INHERITANCE

note:Although the following article has not been revised for this edition of the Encyclopedia, the substantive coverage is currently appropriate. The editors have provided a list of recent works at the end of the article to facilitate research and exploration of the topic.

Statutory and common laws governing inheritance have a profound effect on the formation or dissolution of household structures and the patterns of inheritance transfers over generations. A society, for any political or social reason, can initiate and promulgate a law or civil code controlling inheritance; inevitably, new laws or codes cause drastic change in the structure and functions of family systems.

In order to understand the subject of inheritance, it will be necessary to define many terms. To inherit is by law to receive property, resources, or, often, status from an ancestor at her or his decease or to take by intestate succession or by will. Intestate succession is a transfer of resources according to legal procedures that control distribution of the resources when there is no will. A person is intestate when he or she has not made a will. In a will, usually a written document, a person (or testator) makes a deposition of his or her property, and the deposition takes effect, at least in modern societies, upon the testator's death. A will is changeable and revocable during the lifetime of a testator.

Takers are successors or beneficiaries. Impartible inheritance is a situation, established by statute, in which the property and resources are indivisible and are given to one person (devisee); this type of inheritance is likely to occur within family households primarily in rural areas of most historic societies, especially in Eastern and Western Europe. Partible inheritance, dividing assets for conveyance to one's heirs, is linked to nuclear family households found in those locations where there is accelerated industrialization and urbanization.

Any system of inheritance promotes the continuity of family and societal structures over generations. The transfer of resources from the older to the younger generation helps maintain a family's position and power in the social order. Such transfers also provide stability to existing societal caste and class arrangements and ordinarily are ensconced deeply in tradition and myth. It is more likely that inheritance systems function to perpetuate existing social structures than to change society's organizations and institutions.

INHERITANCE, LAWS, AND CHANGE

Those most affected by changes in laws are persons with little power in society. This is evident in eighteenth- and nineteenth-century European societies, in which the promulgation of laws and decrees drastically affected the structure and practices of rural families. For example, Gaunt (1983) indicates that in nineteenth-century central Europe, in a section that is now part of Czechoslovakia, the decree to conscript unmarried men for military duty caused parents of serf families to encourage early marriage of their children. As a consequence, the incidence of complex family households increased, and there were changes in the patterns of inheritance succession. During a labor shortage in eighteenth-century Poland, fiefdom rulers encouraged endogamous marriage and attempted to restrict their chattels from leaving their villages. The economy was based on household unit sharecropping. The peasants encouraged complex family households consisting of two or more conjugal units, while the Polish lords favored neolocal household residence (Kochanowicz 1983). The inheritance outcomes of these efforts to circumvent political, economic, and social measures, often repressive in intent, affected existent family structures, their forms, and inheritance patterns.

The possession of equities, property, possessions, resources, and land (especially in the case of European peasants) determined whether these would compose an impartible or partible inheritance. For example, an impartible inheritance pattern is most likely when land is the primary family asset. In addition, the transfer of authority over these rights depended upon the timing of "stepping down," a process deeply embedded in the cultural traditions of the society (Gaunt 1983; Plakans 1989). Stepping down usually occurred through a retirement contract, which was essentially a will that indicated the conditions of the transfer from parents to children. This inter vivos phenomenon, a conveyance of property and other equities while the individual was alive and engaged in stepping down, was "one made of preserving intergenerationally the match between family and the property that provides its livelihood" (Sorensen 1989, p. 199). Stepping down, or disengaging, usually occurred when the oldest son married or when the parents were near or at retirement age. Stepping down was invoked by law or tradition, and it resulted in variations in inheritance patterns and in the organization of the life course of various family members.

Sorensen (1989) provides a detailed retirement contract of his great-grandfather on his mother's side, a prosperous farm family in the western part of Jutland, Denmark. The transfer of this medium-sized farm, substantially undervalued, when Laurids Poulsen was fifty-six, was apparently to make certain that the property remained in the family to buttress its position in the society.

It is not explicitly stated in the Poulsen retirement agreement that the heir, Alfred, would care for his parents in their declining years. The contract's provisions enabled Laurids and his wife, Maren, to be independent. Yet in this situation and, as Sorensen (1989) indicates, in Scandinavia since before the Middle Ages, transferring property to a son implied the promise of care in old age. Thus, there existed two motivations for conveying property to a son, but the stronger proved to be the hope of maintaining the property in the family over generations.

In former ages, conveying property and its accompanying position, status, and power was the intent of inheritance patterns. The preservation and maintenance of the family unit was the goal. Impartible inheritance expressed this intent of maintaining the family property and social position, but it created obvious winners and losers. The transfer of a family's property to a single heir meant that other family members could leave to seek opportunities elsewhere or remain with the family enterprise in a subservient position.

The possibilities for fuller expression of family members' abilities, where talent and skills and not family membership determined the individual's life course trajectory, would come at a later period. Partible inheritance reduced the requirement that family members subordinate their desires, interests, and expectations to those of the family unit. Changes in a society's demographics, such as fertility and mortality, and changes in the means of producing goods and services resulted in lessened need for impartible inheritance. Partible inheritance became identified with the modern period of Western civilization.

MODERN PERIOD

Rules for succession and rules for inheritance of property are related but distinct from one another. Inheritance of property usually follows lines of succession to social position. Codified systems of secular law governing inheritance and status succession emerge in complex societies and are sufficiently precise and uniform to meet the majority needs of the population (Radcliffe-Brown 1935).

For urbanites in complex societies such as those in Europe and the United States, the transfer of land, dwelling unit, tools, and equipment is less critical to the survival and maintenance of the family over generations than it is for the rural resident. Economic assets other than land and buildings—that is, personal mementos and possessions—become the content of such transfers.

Rural landholders of modest means were unable to effect a pattern of impartible inheritance. Increasing societal complexity nurtured corporate agriculture in preference to the family farm. The reduction in the number of family-owned farms, the increasing dependence of farm family members upon the larger, mostly urban society for jobs that are not located close to the farm dwelling, price supports, and payment for nonproduction have diminished the possibilities for maintaining the family farm over generations. Partible inheritance has become the norm.

This shift to partible inheritance under new rules of succession meant that for the first time the claims of the surviving spouse outweighed those of the surviving kin of one's lineage, a reversal of the pattern found in eighteenth- and nineteenth-century Europe and in primitive societies (Benedict 1936; Hoebel 1966). Under U.S. state statutes governing intestacy the surviving spouse and children share in the estate. The spouse receives at least one-third, depending on the number of surviving children and the specifics of the state law governing inheritance. If there is no surviving spouse the children share and share alike. Where there is no surviving spouse or children the estate passes to grandchildren. In the absence of grandchildren the next to receive are grandparents, then siblings, then more distant relatives.

Where testacy exists the estate usually passes to the surviving spouse. This horizontal transfer in the generational line of succession is uniquely modern and represents an evolution from the Roman definition of inheritance as "succession to the entire legal position of a deceased man" (Maine 1963, p. 208). In Rome the heir functioned as a guardian or executor of the estate to perpetuate the honor and status of the deceased and family survivors and to keep intact and extend the estate's holdings. The stepwise shift from decedent to surviving spouse and subsequent vertical transfer of equities to children after the death of the surviving spouse resulted in a family system based more on individual relationships, feelings, perceptions, and interactions and less on tradition, primogeniture, and maintaining properties and estates.

The pattern of conveying all property to the spouse, to be discussed further in this article, is a practice seemingly not in consonance with the concept and exercise of testamentary freedom.

TESTAMENTARY FREEDOM

Testamentary freedom, the individual's right to will away property to persons outside the family or to distribute to a number of heirs and legatees related by blood, consanguinity, or adoption, is a fundamental Anglo-American concept of the U.S. inheritance system. The primogeniture system, which passed all property to a single heir and was most suited for the wealthy, was replaced in eighteenth-century England by a new law of the land—testamentary freedom. This occurred at the time England disposed its feudal land tenure system. The 1789 French Revolution, in keeping with its ideology of justice, freedom, equality, and fraternity, and for more concrete social and political reasons, enacted laws requiring equal distribution of a deceased person's assets among surviving children. Colonial settlers in the United States undoubtedly inspired by and enamored of the changes in English and French societies, brought with them this notion of testamentary freedom as part of their intellectual and cultural heritage.

At first blush the practice of testamentary freedom would seem at least to contradict if not destroy the major intent of an inheritance system. According to Edmund Burke, "the power of perpetuating our property in families is one of the most valuable and interesting circumstances belonging to it, and that which tends the most to the perpetuation of society itself" (1910, p. 121). The major question is whether testamentary freedom as it is practiced negates what Burke suggests is a most critical process for generational and societal continuity.

Testamentary freedom, like justice or liberty, is a relative and not an absolute condition. In practice it accommodates to family continuity over generations; a multilineal descent system; a highly differentiated society where the majority of assets owned by individuals are moveable; and values that espouse rationality, choice, freedom, and democracy. The right of an individual to dispose of property according to her or his wishes is recognized if the individual disposes property in a responsible fashion—when one takes care of one's kin, thus maximizing the possibilities of family continuity and orderly social relationships among family members. Testamentary freedom is not exercised absolutely. It accommodates to the norms of responsibility. Empirical data support the idea that compromise occurs among the interests of the individual's family and state in the exercise of testamentary freedom (Sussman, Cates, and Smith 1970).

Courts, social norms, and societal economic patterns limit the expression of testamentary freedom. Courts use the soundness of mind principle in determining if an individual acts in a responsible fashion. Knowing what one possesses, the nature of the business in which one is engaged, and the natural objects of one's bounty are the essential components of the legal definition of a sound mind. In practice, courts almost universally consider the well-being of the family in addition to considering whether an individual has knowledge of his or her assets and successors (Cates and Sussman 1982). Neglecting or abandoning the family is viewed as unnatural, and being unnatural is equated with being of unsound mind. The media abound with cases regarding will contests involving decedents' bequests to loved pets, charities, strangers, or acquaintances. Preventing a distant relative from taking from or receiving an adequate share of an estate may result in legal action. Sussman, Cates, and Smith (1970) report a case in which charities were the major beneficiaries. The sole surviving relative, a niece, contested the will and lost, but she received a large out-of-court settlement of $150,000. Courts, plaintiffs, and defendants normally favor out-of-court settlements because of lower economic and psychic costs. In 1965, when this case occurred, a settlement of $150,000 was judged to be ample compensation. The settlement was also an indication that the well-being of a distant family member had been considered.

Prevalent social norms foster concern for the well-being of surviving family members. Believing that families should take care of their own and that family members should not be pauperized, the state has more than a legal interest in seeing that testamentary freedom be exercised with regard for the well-being of the family.

PATTERNS OF GENERATIONAL DISTRIBUTIONS

Complex societies have developed systems of statewide resource transfers that have replaced in part some of the functions of the family inheritance system. These large-scale transfer systems, based on the principle of serial service, are society's way of taking care of those deemed dependent, those unable to contribute to the gross national product through gainful employment. Preponderant numbers of the very young and the elderly receive support from such transfers. The preeminent philosophical notion is that the young adult and middle age generations, individuals ages sixteen to sixty five, pay through their earnings for governmentinitiated entitlement and needs programs such as Social Security, Medicare, Medicaid, educational grants, welfare payments, and so forth, programs to maintain and enhance the lives of the less fortunate. These generations may participate in these programs somewhat grudgingly, but they do so with the knowledge that they were supported during their childhood and with the expectation of being supported by these same programs in their old age. This pattern of society-wide transfers is characteristic of serial service.

Such massive transfers have reduced the economic burden of families in caring for dependent members. In some instances these programs have diminished the need for family members to provide extensive and intensive social and emotional support (Kreps 1965). Social Security pensions and other vested retirement programs, in both the private and public sectors, provide sufficient income for an increasing number of retirees. Most of them will be able to live independently or with minimal support from their relatives during their later adult years. Being economically independent or quasi-independent in old age with little need to rely upon the family's financial resources is a radical shift from earlier periods, when older relatives depended on the determinations of the inheritance system.

The society-wide transfer system based on universal taxation has not completely replaced the family inheritance system. One can view the former as an impersonal, bureaucratized, and universal system enacted primarily by statute or charter and monitored by official regulations. Participants become part of a large formal system and once they qualify are treated in a uniform manner. The use of computers and identification numbers increases the impersonality of support systems.

The family inheritance system, on the other hand, is influenced but not controlled by large-scale economic transfers. It exists and functions within a set of norms that extoll interpersonal relationships, continuity of marriage and blood lines, symbolic meanings, feelings of filial piety, nurturance, support, care, distributive justice, and reciprocity.

These institutional systems and programs that mutually support families probably condition the bases upon which individual inheritance dispositions are made. Sussman, Cares, and Smith (1970) indicate that as a consequence of the growth of society-wide transfer programs,

Inheritance transfers may be less a consequence of acts of sexual reciprocity, based upon what specific individuals of one generation in a family do for others of another generation, but more a function of serial service. Serial service (a concept elaborated by William Moore, 1967) involves an expected generational transfer that occurs in the normal course of events. It is expected that parents have to help their young children, and middle aged children may be called upon to give care or arrange for care of an aged and often ailing parent. This is within the cycle of life, and services of this kind are expected and are not based upon reciprocal acts. Whatever parents have in the way of worldly possessions will in due course be passed on to lineal descendants. (p. 10)

Since the 1970s there has been a strong movement to reduce, or at best not increase, taxation to support benefit systems that assure society-wide economic transfers across generational lines. Those who support a reduction in such transfers strongly believe that the society has reached its absorptive capacity to pay increasingly higher costs for retirement and a variety of services to dependent family members. The cutback in government programs coupled with the exhortation that the government "should get off the back of family members" and the reglorification of the myth that families in ancient times cared for their own have resulted in increasing burdens for families in the care of their elderly and dependent members. This shift away from society to family responsibility for family members suggests a new look at family economic transfers and the role such inheritance plays in intergenerational relationships and in the solution of long-term care of aged and other dependent members.

INHERITANCE ISSUES FOR THE 1990S

Believing they do not want to be a burden to their children, older adults with economic resources and few relatives for whom to function as primary caregivers when needed are spending their inheritance on travel and other leisure activities and for total medical and physical care as they move from independent to dependent living. A likely result is the diminishing of available funds to heirs and legatees and increased importance of family heirlooms. These gifts express the meaning and significance of the relationships between family members prior to the death of the testator.

The increasing unavailability of relatives, especially children, to care for aged family members will result in the assignment of heirs and legatees who are not related by blood or marriage. These are friends and service providers who supply social support, care, and nurturance and are "like family." Elders in the latter part of this century and well into the twenty-first, that is, members of the World War II baby boom generation, will be searching for a relative or someone to care for them in their declining years. Their drastically low reproduction rates and the consequences of the gender revolution begun in the 1960s will result in the availability of a severely limited number of immediate family members or distant relatives able or willing to provide care. Elderly people will turn to persons not related by blood or marriage—members of the individual's wider family (Marciano 1988). Wider family members are not a certain age, nor must they conform to traditional social norms; they are not related by blood or marriage. Bondings express deep friendship and voluntary informal contracted obligations and expectations. Wider family members today are on call and respond to requests for assistance immediately. They act and feel that they are at home in each other's household.

The consequence is an increasing incidence in the naming of such persons in wills and the probability that courts in the future will uphold their right to share the estate even in the case of intestacy. The courts may rule that traditional patterns of distribution to surviving family beneficiaries be modified to include those who provided care and nurturance to the deceased. The basis for such action is the notion of distributive justice, which invokes fairness. Those who have voluntarily provided services, intimacy, friendship, and care should be recognized and even rewarded. They have fulfilled the role of filial responsibility.

The dark side of this pattern is the exploitative friend, surrogate, or service provider who manipulates the care receiver and takes over the estate through an inter vivos transfer or by a rewriting of the will. This sorcerism, coupled with an increasing number of will contests, will keep attorneys in good financial stead and courts very busy for a long time.

Distributive justice, a just and fair distribution of resources as perceived by a testator or promulgated by the laws of intestacy, will be characteristic of transfers to family and kin as well as wider family members in the coming decades. A component of distributive justice is equality (Piaget 1932), a standard that is invoked by statutes governing intestacy. Equality is determined by the degree of relationship of the deceased to the survivor. Thus, upon the death of a parent intestate, equal shares are given to surviving children. If a spouse survives, she or he usually receives one-third of the estate, and the remaining two-thirds are distributed to surviving children.

Equity is another component of distributive justice. It implies a just and fair condition. It is not the same as equality. It is the "distribution of rewards and costs between persons" (Homans 1961, p. 74). Equity is the component most suited to arrangements for payment of rewards for incurred costs. Hence, both family and nonfamily members can expect rewards in property inheritance in relation to the costs incurred in caregiving and related activities. Such exchanges will become normative and generally accepted. Reciprocity is to be recognized and rewarded.

Portending the future, testators will increasingly enter into contracts with either family or nonfamily (wider) members, setting forth conditions and expectations of needed supports and caregiving (Hanks and Sussman 1990). Such contracts, seemingly as legal as any other contract, are likely to be challenged in the courts by those who stand to inherit under the laws of descent and succession. The rootedness of these contracts in distributive justice, with its fairness and equity principles, suggests that their validity will be sustained.

Contracts can involve inter vivos transfers or declarations in a will. For example, a middle-aged testator can give resources to a young relative or member of a wider family with a pay-back arrangement of care and service when needed by the benefactor. A will provision can readily be made. The exact instrument that will enhance its legality needs to be developed in consultation with attorneys. Such contracts, similar to prenuptial agreements, drawn with both parties of sound mind, should be legal. These contracts will have provisions for modification and cancelation, like any other contract. The major point is that such contracts can reduce the concerns of an aging population regarding their life-style in very old age and diminish their total dependence upon institutional forms of care. The "inheritance contract" fosters independence and utilization of one's resources in meeting health and social needs. Caretaking as a family enterprise can reduce potentially the outlay of public monies currently expended for health and social service programs. This reduction in the economic burden of government is not tantamount to the elimination of current universal support programs. At best the inheritance contract helps provide meaningful relationships, establishes a procedure for planning and expending one's resources, and encourages individuals to rely less upon governmental institutions.

Openness in discussing wills is a new departure from the past and leads to forthrightness in discussing other family matters. Hanks (1989) reports in her sample of 111 corporate family members that 88 percent discussed their wills and 60 percent their funeral arrangements. Such openness with relatives can reduce future will contests and result in conversations and negotiations regarding foreseeable generational transfers and caregiving arrangements. The symbolic meaning attached to the passing on of jewelry, paintings, furniture, books, and other household items has been given very little attention (Sussman, Cates, and Smith 1970). Allocations of these items demonstrated to the recipients the kind of emotional and affective relationship they had with the deceased. More individuals experienced pain and depression from not receiving a promised or expected heirloom than from receiving a lesser share of property or equities. Strong feelings are aroused because of the memories connected with these heirlooms, and the deceased cannot be asked regarding her or his feelings and emotions toward the heir. Openness in discussing wills and related matters can reduce misunderstandings and misinterpretations regarding the meaning attached to the transfer of heirlooms and the trauma of not knowing.

In the 1990s and future decades inheritance will continue its patterns of transfer and distribution of properties and equities over generations. It will be a different system from that found in rural areas in historic or current time. It will continue to be insignificant in the generational transfer of status and power except for the wealthy classes. Things that will distinguish inheritance in the future from that of the past is the emergence of the inheritance contract; openness in discussing will contents with potential beneficiaries; greater inclusion of distant family relatives in wills; increased number of will contests; and increased incidence in the number of older adults who spend money on leisure activities or on their own health care rather than contribute it to their children's inheritance.

(see also: Intergenerational Resource Transfers)

references

Benedict, Ruth 1936 "Marital Property Rights in Bilateral Society." American Anthropologist 38:368–373.

Burke, Edmund 1910 Reflections on the French Revolutionand Other Essays. New York: DuHon.

Cates, Judith, and Marvin B. Sussman 1982 Family Systems and Inheritance Patterns. New York: Haworth Press.

Gaunt, David 1983 "The Property and Kin Relationships of Retired Farmers in Northern and Central Europe." In Richard Wall, Jean Robin, and Peter Laslett, eds., Family Forms in Historic Europe. Cambridge: Cambridge University Press.

Garrett, Mario D. 1995 "Filial Piety or a Filial Pie: Capital Transfer from the Elderly to Their Offspring within Southeast and Eastern Asia." In Stanley R. Ingman, Pei Xiaomei, Carl D. Ekstrom, Hiram J. Friedsam, and Kristy R. Bartlett, eds., An Aging Population in an Aging Planet, and a Sustainable Future. Denton, Tex.: Center Texas Studies.

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——, and Marvin B. Sussman 1990 "Inheritance Contracting: Policy Implications of Inheritance and Caregiving Patterns." Unpublished paper, College of Human Resources, University of Delaware.

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Plakans, Andrejs 1989 "Stepping Down in Former Times: A Comparative Assessment of Retirement in Traditional Europe." In David I. Kertzer and K. Warner Schaie, eds., Age Structuring in Comparative Perspective. Hillsdale, N.J.: L. Erlbaum.

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Rosenfeld, Jeffrey P. 1991 "The Heir and the Spare: Evasiveness, Role-complexity, and Patterns of Inheritance." In Judith R. Blau and Norman Goodman, eds., Social Roles and Social Institutions: Essays in Honorof Rose Laub Coser. Boulder, Colo.: Westview Press.

Schwartz, T. P. 1996 "Durkheim's Prediction about the Declining Importance of the Family and Inheritance: Evidence from the Wills of Providence, 1775–1985." Sociological Quarterly 37:503–519.

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Marvin B. Sussman

INHERITANCE

Julie Hardwick

In the narrowest sense, inheritance concerns the transmission of property from one person to another, whether that property is money, real estate, personal possessions, titles, or kingdoms. This very specific process has, however, the broadest of implications. Every person in European society—men and women, rich and poor, young and old, urban and rural, Catholic and Protestant—was affected by the transmission of property through inheritance. Relations between people, whether parents and children, siblings, or men and women, have influenced and been profoundly influenced by the way inheritance worked. The giving of property provided a symbolic as well as material means for family members to express their affection for each other or to designate their preferences for some family members over others. Moreover, patterns of inheritance were determined by many factors, such as laws, economic practices, social customs, and demographic patterns, as well as personal preferences. Because exploring the way inheritance worked in a particular historical community reveals many different aspects of that community, European social historians have found inheritance to be an invaluable topic.

The approaches of historians to the study of inheritance have changed focus in significant ways since the 1970s. They have moved from examining what might be called the mechanics of inheritance, looking at issues like what the pertinent laws were and what wills of individual testators suggest about how those laws were observed (or not) in practice, to trying to place inheritance more broadly in the context of social relations. In this more recent phase, social historians have looked at inheritance as a key strategy that family members used to deal with the various challenges they faced—to ensure they had support in old age, to provide for all their children, or to guarantee that social status was maintained. This conceptual approach encourages historians to examine the many ramifications of inheritance. As David Warren Sabean has observed, "The way that property is held gives shape to feelings between family members, territorializes emotion, establishes goals and ambitions, and gives to each a sense of dependence and independence" (Medick and Sabean, 1984, p. 171).

Key variables in inheritance practices include laws, timing, the nature of the property, and the status of men and women. The legal regimes governing inheritance varied enormously from country to country or even region to region within countries. Yet as social historians have shown in many studies, families did not always seem to follow the dictates of laws when it came to transmitting their property. They pursued a variety of strategies that tried to ensure that their own goals were met even while legal requirements were being observed or circumvented.

The timing of the transmission of property was enormously influential, shaping individual fortunes and family relationships. Parents might transfer property, in the form of dowries, gifts, or postmortem bequests, to one or more of their children at almost any time between the children's marriages and the parents' deaths. The consequences of these decisions over timing were enormous. A son who had to wait until his parents died to receive his inheritance might, for example, continue living with them and delay marriage. In turn, that pattern of delayed marriage for men was often accompanied by high rates of illegitimacy or prostitution.

When laws required that all children inherit equally or constrained parental testamentary discretion in other ways (as in England, where in many cases the law of thirds prevailed, whereby a widow would get a third, the children would get a third, and the father could dispose of the other third as he chose), parents could still privilege one child over another either by differential timing or by the kinds of property assigned to each child. One child might, for example, receive a substantial portion of their inheritance years earlier than a sibling.

The character of the property that was transmitted—that is, whether it took the form of movables (cash, tools, linen and other household goods, or personal items) or immovables (usually land or houses)—was also a key element in inheritance. When property was transferred on the marriage of a child, it was more likely to be in movables than in real estate.

Gender was another critical element in inheritance: men's and women's right to property often differed, whether as widows and widowers or as sons and daughters. It was possible, for instance, that in a family where the parents' property appeared to be divided equally among all children, sons might receive a preponderance of the land and daughters a preponderance of the movables. In some early modern communities, daughters who received dowries at the time of their marriages were subsequently excluded from any further claim on their parents' estate. Early modern women who were widowed often found themselves in very precarious financial situations because they had to divide their household property with the other heirs.

EARLY MODERN EUROPE

Inheritance shaped the prospects of most early modern Europeans in very profound ways. Although laws about the transmission of property varied enormously, in early modern society a person's life was profoundly shaped by what property he or she might receive through inheritance. The impact was not only economic but also social, framing household structures and kinship relations.

Renaissance Italy. Renaissance Italy provided one of the earliest and most fertile grounds for historians' studies of inheritance. Although the territory of Italy was divided into myriad self-governing republics, Venice and Florence have dominated scholarly attention. In Venice, patrilineal ties (where kinship was figured through the male line) were especially strong. The most striking institutionalization of this dynamic came in the form of the fraterna, a legal device that gave sons equal and joint shares of their fathers' estates while excluding daughters, who had rights only to dowries. Traditionally, this inheritance system was viewed as the key to the persistent close ties between male kin, especially in the Venetian elite, and to the marginalization of women.

Yet as historians have studied inheritance in practice as well as law, a more complex view has appeared because Venetian women's dowry rights were strongly protected, and elite women were key links in the kinship networks that underlay political and economic success. Persistent dowry inflation through the fourteenth and fifteenth centuries made daughters ever more expensive. While some families responded by coercing their girls to pursue the cheaper course of becoming nuns, other families met the clear obligation to dower daughters at whatever cost by drawing on an ever wider group of kin to raise the money.

In Florence, broadly similar inheritance practices prevailed in the fourteenth and fifteenth centuries. Patrilineal tendencies were reinforced as new elite families of merchants and bankers emulated traditional aristocratic practices by giving themselves a family name that was transmitted by male heirs. The marginalization of women was given more material form when families used dowries to fulfill their obligation to daughters while sons were given equal shares of all remaining property. The close ties between brothers that resulted were evident, for example, in the establishment of fraternal communities where all brothers and their families lived together in the same large household, even after the father had died. Even when these groups broke up and siblings established their own households, they often lived in very close proximity, building complex webs of kinship ties that underpinned much economic and political activity. Sisters meanwhile found themselves in limbo, regarded as only temporary members of either their birth families or the families of their husbands.

Partial inheritance. Just as inheritance contributed to the distinctive characteristics of Renaissance Italian city-states, different inheritance laws and practices were similarly pillars that profoundly shaped daily life in the rest of Europe from the late fifteenth to the late eighteenth century. In many parts of Europe, rural and urban, partible inheritance prevailed as a matter of both law and practice. In these kinds of inheritance systems, the property was divided up more or less equally among all heirs. Families observed versions of partible inheritance in much of England, in slightly different forms in western and northern France, and in much of Germany.

In some partible inheritance regions, the commitment to absolute equality was extraordinary. In western France, for example, customary law set very egalitarian standards for inheritance. Daughters as well as sons had equal claims on non-noble estates, and the principle of forced return to the succession required that all property received previously be declared so that the final portion each claimant received would reflect long-term equality. Heirs met to report what each had already received (whether as dowries, loans, or gifts of other kinds), and then the lots were made. Again elaborate safeguards ensured that equality among heirs was preserved. Heirs selected lots according to their sex, with all sons choosing first, and their age, with the oldest having the first pick. This pattern might seem to give the oldest son considerable advantage over the youngest daughter. Yet the person who would choose last was charged with making up the lots from which the heirs would chose, an ingenious device that gave him or her clear incentive to make the lots as fair as possible.

In other partible inheritance regions, the division of property followed what might be termed a different-but-equal pattern. In England, for instance, where many peasant families practiced partible inheritance, an alternative strategy was usually pursued. One heir alone would receive the land, while the others received movable wealth of various kinds.

The consequences of partible inheritance were complex. While it ensured equality among heirs, the constant fragmentation of property endangered the financial viability of family members. Social historians studying some areas, such as rural societies in early modern Germany, have argued that partible inheritance caused intense competitiveness and tension between siblings, whereas elsewhere, as in early modern French towns, it seems to have fostered cooperation. In England, partible inheritance that limited land transfers to one child left the other heirs detached and free to pursue their lives elsewhere, in towns, cities, or even the emerging colonies. In many parts of France, where land was assigned to all children, peasant ties to the countryside remained strong, contributing to greater reluctance to emigrate. Many commentators and historians have linked these differences to subsequent differences in national histories, such as rates of economic development or successes with colonization.

Primogeniture. In other regions, families followed versions of inheritance systems based on the principle of primogeniture, in which one child (usually the eldest son) inherited the bulk of the landed property. Primogeniture was especially widespread among elites throughout European society and in areas where Roman law prevailed, such as southern France and Spain. In Roman law regions, testators had broad discretion to distribute property as they saw fit, which allowed them to concentrate their estate in the hands of a single heir.

Yet even in areas where a basic commitment to primogeniture prevailed, in practice most families showed a clear desire to provide for all of their children. Daughters received dowries and younger sons were helped with education or apprenticeships to provide them with means to make a living otherwise than off the family land. In such areas, families may not have been committed to equality, but they seemed to have pursued equity in their efforts to ensure to the best of their abilities that each child received help. Thus for most nonelite families the legal differences between partible inheritance and primogeniture were not as important in practice as they might seem. Families used inheritance along with many other aspects of their lives to weave strategies that met the varying needs of parents, children, and siblings.

In the late nineteenth century, a French sociologist, Frédéric Le Play, hypothesized that the practice of primogeniture led to the formation of a particular kind of household structure that he called "the stem family." Le Play suggested that in families where only one son would inherit the land, the heir's marriage usually coincided with the retirement of his parents. Subsequently, the two married couples shared a multigenerational household while other noninheriting siblings either went off to establish their own households or remained unmarried in the household of their brother and parents. Since the 1960s, social historians have endeavored to investigate the validity of this thesis, and many doubts have been raised. Historians now think household structures were less stable than this model suggests. Multigenerational households were common experiences at some point for many people, but their formation represented particular moments in the life courses of families. Moreover, families adopted multigenerational households for many reasons besides inheritance systems, including caring for the elderly, redistributing labor needs, and providing child care.

Dowries and inheritance. Marriages were also important moments of property transmission, primarily through families' provision of dowries to their daughters as they married. Realistically, a girl could not marry without a dowry in this period. If a young woman's parents could not afford to provide her with one, she needed to work to save her own dowry. Many female servants, for example, used the payment they received at the end of their term of employment to fund dowries for themselves. (Early modern servants were usually paid only when they left their employers' households, not on a regular weekly or monthly basis.) Most nonelite families seem to have taken their dotal obligations to their daughters very seriously, however, and provided contributions to dowries if they could.

For newly wed couples, dowries were critical elements in establishing the long-term prospects for married life. Young men could use the cash injection dowries represented as an important means of promoting their careers, allowing them to fund their occupations by buying tools or positions. Even the poorest women's dowries also usually included the essential goods to set up a new household—items like a bed, some pots and pans, a stack of linens, and some clothes.

The effect of giving dowries on women's right to inherit varied considerably. In some areas, families considered that their obligations to their daughters had been met by the provision of dowries, and such "dowered off" daughters gave up any further claim on family property. In these cases, dowries were probably a means by which daughters' claims on family property were limited, and consequently girls received smaller shares than their brothers. Elsewhere, though, daughters who had been dowered were still entitled to participate in the division of property after their parents' deaths.

Legally, in most countries, husbands became the managers, if not the outright owners, of their new wives' dowries (and of any other property their wives subsequently inherited). Nevertheless, women's ability to have access to their own property varied from place to place. In England, a married woman's legal identity was consumed by her husband's as she became a feme covert. In the countries of continental Europe, women's dowries were often divided into lineage property and community property, even in nonelite families. Husbands managed both, but could only inherit community property: women's children became the heirs to lineage property, and if wives died childless, husbands had to return lineage property to their in-laws. Everywhere courts were careful to protect women's dowries from the threats posed by husbands who might squander the money.

Noble inheritance patterns. For elite families across Europe, the situation was different. Even in regions where partible inheritance prevailed for commoners, laws permitted noble families to practice primogeniture in some form. Such families often pursued versions of primogeniture most ruthlessly because they felt that their status, with all the privileges it carried, would be jeopardized by dividing their patrimony—their family property in all its forms—among many heirs. The most striking practitioners of primogeniture were of course the European monarchical dynasties, who bequeathed not only personal property but kingdoms, and the contrast between the fortunes of the child who would inherit and other siblings was very dramatic in terms of political power as well as material comfort. The eldest son was the preferred heir in all royal families, and all sons were preferred to all daughters. In France, the early modern observation of Salic law meant in fact that women could not inherit the throne.

Other aristocratic families adopted a number of strategies to circumvent the perils of multiple heirs. The proportion of never married sons and daughters in elite families was high, a tactic that reduced the number of heirs and limited the financial burdens that dowries presented. In many Catholic countries one or more children were encouraged to take religious orders, a step that invalidated their right to inherit. As a result of choices like these, in eighteenth-century Venice, for example, 64 percent of the sons of elite families never married, while in France at the same time 42 percent of the sisters of dukes and peers likewise remained single. The numbers were similarly high throughout European aristocracies.

From the sixteenth century onward elite families also increasingly developed new legal means to avoid the division of their estates. These practices (known as mayorazgos in Spain, fideicommissa in Italy, substitutions in France, and strict settlements or entails in Britain) all sought to preserve intact the landed estate that usually provided the core of both the wealth and the status of great families by forbidding the division of land and putting it beyond the claims of creditors.

All such efforts to preserve the wealth of aristocratic families intact privileged the status of the family as a lineage or dynasty over the interests of individual heirs. Even the son who inherited such an estate was effectively a tenant for life rather than an owner, as his ability to manage the property was severely restricted. He could not, for example, sell the estate, and his ability to mortgage it to raise money was also strictly limited. Younger sons often found themselves with few prospects and needed to seek their fortunes in the army or church. Some historians argue that daughters may have fared better, at least if they were permitted to marry. Although dowry inflation was rampant across Europe in the seventeenth and eighteenth centuries, with the result that the high costs of dowries threatened to ruin many families, the alliances daughters made were essential for the building of kinship and political networks. A daughter who was married with a dowry sufficient to attract a suitor of appropriate status could expect to live a life that was more on a par in material terms, at least, with that of her inheriting oldest brother than her younger brothers could.

Widows. Women who were widowed were also a distinctive group in terms of inheritance. In most countries, widows were entitled to a specific share of the property they and their husbands had accumulated. That share was determined either by law or by specific arrangement at the time of the couple's marriage. It was often around a third, although it could be much less. Although this provision was meant to give widows financial means to support themselves, the splitting of household property between widows and all other heirs left many widows in dire financial straits, as tax rolls from communities across Europe show.

Wills offered one last means for testators to shape the disposal of their property. Historians have shown that besides the practices already described that took care of the majority of property, men and women also used small gifts of personal property, like jewelry, clothes, linen, or books, as a way of expressing particular ties. Women especially seem to have been likely to remember their sisters or nieces, emphasizing a continued sense of the importance of extended kinship beyond the conjugal families they had established with their husbands.

MODERN EUROPE SINCE 1789

The two hundred years or so since the late eighteenth century have seen many dramatic shifts in European society that have had enormous impacts, both directly and indirectly, on the practice and significance of inheritance. These changes have included not only obvious developments like the passage of new laws that explicitly impacted how inheritance worked but also the emergence of new economic and political patterns that have transformed the role of inheritance in most communities.

The era of the French Revolution marked a watershed in many ways, as political changes in many countries were accompanied by shifts toward industrialization, albeit at different rates and by different means in different countries. In France itself, many revolutionary leaders were quick to insist that all children should have equal inheritance rights. The commitment to equal shares for all heirs became a cornerstone of revolutionary legislation, and was enshrined with little alteration in the Napoleonic Civil Code of 1804. For those seeking to reform France, equality of inheritance had political as well as familial goals because they foresaw newly egalitarian families as the building blocks of a democratic nation. This Civil Code, with its emphasis on equal shares for all heirs, became enormously influential in other continental countries in the nineteenth century, providing a model for legal reform. Even regions where its introduction was not the result of Napoleon's conquests chose to adopt the Code, as in the case of the Netherlands, where it became law in 1838.

As Western industrial societies evolved, four key developments dramatically reduced the traditional importance of inheritance in determining individual wealth and life course for all but the richest of families. For most young people, education rather than inheritance became the key to their life fortunes. With the rise of wage earning, children became more independent of their parents. The emergence of a variety of social security systems as elements of welfare state programs also reduced individual dependency on inheritance. The most important of these perhaps has been the establishment of national pension schemes that provide income during old age. Finally, many governments introduced new fiscal regimes, of which estate taxes were the most significant for the role of inheritance.

Nevertheless, the property families transmit from generation to generation still has important material and symbolic roles. Although laws about inheritance still vary from country to country, with France, for example, insisting that all children inherit equally and Britain giving parents the right to divide their property as they wish, in practice equity among all inheriting children, both sons and daughters, has become the rule in all but the most elite families.

Since World War II, as life expectancy has increased, parents have increasingly chosen to pass property to their children at many different stages rather than at the traditional pivotal moments of marriage and death. Parents have underwritten the costs of education as that has become an increasingly important determinant of success for large parts of populations, and offered financial help at many other moments, such as the purchase of houses or the birth of children.

The situation of widows in particular has changed dramatically in many regions since World War II. Increasingly, in countries like England that give wide latitude over the disposal of property, when one spouse dies, the surviving partner, man or woman, inherits the entire estate, leaving the next generation to receive their ultimate share only after the death of both parents. Even in areas like the Netherlands, where inheritance laws are still heavily influenced by the provisions of the Napoleonic Code that were introduced in the nineteenth century, husbands have moved toward choosing to protect the rights of their surviving wives to have at least use of all the property. This pattern, which privileges the interests and competence of both spouses, marks a dramatic change from earlier practices, when widows' portions were assigned along with those of other heirs on the death of the father.

In the countries of Eastern Europe, meanwhile, the post–World War II transition to socialism revolutionized inheritance along with many other aspects of family life. The abolition of private ownership of the means of production ended the role of inheritance as a means by which wealth was transmitted from one generation to the next.

In sharp contrast, elite families have continued to incline toward preserving their estates intact. The legal strategy of entail survived most powerfully in England, where the nobility, like other testators, enjoyed few restrictions on the transmission of their property. For continental nobilities, however, the egalitarian intentions of the French Revolution and Napoleonic Code produced powerful impetuses toward division. Since the early nineteenth century, elites in France and Spain have had only very limited abilities to keep their estates intact. Even in countries like Prussia that at first shared with Britain the survival of the right to entail, noble estates were being divided by the later nineteenth century.

Even in the most aristocratic circles, where primogeniture seems to prevail most stoutly in the transmission of titles and estates, changes in progress suggest that the dramatic transformation of inheritance practices of the last two centuries are reaching the highest levels. In 1999, the British government was preparing a constitutional revolution that designated birth order alone, regardless of sex, as the key to inheritance of the British monarchy. Should a future monarch see a daughter as the firstborn, she will become queen ahead of any subsequently born brothers.

See alsoGender History; Patriarchy (in this volume); and other articles in this section.

BIBLIOGRAPHY

Burguière, André, Christiane Klapisch-Zuber, Martine Segalen, and Françoise Zonabend, eds. A History of the Family. Volume 2: The Impact of Modernity. Translated by Sarah Hanburg-Tenison, Rosemary Morris, and Andrew Wilson. Cambridge, Mass., 1996.

Chojnacki, Stanley. "Dowries and Kinsmen in Early Renaissance Venice." Journal of Interdisciplinary History 5 (spring 1975): 571–600.

Desan, Suzanne. "'War between Brothers and Sisters': Inheritance Law and Gender Politics in Revolutionary France." French Historical Studies 20 (fall 1997): 597–634.

Goody, Jack, Joan Thirsk, and E. P. Thompson, eds. Family and Inheritance: Rural Society in Western Europe, 1200–1800. Cambridge, U.K., 1976.

Gullestad, Marianne, and Martine Segalen, eds. Family and Kinship in Europe. London, 1997.

Kertzer, David I., and Richard P. Saller, eds. The Family in Italy from Antiquity to the Present. New Haven, Conn., 1991.

Klapisch-Zuber, Christiane. Women, Family, and Ritual in Renaissance Italy. Chicago, 1985.

Medick, Hans, and David Warren Sabean, eds. Interest and Emotion: Essays on the Study of Family and Kinship. Cambridge, U.K., University Press, 1984.

Shammas, Carole, Marylynn Salmon, and Michel Dahlin. Inheritance in America: From Colonial Times to the Present. New Brunswick, N.J., 1987.

Spring, David, ed. European Landed Elites in the Nineteenth Century. Baltimore, 1977.

Inheritance

Definition

Inheritance (generally) and heredity (scientifically) both refer to the transmission of genes from parent to offspring, along with the physical, behavioral, and biochemical traits/characteristics they produce.

Description

Throughout history, people have puzzled over heredity. While it is obvious that physical traits and behavioral traits are passed from parent to offspring, both in plants and animals, the puzzling aspects of heredity are the exceptions and quirks in what should be a very basic process. For instance, apparently hereditary characteristics sometimes skip a generation or two, disappear altogether, or appear in an individual or generation for the first time, seemingly from nowhere. A further mystery, the finer points of which are still being unraveled, lay in the mechanism of exactly how traits get from parent to offspring. Even more perplexing to those in the past was the occurrence/recurrence of congenital anomalies and disorders, resulting in physical and/or behavioral disability.

Answers to many of the questions about heredity were deduced in the mid-nineteenth century by a scientist, Gregor Mendel, at a monastery in what is now Czechoslovakia. The deductions he made from the results of his experiments came to be known as Mendel's laws of heredity, since shortened to just Mendelian inheritance. These are the patterns of inheritance, dominant and recessive, with which most people are familiar. However, geneticists have learned in recent years that even Mendel's laws are not immutable, and that other non-Mendelian types of inheritance also exist.

A common misconception is that genetic is synonymous with hereditary, and that the terms can be used interchangeably. In fact, while something that is hereditary is always genetic, something that is genetic is not necessarily hereditary. For example, all cancer at the most basic level is genetic, caused by errors in the genetic control of cell division and proliferation. However, only a small proportion of individuals with cancer inherited the causative gene mutation from a parent. In most cases, an external agent (carcinogen) induces a genetic mutation in a cell somewhere in the body (e.g., tobacco smoke in a lung cell, or ultraviolet radiation in a skin cell), an error in DNA replication results in a mutation, or both. Gene mutations that occur anywhere in the body other than sperms, eggs, or their precursor cells (germline) are called somatic mutations, and are not hereditary. As noted, a small percentage of cases of any particular type of cancer (usually about 5–10% for the most common types) exhibit a hereditary pattern. Most often, it is a predisposition to developing cancer that is inherited, placing someone at increased risk for cancer. In fact, most common diseases, such as diabetes , hypertension, heart disease, etc., are thought to follow the same general pattern as cancer, with a small percentage of cases due to heredity, most due to environmental effects acting on normal, but susceptible, variants of genes, and the remaining proportion caused by purely environmental or purely somatic genetic events.

Classifying the different types of heredity can be done in various ways. However, most people are only familiar with traditional types of Mendelian inheritance, and know little or nothing about other types of heredity. Accordingly, inheritance can be classified as either Mendelian or non-Mendelian, and then further divided and subdivided on that basis.

Mendelian inheritance

All somatic cells in humans (except mature red blood cells) normally contain 46 chromosomes, in 23 pairs. Sperms and eggs carry 23 chromosomes, one of each pair. The process of sperm development is spermatogenesis, egg development is oogenesis, and the general term for both is gametogenesis. The process that is the basis for Mendelian inheritance is meiosis, which takes place only during gametogenesis (chromosome duplication and cell division in somatic cells is mitosis). During meiosis, the 46 chromosomes in precursor cells in testes and ovaries duplicate to produce a total of 92. Two cell divisions then take place, reducing the number of chromosomes per cell to 23. During spermatogenesis, the process results in four sperm, but oogenesis produces only one egg (along with two nonfunctional polar bodies with 46 and 23 chromosomes each). All eggs normally carry a single X chromosome, whereas half of all sperm carry an X, and the other half carry a Y chromosome.

Humans have 24 different chromosomes, the first 22 numbered sequentially, with the twenty-third and twenty-fourth designated as X and Y. Chromosomes 1 through 22 are called autosomes. The X and Y are the sex chromosomes, although only the Y chromosome has any effect in determining sex (gender). Although it is commonly believed that female gender is determined by an XX chromosome constitution, and likewise males are XY, this is misleading. Male and female genders are determined by specific genes and hormonal influences. Certain genetic conditions result in males that are 46,XX, and others in females that are 46,XY. Therefore, it is appropriate and technically more accurate to state that females and males typically or usually have 46,XX and 46,XY chromosome constitutions, respectively.

Mendelian inheritance is either autosomal or sex linked, and dominant or recessive. Given the small number of genes on the Y chromosome and their relative unimportance in producing genetic disease, for all practical purposes, sex-linked inheritance is equivalent to X-linked inheritance. Sex-linked inheritance should also not be confused with sex-influenced inheritance, which involves autosomal inheritance with different phenotypic results in males and females due to hormonal differences. For the purposes of the criteria and terminology that follow, genetic disorders will be assumed, rather than normal characteristics (e.g., blue eyes, brown eyes, etc.) that might follow dominant or recessive inheritance patterns.

Genetic conditions that display Mendelian inheritance are often referred to as single-gene disorders. However, this is somewhat of a misnomer, since these conditions nearly always involve two genes, one on each chromosome of a particular pair. To clarify, the word gene is typically used in a broad context, and includes all the variations of that gene, known as alleles. For instance, there may be a single gene for eye color, with different alleles for brown, blue, green, etc. Some changes in genes result in alleles with no functional difference, or in neutral variations, such as the eye color example. Other genetic changes (mutations) result in alleles associated with disease. If an individual has two identical alleles of a gene, they are considered homozygous; if the alleles are different, that person is said to be heterozygous. Hemizygous refers to the presence of only one allele of a gene, instead of the expected two. Males are normally hemizygous for all the genes on the X and Y chromosomes, since they have only one copy of each. Hemizygosity for an autosomal gene may cause disease, and the symptoms may be different based on whether it is the maternal or paternal allele (gene) that is missing.

Autosomal dominant

The hallmarks of autosomal dominant inheritance include:

• A disorder that is caused by an anomaly in an autosomal gene, requiring only one disease-causing allele to produce symptoms (i.e., heterozygotes affected).
• The condition affects and can be transmitted by both sexes equally.
• A carrier of the gene, whether affected or not, has a 50% chance of transmitting it to each child.
• A later age of onset, with milder symptoms and greater variability, as compared to recessive disorders (on average).
• Sporadic (isolated) cases are not uncommon, and are usually the result of new mutations (no previous family history).

Some conditions (e.g., Huntington disease ) display what has been referred to as true dominance, which means that individuals with one-disease allele (heterozygotes) exhibit the same signs and symptoms as individuals with two-disease alleles (homozygotes). In other conditions, the effects of the gene are additive. For example, it is not unusual for two heterozygous individuals with achondroplasia (a common dwarfing condition) to meet and have children. With each conception, there is a 25% chance the child will receive a normal gene from each parent (homozygous unaffected), a 50% chance of receiving one normal gene and one achondroplasia gene (heterozygous, affected-like parents), and a 25% chance of receiving the achondroplasia gene from each parent, which results in severe limb shortening and other skeletal problems that result in death before or shortly after birth. It could be argued that this type of situation more closely resembles autosomal recessive inheritance, with heterozygotes simply showing more pronounced symptoms than most other recessive disorders.

Other important issues complicating autosomal dominant inheritance include reduced penetrance, variable expression, and possible gonadal mosaicism in the parent of an isolated case. If each person who carries the gene for a particular disorder exhibits symptoms, the gene is said to have 100% penetrance. Therefore, reduced penetrance means that some proportion less than 100% of heterozygotes will develop detectable signs of the condition. For some disorders that have been well studied, penetrance figures at specific ages have been calculated (e.g., a disorder is 50% penetrant at age 30, 70% penetrant at age 50, and so on). Variable expression simply means that two individuals with the same disease allele, even within the same family, may show markedly different ages of onset and/or severity of symptoms.

Autosomal recessive

The hallmarks of autosomal recessive inheritance include:

• The genetic disorder is fully expressed only in individuals homozygous for the disease-causing gene.
• The disorder is usually found only in siblings, with males and females at equal risk.
• When both parents are carriers (unaffected heterozygotes), the risk in each pregnancy of having an affected child is 25%.
• An unaffected sibling of an affected individual has a 66% (two-thirds) chance of being a carrier.
• The incidence of consanguinity in general is increased in autosomal recessive disorders, with a higher likelihood the more rare the condition. Conversely, consanguinity noted in the parents of a child with an unidentified disorder suggests autosomal recessive inheritance as a possible cause.

Many sporadic cases of recessive disorders are noted, because modern families tend to be small and geographically dispersed. Unfortunately, in some cases, the only way that autosomal recessive inheritance is proved is when a second or third affected child is born.

If an affected person has children with a carrier of the same disorder, each child has a 50% risk of being affected, and a 50% risk of being a carrier. If two individuals are affected by the same genetic disorder and have children (rare, but more likely for recessive disorders involving deafness, blindness, or other symptoms that tend to bring people together), all of their children will be affected.

X-linked recessive

The hallmarks of X-linked recessive inheritance include:

• The genetic disorder in which females are carriers, and usually only males are affected.
• There is no male-to-male transmission, since males transmit an X chromosome only to daughters.
• All sons of an affected male will be unaffected, but all daughters will be carriers.
• Typically, carrier females have a 25% chance in each pregnancy of having an affected child (50% chance of transmitting the X chromosome with the disease gene, but only half of those will be passed to boys).

Unlike carriers of autosomal recessive disorders, who, at most, usually show only clinically insignificant biochemical or physical changes, female carriers of X-linked recessive disorders often show mild to moderate effects of the disorder. In rare cases, they may even be as severely affected as their affected male relatives. While it is true that females normally have two X chromosomes and males have only one, it is also true that most of one of the X chromosomes in each cell are randomly inactivated in females shortly after conception. If, by chance, most cells in the body have an active X chromosome that carries the disease gene, a female carrier can show marked symptoms of the disorder. If the reverse is true, she will likely appear completely unaffected.

Women are considered obligate carriers if they have more than one affected son, or an affected male relative and a proven carrier daughter, or an affected son and an affected brother or maternal uncle. Women are at risk for being carriers if they have one affected son, or one affected brother, or one affected maternal uncle, or a sister with an affected son (since any of them may have a new mutation).

An isolated case of a diagnosed X-linked disorder may be the result of a new mutation in the affected individual or in the mother, or may be the result of carrier status transmitted to the mother by her mother. Clarification of this point can make the difference between 50% (mother is a carrier) and negligible (new mutation) in the recurrence risk for the next male pregnancy, and in the risk for daughters or sisters to be carriers. Unfortunately, carrier status of the mother may be very difficult to determine unless reliable carrier testing is available. Molecular genetic testing is making this possible for an increasing number of diseases (e.g., Duchenne muscular dystrophy and fragile X syndrome ). An isolated case of an X-linked disorder may also be the result of germline mosaicism in the mother in which some of her eggs carry the mutation and others do not.

X-linked dominant

X-linked dominant disorders are rare. Females are usually affected more mildly than males. However, since both males and females can show symptoms, the inheritance pattern may resemble autosomal dominant inheritance, with the critical difference being no male-to-male transmission in X-linked inheritance. A few X-linked dominant disorders are lethal in males (e.g., incontinentia pigmenti ).

Non-Mendelian inheritance

Chromosomal heredity

For purposes of broad classification, chromosome anomaly syndromes are considered either numerical or structural. As the term implies, numerical chromosome anomalies involve a change in the total number of chromosomes in each cell, most often presenting as a trisomy, such as the most common type of Down syndrome (trisomy 21). Numerical chromosomal syndromes are not considered to be hereditary. Structural chromosome anomalies can take various forms, but most often involve a translocation of some type, either an exchange of chromosomal material between two chromosomes (reciprocal translocation), or two chromosomes attached to each other to form a single chromosome (Robertsonian translocation). Either type of translocation can be balanced (no extra or missing chromosomal material, just rearranged) or unbalanced (missing and/or extra chromosomal material). Individuals who carry a balanced translocation have no ill health effects from it, but when they produce sperms or eggs, the translocation can be passed on in an unbalanced form, which can produce a syndrome of some type in a child, or very often results in repeated pregnancy loss. There is also an equally likely chance that a sperm or egg will receive the translocation in the balanced state as the parent carries it, or receive a normal chromosome complement. Other types of structural chromosome anomalies include ring chromosomes, and different types of inversions of material within a single chromosome, each of which can be hereditary, and present reproductive risks.

Mitochondrial inheritance

Mitochondria are tiny structures (organelles) in the cytoplasm of cells that are the primary site of energy production. They are also the only location outside of the nucleus that contains DNA. The DNA exists in a ring structure, with about 2–10 rings per mitochondrion. Additionally, depending on cell type, there may be anywhere from several dozen to more than a hundred

mitochondria per cell. Approximately 70 mitochondrial genes have been identified, many of which are associated with specific genetic disorders.

Mitochondrial inheritance is unusual in that, with rare exceptions, a person inherits all their mitochondria through the egg from their mother. Again, with few exceptions, a typical mitochondrial inheritance pattern involves an affected female who transmits the condition to all of her children, but none of her affected sons will pass on the disorder. However, this straightforward pattern is usually complicated by the fact that each mitochondrion may be mosaic for the gene mutation (e.g., five rings with the mutation and five rings with normal DNA), any particular cell is likely to be mosaic for mitochondria that are themselves mosaic, and this complicated mosaic pattern can apply to any egg that results in conception. All of which makes it nearly impossible to predict specific recurrence risks or the degree of severity if a child is affected.

Uniparental disomy

Uniparental disomy (UPD) refers to an unusual genetic status in an individual in which one parent is the source of both chromosomes of a pair. Again, before it was possible to analyze chromosomes at the microscopic (DNA) level, the logical assumption was that one chromosome of each pair is always maternal in origin, and the other one paternal in origin. Through various means, this supposed rule of heredity was found to have exceptions.

There are two possible mechanisms for the occurrence of UPD. The first involves a conception in which the embryo is trisomic for a particular chromosome and, at some early stage one of the extra chromosomes is "lost" during mitotic cell division. The result is a normal, diploid cell, and every cell produced from it from that point on will also be diploid. The remaining trisomic cell(s) may produce only a small percentage of the total cells in the body (mosaicism), or they may die off completely. In any case, if the disomic (diploid) cells contain the two chromosomes contributed by either the sperm or the egg, the result is UPD. This process is sometimes referred to as trisomy rescue. The other, less likely possibility is that one gamete at conception carries an extra chromosome (24 total, which is common), but the other gamete is coincidentally missing that same chromosome. If UPD involves two identical chromosomes (the first stage of meiosis produces two pairs of identical chromosomes at each position/number), the situation is further distinguished as uniparental isodisomy. On the other hand, if the chromosome constitution at that position is the same as the parent's, it is termed uniparental heterodisomy. Uniparental isodisomy presents a much greater risk of transmitting an autosomal recessive disorder than does the alternative.

Gonadal mosaicism

Just as individuals may be mosaic for chromosome anomalies, such as mosaic Down syndrome, so also can mosaicism for single-gene anomalies exist. Mosaicism can have both medical (the individual's health) and reproductive (the individual's children's health) implications. Medical significance is determined by the degree of somatic mosaicism, while reproductive risks depend on the presence or absence of gonadal (germline) mosaicism. A person may have either type of mosaicism, but most cases probably involve both. While the presence of mosaicism in a particular tissue can be proved, the actual level (percentage of abnormal cells) can never be determined, since doing so would require genetic testing of every cell. Likewise, without testing every cell, the absence of mosaicism cannot be confirmed.

Epigenetic effects (imprinting)

Epigenetics is the study of heritable changes in gene expression that occur without a change in DNA sequence. Imprinting , the selective deactivation of certain genes in sperm, and others in eggs, is the best-known and most dramatic epigenetic effect. There is also mounting evidence that certain maternal biochemical or physical influences on the embryo/fetus may alter the function of some genes. As a general rule, imprinting is removed and then reapplied during gametogenesis. It remains to be seen whether other types of epigenetic effects are similarly reversible. However, the discovery of new exceptions to old rules continues, with no evidence of slowing down.

Resources

BOOKS

Connor, Michael, and Malcolm Ferguson-Smith. Essential Medical Genetics, 5th edition. Oxford: Blackwell Science, Ltd., 1997.

Harper, Peter S. Practical Genetic Counselling, 5th edition. Oxford: Butterworth-Heinemann, 1998.

ORGANIZATIONS

Alliance of Genetic Support Groups. 4301 Connecticut Ave. NW, Suite 404, Washington, DC 20008. (202) 966-5557.

March of Dimes Birth Defects Foundation. 1275 Mamaroneck Ave., White Plains, NY 10605. (888) 663-4637. (April 20, 2005.) <http://www.marchofdimes.com/>.

National Society of Genetic Counselors. 233 Canterbury Dr., Wallingford, PA 19086-6617. (610) 872-1192. (April 20, 2005.) <http://www.nsgc.org/>.

WEBSITES

National Library of Medicine: Genetics Home Reference. (April 20, 2005.) <http://ghr.nlm.nih.gov>.

Scott J. Polzin, MS

Inheritance

Definition

Inheritance refers to the transmission of genetic information across generations. There are two types of inheritance patterns in humans: Mendelian nuclear inheritance and non-Mendelian mitochondrial inheritance. The 23 pairs of human chromosomes located in the nucleus of the cells make up the human nuclear genome. This genome contains an estimated 30 to 40 thousand genes that we inherit in combination from our parents. These genes are called Mendelian-inherited nuclear genes, after Gregor Mendel, the Austrian monk who first established the laws of inheritance in the late 1800s. There is also DNA , called mitochondrial DNA, or the mitochondrial human genome, in the cytoplasm that we inherit almost exclusively from our mothers. These mitochondrial genes are called non-Mendelian-inherited mitochondrial genes.

Mendelian inheritance

Mendelian type inheritance is the more familiar form of genetic inheritance. During reproduction, genetic material is passed from the mother and the father to the offspring. These genes are inherited according to the laws of segregation established by Gregor Mendel, and are called Mendelian-inherited nuclear genes.

A chromosomally normal human carries 23 pairs of chromosomes in the nucleus of each cell: 22 pairs of autosomes and one pair of sex chromosomes. An individual inherits one of each paired chromosome from each parent. Each of these chromosomes is made up of thousands of genes. Genes are the chemical sequences which together control all characteristics and functions of the body. A particular characteristic controlled by a single gene is called a trait.

Almost all genes are located on each of the two copies of the paired chromosomes. The two copies of these genes, taken together, are called an allele. If the two copies of this gene are identical to each other, this person is said to have a homozygous allele for that gene. If the two copies of this gene are not the same, this person is said to have a heterozygous allele for that gene.

The only genes that are not located on two copies of paired chromosomes occur when there is not a matching pair of chromosomes, such as those genes on the single X chromosome in an XY male. When only one chromosome carries a gene, this gene is called a hemizygous allele. A hemizygous allele is made up of only the one copy that is present.

There are three modes of Mendelian inheritance: dominant, semi-dominant, and recessive. Additionally, a trait may be sex-linked, or non-sex-linked (autosomal). A sex-linked trait is conferred from parents to their child on the X or Y chromosome. An autosomal trait is transmitted from parents to their child on one of the other 22 pairs of chromosomes (the autosomes).

Recent advances in molecular genetics have tended to blur the line between dominant and semi-dominant inheritance. It is now believed that semi-dominant inheritance is almost always observed in traits once felt to be strictly dominant traits. These research findings are in direct opposition to current clinical practice. Genetic counselors and other health care professionals prefer not to confuse their patients by referring to semi-dominant inheritance of a particular trait. Therefore, in a research setting, one is unlikely to discuss true dominance of a trait, while in a clinical setting, one is unlikely to encounter the usage of semi-dominance.

Autosomal Mendelian inheritance

AUTOSOMAL DOMINANT In autosomal dominant inheritance, only one copy of the gene that causes a specific trait must be present in order for the person to display (express) the trait. The gene is said to dominate the expression of the trait because its effects outweigh that of the corresponding gene on the other half of the chromosome pair. Thus, in the case of a genetic mutation, one parent may pass the mutation to his or her offspring. Homozygous and heterozygous individuals will be affected equally by the mutation and both will express identical forms of the trait. The second copy of the mutated gene in the homozygous individual does not affect them more severely than the single copy of the mutated gene affects the heterozygous individual.

By definition, parents who pass on an autosomal dominant mutation to their offspring express the characteristics of that mutation. These parents are not called carriers because they are already fully affected with the trait. In the case of one heterozygous affected parent, the probability that a child will inherit this trait is 50%. In the case of two heterozygous affected parents, the probability that a child will inherit this trait is 75%. In the case of one homozygous affected parent, regardless of whether or not the other parent is affected, the probability that a child will inherit this trait is 100%.

AUTOSOMAL SEMI-DOMINANT If a particular trait is an autosomal semi-dominant trait, homozygous and heterozygous individuals will both experience characteristics of the trait. The gene for this trait still dominates the expression of the trait, but the effect of the corresponding gene on the other chromosome is noticeable. In diseases caused by a genetic mutation, the homozygous individual will experience more severe characteristics of that disease than the heterozygous individual because of the extra copy of the mutated gene that the homozygous individual possesses. Heterozygous individuals are carriers of the trait. Because these heterozygous individuals will exhibit some symptoms of the trait, they are also called symptomatic carriers.

In the case of one carrier parent and one non-carrier parent, the probability that a child of these parents will be a carrier of the trait is 50%, but their child cannot be homozygous for the trait. In the case of a homozygous affected parent and a non-carrier parent, the probability of a child being homozygous for the trait is also zero. The probability that this child will be a carrier of the trait is, however, 100%. In the case of two carrier parents, the probability that a child will be homozygous for the trait is 25%. The probability that this child will be a symptomatic carrier is 50%. In the case of one carrier parent and one affected parent, the probability that a child will be affected is 50%. The probability that this child will be a symptomatic carrier is also 50%. In the case of two affected parents, the probability that a child will be affected is 100%.

AUTOSOMAL RECESSIVE If a particular trait is an autosomal recessive trait, two copies of the mutated gene that causes this trait must be present in order for the person to possess the trait. The effect of the recessive gene is less than that of the corresponding gene on the other half of the chromosome pair. Therefore, only homozygous individuals will be affected with the trait. Heterozygous individuals will not exhibit characteristics of the trait. These heterozygous individuals are called carriers because they carry the trait and can pass it on to their children. Because these heterozygous individuals do not show characteristics of the trait that they carry, they are also called asymptomatic carriers.

A child cannot exhibit the symptoms of a recessive trait unless her or his parents are either both carriers of the trait or one is a carrier of the trait and the other is affected with the trait. In the case of one carrier parent and one non-carrier parent, the probability of a child being affected with the trait is zero. However, the probability that a child of these parents will be a carrier of the trait is 50%. In the case of an affected parent and a noncarrier parent, the probability of a child being affected with the trait is also zero. The probability that this child will be a carrier of the trait is, however, 100%. In the case of two carrier parents, the probability that a child will be affected with the trait is 25%. The probability that this child will be an asymptomatic carrier is 50%. In the case of one carrier parent and one affected parent, the probability that a child will be affected is 50%. The probability that this child will be an asymptomatic carrier is also 50%. In the case of two affected parents, the probability that a child will be affected is 100%. The probability that an autosomal recessive trait will be passed to the child of consanguineous parents is much higher than it is in non-consanguineous parents.

Sex-linked Mendelian inheritance

Sex-linked traits are carried on the X and Y, or sex, chromosomes. Sex-linked traits may be linked to either the X or the Y chromosome and may also be either dominant, semi-dominant, or recessive. Many more X-linked traits have been identified than Y-linked traits.

The sex chromosomes control the biological sex of an individual. Individuals with XY chromosomes are male, and individuals with XX chromosomes are female. The chromosome inherited from the mother is always the X chromosome, while the chromosome carried by the father's sperm may be either an X or Y chromosome.

X-LINKED DOMINANT Chromosomally normal females possess two X chromosomes; therefore, they can be homozygous or heterozygous in a trait that is caused by a gene mutation on the X chromosome. In the case of X-linked dominant traits, only one copy of the mutant gene must be present for the trait to be fully expressed. A female child affected with an X-linked trait may inherit this trait from either her mother or her father. In cases of an affected heterozygous mother and an unaffected father, the probability that a female child will be affected with an X-linked dominant trait is 50%. In cases of an affected homozygous mother, the probability that a female child will be affected is 100%, regardless of whether or not the father is affected. In cases of an affected father, the probability that a female child will be affected is 100%. This is because the father is hemizygous for the mutant allele. His only copy is affected and he must pass that copy on to his daughters.

A chromosomally normal male child must receive his only X chromosome from his mother. He gets his Y chromosome from his father. Therefore, in cases of X-linked dominant traits, a male child has a 50% chance that he will receive the mutant gene from his heterozygous affected mother. If his mother is homozygous, this male child has a 100% likelihood of being affected with the trait. Therefore, while X-linked dominant traits are passed on equally from mothers to daughters and from mothers to sons, females may also inherit X-linked dominant traits from their fathers.

In some instances of dominant X-linked inheritance, the lack of the presence of a copy of the normal gene causes embryonic, fetal, or neonatal death. Therefore, in these cases, only very few affected males are born alive, and those that are generally die within a few hours of birth. This inheritance pattern is also known as male-lethal X-linked dominant inheritance. Since there are no affected males to contribute to the inheritance patterns of these traits, inheritance from father to daughter is not possible. Likewise, homozygous females are not possible. Only heterozygous females survive. In this form of inheritance, all affected males will inherit this trait from their heterozygous mothers. These males will either become miscarriages, they will be stillborn, or they will die shortly after birth. Heterozygous females can inherit male-lethal X-linked dominant traits from their heterozygous mothers. Therefore, the inheritance of these traits has an overall 50% probability of occurrence.

X-LINKED RECESSIVE In cases of X-linked recessive traits, female children can only be affected if their mothers are carriers and their fathers are affected with the trait. The inheritance patterns in females of X-linked recessive traits are identical to the inheritance patterns of autosomal recessive traits. However, because the odds of a carrier mother producing offspring with an affected father are extremely low, X-linked recessive traits are characterized by the general absence of affected females. Because males are hemizygous in all X-linked traits, they have a 50% probability of inheriting an X-linked recessive trait from their carrier mothers. In the rare instances of affected mothers, males have a 100% chance of inheritance. Fathers cannot pass any X-linked trait to their XY sons. When affected fathers produce female children, 100% of these girls will be carriers of this trait. Almost all cases of females affected by an X-linked recessive trait are the result of consanguineous parents.

X-LINKED SEMI-DOMINANT A few examples of X-linked semi-dominant traits exist. In these cases, the carrier females are generally affected with a milder form of the trait than the affected males. Occasionally, some females show mosaicism of their X chromosomes that causes an activation of one of the X chromosomes in preference to the other. In these cases, heterozygous females show characteristics of the trait caused by the mutant gene that are identical, or nearly identical, to those characteristics seen in hemizygous affected males. Examples of this type of X-inactivation are a form of hereditary mental retardation called fragile X syndrome , and both Duchenne type and Becker type muscular dystrophies.

Mitochondrial inheritance

A human being is conceived by the joining of the egg from the mother and the sperm from the father. Relative to the egg, the sperm is extremely small. It contains almost no cellular material outside the nucleus (cytoplasm) and very few mitochondria. In the cytoplasm of the egg, cellular components called mitochrondria are present. These mitochondria carry mitochondrial DNA, which is circular and contains 16,569 base pairs. Each mitochondrion contains between two and ten copies of this mitochondrial DNA. This separate genome codes for two ribosomal RNAs, 22 transfer RNAs, and 13 proteins that are used as enzymes in oxidative phosphorylation (cellular metabolism). Almost all the mitochondria in a person are derived from maternal mitochondria. Therefore, traits that result from mutations in mitochondrial DNA are exclusively inherited from the mother. These traits are not characterized by dominant, recessive, or semi-dominant patterns.

Most often, mitochondrial DNA is mosaic for a particular trait. That is to say, the trait exists on some, but not all, of the mitochondrial DNA in each cell. There can be as few as two or as many as ten copies of this mitochondrial DNA in a single cell. When cell division occurs, these mitochondrial DNA are randomly distributed into the newly formed mitochondria of the daughter cells. In most cases this mosaicism is such that only certain cells of the body contain the mutant DNA forms while other cells of the body are normal.

Human pedigree analysis

A pedigree analysis is the inspection of a family tree to look for the inheritance pattern of a trait associated with a mutant gene or a chromosomal aberration. Because the size of human families is usually quite small, it is often impossible to determine the inheritance pattern of a particular trait by performing a pedigree analysis on a single family. Other complications arise when analyzing human pedigrees. Among these are: anticipation; de novo mutations, improper identification of members of the pedigree; mosaicism; penetrance; variable expression; and recessive conditions appearing dominant, or pseudo-dominant.

Anticipation is the tendency of a trait to become more severely expressed in succeeding generations. This is called anticipation because the more severely affected child is discovered first, then other members of the pedigree are often "anticipated" as having to be affected with milder forms of that trait. While this anticipation was originally thought to be an error in backward identification of a trait in preceding generations caused by the identification of that trait in succeeding generations, it is now recognized as a true genetic characteristic. As an example, fragile X syndrome has been demonstrated to affect each succeeding generation more severely than the preceding generation within the same family.

De novo mutations, or mutations that were not inherited from either parent, can cloud the pedigree analysis within a family. The individual who is affected did not inherit these de novo mutations but he or she may pass them on to his or her children. In these cases, if the pedigree analysis does not span a significant number of generations after the de novo affected person, the true genetic inheritance pattern of this new trait may not be able to be identified. In such cases of a lack of succeeding generations, the cause can often be mislabeled as not of hereditary origin (sporadic).

Improper identification of members of the pedigree has to be avoided when performing a pedigree analysis. This occurs most often when the father of a particular child is misidentified.

Mosaicism often causes traits to appear to have a dominant inheritance pattern in some families while that same trait appears to be recessive in other families.

Penetrance is the term used to describe the probability that a person possessing a genetic mutation will express that mutation. A true dominant trait will have a penetrance of 100%. However, many traits that are termed dominant do not have complete penetrance. Therefore, some individuals with an otherwise dominant seeming trait may be asymptomatic for that trait. Penetrance is often also problematic in age-related traits. In these traits, a dominant inheritance pattern may be missed because members of the pedigree died of causes unrelated to the dominant trait prior to developing symptoms of the trait.

Variable expression is extremely common in dominant traits. In these cases, identical mutant alleles cause different characteristics of expression in different people. This may be a variance of symptoms from one affected family to another affected family or it may be a variance of symptoms from one individual to another within a single family.

Recessive traits may appear to be dominant, or pseudo-dominant, within a pedigree. If a particular trait has a high frequency in the population, it is likely that two or more people may have independently introduced this trait into a single pedigree. This is in contrast to the typical founder effect, in which a single "founder" individual introduces the trait into the pedigree. A "founder" may be a person who is affected with a de novo mutation that enters the pedigree with them. Or, it may be a person who comes from a relatively separate gene pool , such as a European explorer entering the formerly isolated gene pool of a remote tribe or race of people.

Resources

BOOKS

Campbell, Neil A. Jane B. Reece, and Lawrence G. Mitchell. Biology. 5th edition. New York, New York: Benjamin Cummings, 1999, pp. 239-293.

PERIODICALS

Wilkie, A. O. "The molecular basis of genetic dominance." Journal of Medical Genetics 31, (1994): 89-98.

ORGANIZATIONS

Genetic Alliance. 4301 Connecticut Ave. NW, #404, Washington, DC 20008-2304. (800) 336-GENE (Helpline) or (202) 966-5557. Fax: (888) 394-3937 info@geneticalliance. <http://www.geneticalliance.org>.

WEBSITES

"Complications to basic Mendelian inheritance." <http://www.ich.ucl.ac.uk/cmgs/compmend.htm> (February 28, 2001).

"Mitochondrial inheritance and oxidative phosphorylation (oxphos) diseases." <http://www.emory.edu/WHSC/GENETICSLAB/dna/mito.htm> (February 28, 2001).

MITOMAP: A human mitochondrial genome database.<http://www.gen.emory.edu/mitomap.html> (February 28, 2001)

OMIM—Online Mendelian Inheritance in Man.<http://www.ncbi.nlm.nih.gov/Omim/> (February 28, 2001).

Patterns of inheritance.<http://www.ich.ucl.ac.uk/cmgs/modes.htm> ( February 28, 2001).

Paul A. Johnson

Chapter 1
Inheritance

In legal terms, inheritance describes the wealth that parents leave, or pass on, to their children. In biology, the word refers to the genetic processes by which parents endow their children with specific traits, like height, hair color, eye color, and other characteristics. It is an everyday observation that children tend to resemble their parents in many ways, but it is equally obvious that the resemblance is not a simple one. Tall parents do not always produce tall children; children with blue eyes do not necessarily have parents with blue eyes.

Similarity and Difference

Yet, in humans and all other biological organisms, there are clear similarities between one generation and the next: Dogs always give birth to dogs; acorns always develop into oak trees; when a chicken's egg hatches, an infant chicken—not a duck or an eagle—emerges from the shell. In other words, each species, or type of organism, produces members of the same species. Striking similarities also exist within species. Every normal cat has four legs, humans have two, and spiders have eight. Despite these similarities, no two individuals of the same species (except for identical twins or triplets) are carbon copies of each other.

Genetics is the science that explains these similarities and differences. It even explains those rare and often tragic cases where a member of a species is born with physical abnormalities. It does so through what are called the laws of inheritance. These laws were worked out in the middle years of the nineteenth century by an Austrian monk named Gregor Mendel, who devised them after conducting an extensive series of experiments on garden peas. In the years since Mendel did his work, countless experiments with other types of plants and animals have confirmed that the laws of inheritance apply to even extremely complex biological organisms like human beings.

Although Mendel published the results of his work with peas almost a century and a half ago, his experiments are still regarded as masterpieces of scientific method. "While others . . . pondered the huge complexities and confusions of heredity, Mendel perceived that no one would make any progress until they first identified and worked out the simplest possible cases," says science writer Colin Tudge. "Never, in short, in all of science have experiments been more beautifully conceived and executed. This is not the simplicity of a simple man, but of a genius, who sees the simplicity that lies behind the surface incoherence."²

That in essence is what science does. It seeks to find simple rules to explain phenomena that on the surface appear to be very complex. When these rules are confirmed by numerous experiments, they are classified as laws. Most scholars agree that the paper in which Mendel recorded the results of his eight years of work with garden peas remains a completely up-to-date discussion of the laws of heredity. "With peas, the simplest possible case, he worked out the ground rules," Tudge says. "But he knew . . . that most cases were more complicated. He wanted to find rules that were universal. I'm sure he felt in his bones that the rules he derived from peas were universal, and that with a little tweaking they could explain the odd patterns of inheritance in beans and bees and, indeed, in human beings."³

Reproduction

Some organisms—bacteria, for example—are made up of just a single cell. These life-forms reproduce by a relatively simple process of cell division. Stimulated by internal chemical processes, the solitary cell divides to produce an exact replica, or clone, of itself. The genes contained in the replica are identical to those in the original. Depending on the type of organism involved and the environment in which it is living, cell division can recur a virtually limitless number of times, creating countless identical living things.

However, plants and animals—including human beings—reproduce sexually. That is, genetic material from both a male and a female parent is required to produce a new member of the species. One of the consequences of sexual reproduction and the transfer of genetic information that it entails is that offspring are different from their parents and from each other. Every individual produced in this way, except for identical twins and triplets, is unique.

To arrive at the principles underlying the transmission of traits from parents to their offspring in sexual reproduction, Mendel chose to study garden peas. He focused on seven traits that were easy to observe and, in garden peas, come in two distinct varieties: (1) seed shape, which is either round or wrinkled; (2) seed color, yellow or green; (3) flower color, purple or white; (4) pod shape, inflated or pinched; (5) pod color, green or yellow; (6) stem height, tall or short; and (7) flower position, at the end of the stem or off to the sides.

In order to fully appreciate the laws of inheritance, it is necessary to understand that plants, like people, have male and female sexual organs. However, unlike in people, both sex organs can occur on the same plant. This allows the plant to fertilize itself as well as be cross-fertilized by another plant. The sex organs are located in the flower of the plant. The male organs, called the anthers, produce pollen, which contains the male sex cells, or sperm. When fertilization occurs, pollen is transferred from the anthers to the female organ, called the stigma, where it combines with a female sex cell, or egg.

Mendel's Experiments

In nature, cross-fertilization (also called crossbreeding) occurs when a bee or other insect (or even the wind) transfers pollen from one plant to another. Mendel used a small brush to accomplish this chore. He set himself the task of cross-fertilizing plants with contrasting traits—tall with short, yellow-seeded with green-seeded, and so on—and tabulating the results. He realized that he would have to observe a large number of crossbreedings to compensate for the possibility that some of the results might be due to flukes like unintended wind-borne fertilization. By the time his eight-year experiment was over, he had carefully recorded the results of twenty-eight thousand crossbreedings.

As further insurance against false results, he also made sure that the plants he started with were pure-breeding, another way of saying that for generations they had produced only one version of each of the traits he was interested in. "He had a bunch of plants with yellow seeds that produced only plants with yellow seeds when bred to each other," explains geneticist R. Scott Hawley. "Similarly, he had a bunch of plants with green seeds that produced only plants with green seeds when bred to each other."⁴

When Mendel looked at the first generation of plants produced by his pairings, he found immediate confirmation of his belief that the blending theory of heredity was wrong. The first trait he looked at was seed shape. All the plants produced by crossbreeding round-seed and wrinkle-seed plants had round seeds. Similarly, the mating of tall plants with short ones produced plants that were all tall. There were no intermediate specimens—no seeds that were somewhat

[Image not available for copyright reasons]

wrinkled and no plants whose height fell between that of its tall and short parents.

Discussing seed color, Hawley says,

In the first generation, when Mendel crossed plants with green seeds to plants with yellow seeds, all he saw in the progeny were plants with seeds identical in color to those of the green-seeded parent. It didn't matter which way the cross was made (i.e., green males crossed to yellow females or vice versa), all the offspring had green seeds. . . . A real adherent to blending would have postulated that the progeny of the first generation should have been yellowish-green, not true green.⁵

One Gene, Two Variants

This part of Mendel's experiment established the first principle of inheritance. If genetic material was not blended, then it had to be passed on in discrete units. These units—Mendel called them factors, but they are now referred to as genes—retained their individual identity in the newly created organisms, which could then pass them on to succeeding generations. The second, and equally important, principle of inheritance came to light when Mendel bred second-generation plants to themselves and each other. Traits—yellow seeds and shortness, for example—that had vanished in the second generation suddenly reappeared in the third.

With respect to height, one out of every four third-generation plants was short. Mendel obtained similar ratios for the other traits he was studying. "In the subsequent generation, green-seeded plants crossed to themselves or each other produced both yellow and green-seeded plants," Hawley explains.

In order to explain differences in traits, Mendel supposed that genes could take different forms, called alleles, that specified different expressions of the trait. For example, Mendel claimed that there was a gene that gave seed color and two different forms or alleles of that gene: one specifying green color and one specifying yellow color.

In addition, an individual must be able to carry genetic information for a trait it does not express. (We know that the green-seeded progeny [offspring] produced by the first generation carried the information to produce yellow seeds because they were able to pass it on to the yellow-seeded progeny in the next generation.) Mendel's second insight was that this pattern of inheritance could only be explained if the green alleles could mask the expression of the yellow alleles, such that individuals getting a green allele from mom and a yellow allele from dad would be just as green as those that got green alleles, and only green alleles, from both parents. . . . To denote this difference between the ability of alleles to determine a [trait], Mendel introduced the terms dominant and recessive.⁶

That every gene comes in two varieties, a dominant form and a recessive form, explains the ability of traits to skip generations. Each parent passes on one allele of each gene to each child. A child who inherits two dominant alleles for a given trait will exhibit that trait. A child who inherits one dominant and one recessive allele will also exhibit the trait because the dominant allele will mask the presence of the recessive allele. Only if the child inherits two recessive alleles will the dominant trait not appear.

Dominant and Recessive Traits

The propensity for red hair, for example, could be lurking in recessive form in the genes two nonredheaded parents pass along to their children. Those children will, in turn, pass it on to their children. Should one of those children grow up and marry someone also carrying the recessive form of the gene, and should they both pass on the recessive form to their offspring, redheadedness will suddenly emerge after an absence of several generations in the family tree.

The height of garden pea plants illustrates this point clearly. With respect to the gene that governs this trait, the dominant allele leads to tallness and the recessive allele leads to shortness. A pea plant will be tall if it inherits the dominant (tall) allele of the gene from each of its parent plants. It will also be tall if it inherits one dominant (tall) allele and one

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recessive (short) allele. Only if it inherits two recessive alleles will it be short.

This insight is one of the cornerstones of modern genetics: Rather than blending in offspring, genes are inherited from parents as discrete units that come in two forms, each of which is passed on separately and remains separate in the genetic makeup of the new individual. This means that recessive traits can be passed on from generation to generation without leaving a visible trail.

"Mendel concluded that the hereditary determinants for the traits in the parental lines were transmitted as two different elements that retain their purity in the hybrids [the plants resulting from crossbreeding]," says geneticist Daniel L. Hartl. "In other words, the hereditary determinants do not 'mix' or 'contaminate each other.' The implication of this conclusion is that a plant with the dominant trait might carry, in unchanged form, a hereditary determinant for the recessive trait."⁷

Segregation and Assortment

In humans, for example, a dimpled chin is caused by a dominant allele. A person with a dimpled chin has either one or two dominant alleles for this trait. The absence of a dimple indicates that two recessive alleles are present in the person's genetic makeup. However, since someone with a dimpled chin might have a recessive allele for that trait, it is possible for that person to pass it on to a child. Should that child's other parent also contribute a recessive allele for this trait, the child would have a dimpled chin.

This property of genes, that they each come in two different forms or alleles and that each parent contributes one allele to the genetic makeup of his or her children, is called the principle of segregation. Support for this principle came in the twentieth century when biologists discovered that when sex cells—male sperm and female eggs—are formed, each carries only one allele for each gene. When the sperm fertilizes the egg to form a new individual, these alleles recombine and, according to the rules of dominance and recessiveness, produce the traits the new individual exhibits. Each egg cell and each sperm cell are just as likely to contain one allele as they are to contain the other.

Another important law of inheritance, also discovered by Mendel and proved by subsequent research, is called the principle of independent assortment. In conducting his experiments with peas, Mendel noticed that traits were inherited independently of each other. In other words, whether a plant was tall or short had no effect on whether its seeds were green or yellow. This is because the traits he was looking at are caused by genes that are located on different chromosomes (structures in each of an organism's cells). Mendel was not in a position to know this fact, but it was proved when the structure of cells was determined in the decades after he completed his work.

Exceptions to Mendel's Rules

While it is true that Mendel's description of the principles of genetic inheritance has been confirmed by tens of thousands of experiments on plants and animals, it is also true that the transmission of genetic information from one generation to another is somewhat more complex than he realized. In certain cases, called incomplete dominance, the inheritance of both a dominant and a recessive allele of a gene does produce what appears to be a blend of traits.

Certain flowers, for example, come in three colors. When a dominant and a recessive allele for the gene that determines color combine, the result is a shade intermediate between the two other colors that the flower can assume. Although this seems to suggest that a blending of genetic material has occurred, it has not. It is simply a property of these flowers' genes to produce three colors and not two, as Mendel's peas did.

Another subtlety that escaped Mendel has come to be known as quantitative inheritance. The traits he studied were very easy to observe: A seed was either green or yellow, a plant was either tall or short. Each of these traits was caused by a single gene. Some characteristics, however, do not lend themselves so easily to either-or classification. Human skin color, to take an obvious example, comes in a wide range of shades. Scientists now know that these traits are determined by more than one gene. It is the combined effect of all the relevant genes that produce the characteristic, allowing for more variation than occurs when only single genes are involved.

The rules of inheritance that Mendel discovered—that genetic material is passed from generation to generation in discrete units, that these units come in two forms called alleles, that each parent contributes one allele for each gene to the offspring, and that the units operate independently of each other—laid the groundwork for twentieth-century genetics. From his modest experiments with garden peas have come the breakthroughs that are revolutionizing biological science and our understanding of life itself.