CONSANGUINITY

kinship characterized by the sharing of common ancestors. The word is derived from the Latin consanguineus, of common blood, which implied that Roman individuals were of the same father and thus shared in the right to his inheritance. Kin are of two basic kinds: consanguineous (sharing common ancestors) and affinal (the kinsmen of one's spouse). In some societies other pairs of individuals also treat each other as relativesfor example, the wives of a pair of brothers, relatives by adoption, and persons such as godparents who have special kinlike relationships (fictive kin). Consanguineous kinship is a universal type; it includes those with common ancestors and it excludes individuals who lack ancestors in common. In the modern sense, consanguinity is a genetic concept. From the biological point of view the term itself is unsuitable (as are the terms mixed blood and good blood) because the genetic contributions of ancestors do not flow to their descendants as blood, but through the genes contained in the chromosomes in the cell nuclei. The chromosomes are made up of nucleic acids (DNA, or deoxyribonucleic acid) and proteins. The DNA is the part of the chromosome that carries the genes and is coded in specific ways to produce and control protein synthesis; parts of each parent's coded message are transmitted to the offspring. Besides genetic determinants carried in the DNA, there are other biological influences of parents on offspring, such as the environment in the mother's womb; indeed, there is even cultural inheritance through learning, which in turn influences nutritional and other habits and hence affects growth and development. In genetics, consanguinity affects the probabilities of specific genotypesthe combinations of genetic characteristics. Consanguinity results in inheritance, from common ancestors of both parents, of their transmissible capacities to synthesize and control nucleic acids and proteins, the essential substances of all organisms. Consanguineous relatives are said to be of various degrees, according to the likelihood of their sharing genetic potentialities from the common ancestors. Thus, pairs of brothers and sisters (siblings) have all the same ancestors, whereas pairs of first cousins (cousins german) who are not otherwise related share only one-half of their ancestors. A child inherits only about one-half of the coded information from each parent; hence a pair of brothers or sisters have about half of their chromosomal constitution in common (the doubt about the exact fraction is due to the chance element in transmission of material during meiosis, the cell division that produces sperm and egg, each possessing a haploid number of chromosomes). Genetically the degree of consanguinity of siblings is thus the same as that between a parent and child, and both may be called consanguineous in the first degree. An aunt or uncle shares with his niece or nephew about half the chance of common inheritance of a pair of siblings, and aunts and uncles may be called consanguineous kin of the second degree. First cousins may be called consanguineous of the third degree. Legal systems of designating degrees of consanguineous kin exist in Roman law, common law, and canon law. None of these systems depends on the genetic facts, however, and they have no place in a classification of the biological relationships between individuals. A great-grandparent and great-grandchild are genetically related to the same degree as a pair of first cousins. The grandparent is a linear kinsman, however, whereas the cousin is a collateral one. In genetics the degree of consanguinity is the sole factor of significance, but in social relations in various societies important considerations of collateral versus lineal types of relationship as well as age, birth order, and other factors may determine social behaviour. In fact, consanguineous kin of various degrees and even nonconsanguineous kin may be called by the same name and treated similarly by custom or law (the term uncle, for instance, may be applied to a granduncle or to the husband of an aunt). One of the two main applications of data on consanguinity is with respect to the probability that two individuals of a known degree of consanguinity with another individual will share the traits of that person. This probability depends upon the mode of inheritance and on the degree of penetrance or expressivity of genetic factors. The mode of inheritance may, for example, be dominant or recessive. A pair of genes in the same relative positions in a set of two chromosomes in the cell nucleus (these genes are called alleles) are for two alternative traits, such as greenness and yellowness in peas; both alleles may be for one of the traits, or they may differ, so that one allele is for one trait and one for the other. The trait that appearsis expressedin an individual, when the pair of alleles differ, is the dominant trait and the inheritance is dominant; if a trait does not appear unless both alleles are for that trait, it is recessive. Another mode of inheritance is sex-linked inheritance. Genes for hemophilia, for example, are present in both males and females, but the disease tends to affect only males. The degree of penetrance is the frequency with which any trait or effect is shown in a group or population that has the gene corresponding to that trait. Expressivity is the degree to which traits are shown in an individual. Another significant application of data on consanguinity is the measurement of inbreeding by the degree of consanguinity between two parents. The coefficient of inbreeding (F ) is used. This is the probability that two alleles will be identical and derived from the same forebear. The application of this principle is most easily demonstrated by example. If brother and sister marry, the offspring would have one chance in four of inheriting a pair of identical alleles from the grandparent. With each further degree of consanguinity the likelihood is halved, so that the likelihood in the child of a mating between aunt and nephew would be 1 in 8, and in a child of first cousins, 1 in 16. (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with In construction of pedigrees, if horizontal lines are used to connect symbols of siblings and mates and vertical lines to connect parents with their offspring, all inbreeding is represented by one or more loops (see the Figure), each of which involves consanguinity. The coefficient of inbreeding for the individual is the sum of that calculated for all the loops that include the individual's parents. The inbreeding coefficient of a population is the average F of its members. The highest values of F are found in small populations whose members marry one another over many generations. Such groups are called isolates. Thus, the Samaritans, who remained a small but distinctive group since before the time of Christ, are considerably inbred. In the United States some religious groups also live in agricultural colonies as isolates (for instance, the Amish and Hutterites). In genetics an allele that is carried at the same position in both of a pair of chromosomes is called homozygous. An allele may be rare in the general population, but, if the parent possesses it, it is transmitted from parent to child with the same probability as a common one. Therefore the chance of receiving a rare allele in the chromosomes derived from both mother and fatherthat is, the chance of being homozygous for that alleleis greatest in the offspring of consanguineous mating. In theory, since repeated mutations are rare, homozygosity of even common alleles may be ascribed to distant consanguinity. Mendel's classical experiments with peas and much subsequent work show that when an allele is present in double dose (homozygous), the effects may be very different from those when it is inherited only from one parent (heterozygous). In medical genetics there are many proteins, especially enzymes, that are produced in adequate amounts if either chromosome carries the appropriate allele. Absence of the gene in both of a pair of alleles produces a deficiency in the protein it determines. The rare diseases and anomalies of this kind are relatively less rare in the offspring of consanguineous unions. In fact, in 1902, within two years after the rediscovery of Mendel's laws, the high frequency of consanguinity in the parents of individuals with three inborn errors of metabolism was used as evidence of recessive Mendelian inheritance in man. One of the defects noted was albinism, a condition in which the skin is pink and the hair white, the eyes lack pigment, and the subjects are uncomfortable in bright sunlight and usually squint. Such adverse genetic effects in the offspring of consanguineous unions is appreciable only in rare hereditary diseases; the rarer the occurrence, the more frequently the parents are found to be consanguineous. A large proportion of offspring of consanguineous mating of the first degree die or have serious defects by six months of age. In the offspring of first-cousin marriages and in other instances of similar or lesser consanguinity, few deleterious effects have been observed. Furthermore, because rarity is a factor in this effect, the overall influence of inbreeding in the general population is very limited. The modest socioeconomic differences between the kinds of people who mate with consanguineous kin and those who do not may account for some of the increase in disease and mortality that has been ascribed to consanguinity. From the social, as opposed to the individual, point of view, close inbreeding does not increase the number of deaths from recessive disease; it merely precipitates these so-called genetic deaths in earlier generations. In heterozygous form, with no adverse influence on the individual who carries them, such alleles contain the possibility of future deaths from recessive disease; and death for infant offspring of consanguineous parents reduces that possibility for the next generation. The principle of deliberate inbreeding is used with domestic animals to eliminate such covert recessive alleles from the stock. Nevertheless, there seem to be problems of health from very highly inbred pure lines, and heterozygosity in some alleles seems to be advantageous. Many species, including man, have been established by episodes of isolation and inbreeding interspersed with outbreeding; they apparently thrive in this way. All human societies have some incest taboos. These are rules and laws that prohibit marriage or sexual relations, or both, between certain kinds of kin. The kinds of kin always include some consanguineous classes, and one theory of the establishment of incest laws is folk knowledge of undesirable inbreeding effects in offspring of such unions. Incest taboos are not uniform restrictions to a particular grade, however, and often extend to nonconsanguineous relations. Thus, in traditional Chinese society a man may marry his mother's brother's daughter, for instance, but he may not marry any person with his own surname. Other theories of the origin of incest, therefore, include analysis of its effects on stability of the family as an economic and educational unit and ascribe the definition of incest in various societies to social and psychological motives.