Genetic Implications of Marriage and Mating Customs
The study of the total genetic information, or gene pool, of a group, is called population genetics. Among the variables important to population genetics are the frequencies of particular alleles and genotypes. If these frequencies remain stable over time, the population is not evolving, and is said to be in genetic equilibrium.
A fundamental law of population genetics, describing the conditions under which genetic equilibrium can occur, is called the Hardy-Weinberg equilibrium principle. According to the Hardy-Weinberg principle, genetic equilibrium requires a situation in which mating is random. This is a situation that hardly ever occurs in human populations. Human societies generally develop complex rules about marriage and mating, including customs and taboos regarding the selection of partners from within the group (inbreeding) or outside it (outbreeding).
Outbreeding increases genetic diversity by introducing new gene versions, or alleles, into the population. This is generally healthy for a population because it offers a wider variety of potential traits by which an evolving population can adapt to a changing environment. Outbreeding is seen in many animal populations. For example, elephant herds are matriarchal. Adult male elephants do not live with the herd, but wander on their own and find mates where they may. Similarly, in human societies, when resources and opportunities are limited, young people will often migrate to seek their fortunes elsewhere.
Inbreeding, by contrast, decreases genetic diversity, and increases the proportion of individuals who are homozygous for particular alleles; that is, who received the same allele from both parents. Many genetic disorders are associated with recessive alleles. They only show up if an individual is homozygous for the responsible allele. A heavily inbred population is more likely to experience the occurrence of such disorders.
The more closely related a mating pair is, the more likely they are to have offspring homozygous for particular alleles. Suppose, for example, that one of 40,000 individuals within a population are homozygous for the recessive allele that causes albinism. The offspring of first cousins within that population would have a risk more than 13 times higher.
For this reason, most human societies, even those in which inbreeding is the norm, have developed incest taboos against mating between closely related individuals. The extent of the relationships proscribed by these taboos varies by society. They usually exclude marriage between parents and children, and between siblings. Taboos usually extend to marriages between aunts and nephews, or uncles and nieces. Often they forbid marriages between first cousins. In Navajo society, even strangers may be strictly forbidden to marry if they trace their roots to the same ancestral subgroup, or clan.
Despite the incest taboos, many human societies encourage some degree of inbreeding in order to preserve their cultural identity. Families in which both parents are group members are generally more likely to raise their children within the culture of the group. For example, the Jewish community, until recently, maintained a strict taboo against intermarriage, allowing it to remain a genetically and culturally distinct population despite 2000 years of geographic dispersion. This taboo has only been relaxed in the past few generations, a change that is one of the most controversial issues within the community.
In many cases, social rather than (or in addition to) ethnic qualifications for mate selection are important. In European royal families, marriage partners were expected to be chosen from within the aristocracy until recently, and were often used to cement political alliances. An English princess was far more likely to marry a European prince than she was to marry an Englishman who was not of royal ancestry. The nobility of England, Germany, France, and other European countries married each others' children with regularity. Tzar Nicholas II of Russia and King George V of England were first cousins to each other as well as to Kaiser Wilhelm II of Germany, whom they opposed in World War I. The inbred nature, or consanguinity, of the aristocracy increased the prevalence within its ranks of genetic diseases such as porphyria and hemophilia.
When discussing human population genetics, it is important to include both marriage, and mating outside of marriage. In many historical cases, relationships that were officially forbidden, or at least not sanctioned, nevertheless occurred with some frequency. For example, in the pre-Civil War American South, miscegenation laws forbade marriage between blacks and whites. These taboos were strictly enforced as far as white women mating with black men. However, while male slaveholders did not marry their female slaves, they fathered children by them often enough that over half of all African-Americans today are of mixed race.
Personal preferences, often driven by cultural mores, are also extremely important in mate selection. This results in assortative mating, in which particular traits are valued, rather than random mating. Depending on the culture, strong warriors or intelligent scholars may be particularly sought-after. In a society in which food is scarce, a large woman who looks well-fed may appear to be the picture of health and strength, more likely to bear and nurse viable children, and therefore more desirable as a mate. In today's modern Western civilization, with its over-abundance of food, the opposite appearance is often considered desirable.
With modern reproductive technologies, new options for selecting or de-selecting traits in offspring are afforded, in addition to those that arise with choosing a mate. The ethics of using those capabilities involve issues that will be debated for years to come.
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