases. For example, the recurrence risk for siblings in neural tube defects is almost 4 percent, or ten times greater than the risk in the population as a whole.

CHROMOSOMAL DISORDERS

Failure of appropriate segregation (nondisjunction) during meiosis by allelic chromosomes or by sister chromatids can lead to an imbalance in the number of chromosomes present in a gamete. The frequency of chromosomal nondisjunction increases with increasing maternal age, with up to 1 percent of the offspring of mothers aged thirty-five and 10 percent of the offspring of mothers aged forty-five or older exhibiting such abnormalities. A zygote with only one copy (monosomy) of any one autosome is nonviable, and three (trisomy) or more copies of any one type of autosomal chromosome are also typically lethal. Exceptions do occur, the most common of which is trisomy 21, or Down syndrome.

Due to lyonization, nondisjunction of the X chromosome is better tolerated. XO individuals (Turner's syndrome) are phenotypically female, but are typically sterile. The Y chromosome is dominant, hence XXY individuals (Klinefelter's syndrome) are phenotypically male. However, affected men are also commonly sterile. Rare individuals with apparent Klinefelter's syndrome have been fertile, but these individuals are typically mosaics, with a sufficient population of chromosomally normal cells to yield viable gametes.

Harry W. Schroeder, Jr.

(see also: Congenital Anomalies; Genes; Genetics and Health; Human Genome Project; Medical Genetics )

Bibliography

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Murcia, N. S.; Woychik, R. P.; and Avner, E. D. (1998). "The Molecular Biology of Polycystic Kidney Disease." Pediatric Nephrology 12(9):721–726.

Nicolaidis, P., and Petersen, M. B. (1998). "Origin and Mechanisms of Non-Disjunction in Human Autosomal Trisomies." Human Reproduction 13(2):313–319.

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Ogata, T., and Matsuo, N. (1995). "Turner Syndrome and Female Sex Chromosome Aberrations: Deduction of the Principal Factors Involved in the Development of Clinical Features." Human Genetics 95(6):607–629.

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Vogel, F., and Motulsky, A. G. (1997). Human Genetics: Problems and Approaches, 3rd edition. Berlin: Springer-Verlag.

Genetic disorders

Genetic disorders refer to medical conditions that develop as the result of abnormalities in an individuals genetic material, usually that is inherited. Inheriting or developing a genetic disorder leads to a collection of clinical manifestations known as a syndrome . These clinical manifestations can vary from person to person with the same genetic defect or have similar presentations.

Principles of genetic inheritance patterns

Genetic information is packaged into chromosomes that are found in thecells nucleus, or DNA containing organelle. Virtually every human cell has 46 chromosomes, except for the sperm and egg (reproductive cells) which each have 23 chromosomes. Fertilization of the egg by the sperm results in a newly formed cell called the zygote. Each zygote receives 23 chromosomes from the egg and 23 chromosomes from the sperm. All but one of the 23 chromosomes are called autosomes. The remaining chromosome is the sex chromosome and it is either an X or a Y. After these sex cells unite, gender is determined. Females have two X's (XX), and males have one of each (XY). Females can only pass an X to their offspring, and males can pass either an X or a Y. Therefore, the male sperm is responsible for gender selection .

The duplicated set of 22 autosomes, numbered 1 through 22, are called homologous pairs in that there are two chromosome number 1 with similar genes and genetic material. There are approximately 50,000 genes on all the chromosomes. Individual genes are made of deoxyribonucleic acid (DNA) , which makes up the genetic code or alphabet that produces specific proteins . Proteins can play important structural and functional roles in the body. Each gene has a set locus , or position, on a particular chromosome. Identical genes that are located on the same locus on corresponding chromosomes are called alleles .

A persons genotype represents the genes that they inherited. If is an autosomally inherited disease , the genotypes are written as a lower or an upper case letter A (such as AA, aa or Aa to represent both alleles known as the genotype), where capital letters define dominant genes and lower case letters define recessive genes. Genotypes are either homozygous or heterozygous. Having two identical alleles, such as AA or aa, makes the genotype homozygous for that locus. Having different alleles (for example, Aa) at a locus represents a heterozygous genotype. The observable features that characterize an individual are collectively called the phenotype. A phenotype can also extend to observable characteristics that can only be visualized with the help of a microscope or other equipment.

Types of genetic inheritance

There are many types of genetic disorders. Mendelian disorders are a group of disorders that are inherited either as an autosomal (through one of the 44 chromosomes, excluding the X or Y chromosome) or X-linked (through the X chromosome) defect in a dominant or recessive pattern. Although most of them are commonly associated with inherited defects, many of them can involve spontaneous or de novo alterations in an individuals genetic material. There are also genetic disorders that have an unknown etiology and the mechanisms for how a disease develops have not been clearly elucidated. Some genetic disorders can be induced by environmental factors and are collectively called multifactorial disorders. Mitochondrial disorders involve disorders in the mitochondrial genome and are inherited only from the mother. The mitochondria are tiny compartments found in almost every cell and has a distinct genome from the DNA found in the nucleus and is involved primarily in producing energy for all the tissues of the body. Finally, polygenic disorders mean that many genes act together to produce a genetic disease.

Dominant and recessive

A dominant gene means that a single allele can control whether the disease develops. If only one parents (usually affected) passes on an autosomal, defective gene which results in the child having a genetic disorder, then the disorder is called autosomal dominant. A recessive gene means that there is enough normal protein product to function properly from the normal gene and, therefore, two copies of the defective gene are necessary for the disease to develop. If both parents are unaffected and they each pass on a defective gene causing their child to be affected, then the genetic disorder is autosomal recessive. The parents are called carriers. For example, sickle-cell anemia is a recessive disorder characterized by abnormal hemoglobin production. The genetic defect involves a gene that produces hemoglobin. Although sickle-cell carriers produce, in part, abnormal hemoglobin, although they usually do not experience clinical manifestations since the normal hemoglobin produced from the normal gene is enough to function normally.

However, Many other genetic disorders are caused by defects related to the sex chromosomes, or the X and Y chromosomes. If a defective gene on the X-chromosome are inherited, it is called X-linked. Like autosomal disorders, X-linked genetic diseases also can be inherited by dominant and recessive mechanisms. X-linked dominant means that if the father passes on the defective gene on his only X chromosome, all his offspring (which will be females) will be affected. If he passes on his Y chromosome, none of these males will be affected. Therefore, there is no male-to-male transmission. If it is X-linked recessive, all daughters will be carriers. If the mother passes on a recessive X-linked gene, then all her sons will be affected and all her daughters will be carriers. Understanding the mechanisms by which genetic disorders are inherited are very important for interpreting recurrence risks.

A variation of Mendelian patterns of inheritance is called incomplete dominance. Incomplete dominance occurs when both alleles are expressed. An example of this is observed in Four-O-Clock flower color . A white and a red phenotype are neither dominant nor recessive. If a flower is heterozygous and carries genes that produce both the white color and the red color, the flower color results in pink. Another variation is codominance. In incomplete dominance, the phenotype is a blending of the two different gene effects. In codominance, both gene variants are expressed at the same time, representing a third phenotype. For example, a flower with alleles that can produce either white or red in the homozygote form will produce both colors in the heterozygote form expressed as spotted flowers.

Genetic analysis

Chromosomal analysis can be performed on cell samples from an individual, a technique called karyotyping. A karyotype involves visualizing the chromosomes using a specific dye and a high-resolution microscope. The chromosomes can be distinguished from one another in size and staining pattern. Corresponding chromosomes 1 through 22 and the sex chromosomes can be lined up and visually inspected for abnormalities. Any obvious defect can indicate a diseased state. Sometimes it is apparent that a part of one chromosome was incorrectly combined with a different chromosome during cellular division. This is called a translocation and represents a structural abnormality. Numerical abnormalities occur when an extra chromosome is present. When more than 46 chromosomes are observed in chromosome, it is called aneuploidy. Most aneuploidies that occur are called trisomies which involve three homologous chromosomes, or the presence of extra chromosome. Numerical abnormalities are typically incompatible with life, with the exception of Trisomy 21 (Down Syndrome ) and a few other rare genetic disorders.

Dominant genetic disorders

If one parent has an autosomal dominant disease, then offspring have a 50% chance of inheriting that disease. There are roughly 2000 autosomal dominant disorders (ADDs) with effects that range from mild clinical manifestations to death. These diseases may develop early or late in life. ADDs include Huntington disease , Marfan syndrome (extra long limbs), achondroplasia (a type of dwarfism), some forms of glaucoma, most forms of porphyrias, and hypercholesterolemia (high blood cholesterol ).

Huntington Disease is a late onset nuerodegenerative disease. It is characterized by progressive chorea (involuntary, rapid, jerky motions) and mental deterioration that often develop after the fourth decade of life, eventually leading to death approximately 15 years later. The Huntington disease gene locus is on chromosome 4, and can be identified.

Marfan syndrome is an ADD characterized by long arms, legs, and fingers. People with Marfan syndrome also have a blue sclera that represents a hallmark clinical feature that can be detected when observing the eyes. In addition, these individuals have a high incidence of eye and aortic heart problems. The elasticity of the vessels in the aorta are susceptible to rupture, which can cause death. Not all people with Marfan syndrome inherit it from a parent, about 15% of Marfan syndrome cases are caused by a spontaneous mutation .

Recessive genetic disorders

Recessive genetic disorders (RGD) result from inheriting two defective recessive alleles of a gene, one from each parent. RGD often require careful molecular or biochemical genetic analyses to determine carrier status. Hence, the birth of a child with a recessive disorder may surprise unknowing parents. The probability of two carrier or heterozygous parents having an affected child is 25% each time they conceive. The chance that they will have a heterozygous (carrier) child is 50% for each conception. And the chance of having an unaffected homozygous child is also 25% for each pregnancy. More than 1,000 RGDs have been identified and include: cystic fibrosis , phenylketonuria (PKU), galactosemia, retinoblastoma (Rb), albinism , sickle-cell anemia, thalassemia, Tay-Sachs disease , autism , growth hormone deficiency, adenosine deaminase deficiency, and Werner's syndrome (juvenile muscular dystrophy).

A number of eye disorders are RGDs and are often associated with a mutant gene on chromosome 13. The Retinoblastoma (Rb) gene was the first human gene found to cause cancer and the first human cancer gene in which its location on a chromosome was determined. Rb is a gene that can cause a tumor in the retina, called a retinoblastoma. Most retinoblastomas are inherited; however, in some cases, inheriting a mutation on one allele combined with a spontaneous mutation on the other allele can result in retinoblastoma. Environmental carcinogens (cancer causing agents) can induce a spontaneous mutation. Other recessive eye disorders include myopia (nearsightedness), albinism, day blindness, displaced pupils, and dry eyes.

Some RGDs affect people having a particular ethnic background. For example, cystic fibrosis (CF), sicklecell anemia (SCA), and Tay-Sachs disease (TSD) all have specific mutations in the gene that causes each disorder that are preferentially found in individuals of a certain ethnic background. CF is a common autosomal recessive disease in individuals of Northern European decent and one in every 25 people in this population are carriers. SCA usually is most common in black and Hispanic
populations; however, some mutations in the gene that causes SCA are also found in Italian, Greek, Arabian, Maltese, Southern Asian, and Turkish populations. About 1 in 12 blacks are carriers for SCA gene mutations in one of the two hemoglobin genes. Hemoglobin carries oxygen in red blood cells to tissues and organs throughout the body. SCA patients have red blood cells that live only a fraction of the normal life span of 120 days. The abnormal blood cells have a sickled appearance and cannot transport oxygen efficiently. TSD gene mutation carriers are commonly found in Ashkenazi Jewish populations and approximately 1 in 30 are carriers of a mutation.

Galactosemia and PKU are examples of metabolic RGDs and are commonly called biochemical disorders. Both these disorders result from mutations in two different genes, both of which produce dysfunctional enzymes that are important in metabolism . Enzymes speed up chemical reactions and are essential for many cellular processes. People with galactosemia are enzyme deficient for Galactose-1-phosphate uridyl transferase, without which they can not metabolize galactose, a sugar found in milk. If milk and other galactose-containing food items are consumed, these individual cannot digest this compound properly and the result is severe developmental delay. Individuals that inherit PKU are deficient in the enzyme Phenylalanine hydroxylase, which is responsible for converting phenylalanine (an amino acid , a building block of protein) to tyrosine. The build-up of phenylalanine leads to severe developmental delay, and preventing clinical manifestations can be achieved by dietary modification. A phenylalanine-free diet containing sufficient amino acids is available for people diagnosed with PKU. Both of these metabolic disorders result from mutations in two different genes. Carriers are detected through a nationwide newborn screening programs, which differ in testing services from state to state.

Adenoside deaminase deficiency another autosomal recessive genetic disorder that is also called severe combined immunodeficiency. It is caused by a single mutation on chromosome 20 in a gene that encodes an enzyme important for proper functioning of the immune system . Adenosine deaminase converts breaksdown a small molecule called adenosine. Gene therapy has been successful in delivering a normal gene and prevents the damaging effects of adenosine on specific cells important for immune system function.

X-Linked genetic disorders

X-Linked genetic disorders (XLGDs) can be either dominant or recessive. Dominant XLGDs affect females and males. Dominant XLGD's include: Albright's hereditary osteodystrophy (seizures, mental retardation, stunted growth), Goltz's syndrome (mental retardation), cylindromatosis (deafness and upper body tumors), oralfacial-digital syndrome (no teeth, cleft tongue, some mental retardation), and incontinentia pigmenti (abnormal swirled skin pigmentation).

Recessive XLGDs are passed to sons through their mothers, who are carriers. Often, a carrier mother will have an affected male relative. Major XLGDs include: severe combined immune deficiency syndrome (SCID), color blindness , hemophilia , Duchenne's muscular dystrophy (DMD), some spinal ataxias, and Lesch-Nyhan syndrome. Roughly one third of these XLGDs result from a spontaneous mutation. Of these disorders, color blindness is the most benign.

Hemophilia is a more serious XLGD caused by failure of one of the clotting proteins, which serves to prevent an injured person from bleeding to death. Hemophilia A is the most severe form of this disease, and is characterized by extreme bleeding. It primarily affects males, although a few females can develop symptoms. This disease has been associated with royalty, as England's queen Victoria was a carrier and her descendants became rulers in several European countries.

Multi-factorial genetic disorders

Statistics and twin studies are often used to determine the genetic basis for multi-factorial genetic disorders (MFGDs). Because environment can play an important role in the development of these diseases, identical and fraternal twins who have been raised in different homes are ideally studied. MFGD include some disorders associated with diet and metabolism, such as obesity , diabetes, alcoholism , rickets, and high blood pressure . And the tendency of contracting certain infections such as measles, scarlet fever , and tuberculosis can be considered MFGDs. In addition, schizophrenia and some other psychological illnesses represent as well as congenital hip, club foot, and cleft lip are also inherited in this manner. Cancer, where the risk is associated with the environmental exposure also falls into this class of disorders. Certain breast, colon, skin, and small-cell lung cancers have been shown to have a genetic link. Certain genes predisposed people to a certain type of cancer and this risk is enhanced when there is a specific environmental exposure. This susceptibility is influenced by inherited variations in genes, which encode proteins that may be more or less functional. For example, if the protein is involved in the metabolism of a carcer causing substance and an individual inherits a variation in the gene sequence, this might affect the function of the protein it encodes. If it reduced its function, this might lead to more damaging effects of the cancer causing substance.

Aneuploidy

The two most common aneuploidies, trisomies and extra sex chromosomes, can be due to maternal or paternal factors including advanced age. A number of aneuploidies can be attributed to dispermy, or when two sperm fertilizes one egg. The resulting genetic disorders can result in a spontaneous mutation. Live-born children with autosomal aneuploidies can have trisomy 13, 18, or 21, and all have some form of developmental delay, while trisomy of any other chromosome is usually always fatal. Trisomy 13 (Patau's syndrome) is characterized by retarded growth, cleft lip, small head and chin, and often polydactyly. Trisomy 18 (Edward's syndrome) is marked by severe, variable abnormalities of the head, thumbs, ears, mouth, and feet. Trisomy 21 (Down's syndrome) occurs equally in all ethnic groups, and is closely related to increased maternal age.

Aneuploidy of the sex chromosomes can cause abnormal genital development, sterility, and other growth problems. The most common aberrations involve multiple X chromosome syndromes. Males with an XXY aneuploidy have Klinefelter's syndrome, and have small testes and typically no sperm. Hermaphrodites are individuals that have both male and female genitals, are extremely rare, and result from cell lines that have two different chromosome patterns with both XX and XY cells.

Genetic screening

Genetic tests are available and can reveal varying degrees of genetic information depending on the test. Most of these tests are performed by isolating chromosomes or by measuring a protein product that is excreted in the urine to test for biochemical defects. These tests can be used prior to conception to determine a couple's risk for having an affected child, during pregnancy, at birth or later in life.

The most successful wide-spread test for genetic disorders is the newborn program that tests for diseases such as PKU. Newborn screening for hypothyroidism and galactosemia are also performed in several states. Prenatal testing in embryos and fetuses include chorionic villus sampling (CVS) , amniocentesis , and ultrasound. CVS can detect Down syndrome, hemophilia, DMD, CF, SCA, and sex chromosomal aberrations. Amniocentesis can detect Tay-Sachs disease, Down syndrome, hemophilia, spina bifida , and other abnormalities. Ultrasound is used to visualize the developing baby; it can detect spina bifida, anencephaly (no brain ), and limb deformities.

Genetic counseling and testing can help people find out if they carry the gene for some disorders, or whether they will develop a late-onset genetic disorder themselves. Genetic probes can identify the genes for Huntington's disease, cystic fibrosis, Tay-Sachs, sickle-cell, thalassemia, and abnormalities associated with growth hormone. Genetic testing capabilities increase each year as additional genetic disorders are better characterized and the gene localization and protein function is determined.

See also ADA (adenosine deaminase) deficiency; Albinism; Birth defects; Embryo and embryonic development.

Resources

books

Lodish, J., D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell. Molecular Cell Biology. New York: Scientific American Books, Inc., 1995.

Nussbaum, Robert L., Roderick R. McInnes, and Huntington F. Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.

David L. Rimoin Emery and Rimoin's Principles and Practice of Medical Genetics. London; New York: Churchill Livingstone, 2002.

Louise Dickerson
Bryan Cobb

KEY TERMS

Allele

—Any of two or more alternative forms of a gene that occupy the same location on a chromosome.

Chromosomes

—he structures that carry genetic information in the form of DNA. Chromosomes are located within every cell and are responsible for directing the development and functioning of all the cells in the body.

Dominant trait

—A trait which can manifest when inherited from one parent.

Gene

—A discrete unit of inheritance, represented by a portion of DNA located on a chromosome. The gene is a code for the production of a specific kind of protein or RNA molecule, and therefore for a specific inherited characteristic.

Heterozygote

—A person possessing two non-identical alleles.

Homozygote

—A person possessing two identical alleles.

Multifactorial trait

—A trait which results from both genetic and environmental influences.

Recessive trait

—A trait which is not expressed in heterozygotes but is expressed if two defective genes are inherited by carrier parents.

Sclera

—White of the eye.

X-Linked trait

—A trait that is inherited due to mutations in genes on the X-chromosome.