Mendel’s Law

BIBLIOGRAPHY

Gregor Mendel (1822–1884), an Austrian monk, conducted experiments on 28,000 pea plants in his monastery garden between 1856 and 1863. His results, outlined in his essay “Experiments on Plant Hybridization,” were read to the Natural History Society of Brno (Brunn) in February and March of 1865 and were published in 1866 in the proceedings of the society. These experiments involved studying variations in peas across successive generations of true breeding plants in terms of their shape, size, and color. In simple terms, through close observation and good record keeping, he discovered that, for example, the size of a plant was passed on as a separate trait in a competition for the dominance between traits. These variations were caused by what Mendel called “factors” but what we now call “genes.”

The results of these experiments were initially neglected, but then “rediscovered” in 1900 by three European scientists, Hugo de Vries, Carl Correns, and Erich von Tschermak. However, it was William Bateson (1861–1926) who promoted these research findings and who coined the key terms of modern genetic science, namely “genetics,” “gene,” and “allele.” Genetics is that branch of biology that studies both hereditary and variation in organisms. It was Thomas Hunt Morgan (1866–1945) who would later integrate Mendel’s theoretical model with the chromosome theory of inheritance. Morgan’s experiments with the fruit fly showed that genes are carried by chromosomes, which are the mechanisms of hereditary. His account of hereditary particles created what is now referred to as “classical genetics.”

Mendel’s First Law, or Law of Segregation, states that members of a pair of homologous chromosomes separate during the formation of gametes such that every gamete receives only one member of the pair. The law can be broken down into four components: (1) alternative versions of genes account for variations in inherited characters; (2) for each characteristic, an organism inherits two alleles, one from each parent; (3) if the two alleles differ, then the dominant allele is fully expressed in the organism’s appearance, while the recessive allele has no noticeable effect; and (4) the two alleles for each characteristic segregate during gamete production.

Mendel’s Second Law, or Law of Independent Assortment, states that the emergence of one trait will not affect the emergence of another. For example, the eye color and height of a human are not necessarily connected but random.

These laws can only be fully understood in terms of the behavior of chromosomes in reproduction. A cell nucleus is composed of several chromosomes that carry genetic traits, and in a normal cell these chromosomes have two parts, or chromatids. A reproductive cell contains only one of these chromatids, but when two cells (normally male and female) are merged, the genes are mixed and the new cell becomes an embryo. Mendelian laws explain how this new cellular life has half the genes of each parent, and also explain the varying dominance of different genes, resulting in the uneven distribution of traits across generations.

The reproductive advantages of Mendelian-type hereditary are that it creates greater evolutionary opportunities that are beneficial. For example, cell mutations can produce positive side effects such as disease resistance. It is also the case that mutation in a single gene can cause an inherited disease such as sickle-cell anemia or cystic fibrosis. However, this outcome can also be treated as consistent with Mendelian hereditary advantages. Whereas sickle cell disease is a crippling ailment, the sickle cell trait, more prevalent in populations that have a greater geographic likelihood of exposure to mosquitoes, is associated with some resistance to malaria.

Mendelian theories of hereditary have proved to be hugely controversial in modern society. For one thing, Mendelian genetic theories offered additional support to the theory of natural selection in the evolutionary science of Charles Darwin (1809–1882) in which human development was to be explained by secular causes such as the blind adaptation of species to the natural environment rather than by conscious or intentional design. In addition, following Francis Galton (1822–1911), Mendelian theories have become associated with eugenics, or the science that aims to improve the quality of the human stock by increasing “good genes.” Eugenics, though, also became an arm of European fascism in which sterilization and selective breeding would improve the Aryan race. Mendelian laws also underpin the pressure from parents in affluent societies for so-called “designer babies,” leading critics to fear the creation of a master race. Application of the findings of the Human Genome Project will give scientists increasing control over reproductive outcomes. However, these fears can be exaggerated, since genetic counseling is directed at the prevention of crippling disease (specifically Huntington’s disease) rather than designing aesthetically pleasing or highly intelligent offspring. These fears of course are influenced by the accuracy of the content of the genetic counseling and on the impact and effectiveness of such counseling.

SEE ALSO Eugenics; Galton, Francis; Genetic Testing; Genomics; Heredity; Phenotype

BIBLIOGRAPHY

Bowler, Peter J. 1989. The Mendelian Revolution: The Emergence of Hereditarian Concepts in Modern Science and Society. Baltimore, MD: Johns Hopkins University Press.

Fisher, Ronald A. 1936. Has Mendel’s Work Been Rediscovered? Annals of Science 1: 115–137.

Fukuyama, Francis. 2002. Our Posthuman Future: Consequences of the Biotechnological Revolution. New York: Farrar, Straus and Giroux.

Warnock, Mary. 2002. Making Babies: Is There a Right to Have Children? Oxford: Oxford University Press.

Bryan S. Turner