Genetic Effects of Consanguineous & Cousin Marriages | Anthropology Optional for UPSC PDF Download

Non Random Mating

Genetic recombination does not by itself alter allele frequencies. However, any consistent bias in mating patterns can alter the genotypic proportions. By affecting the combination frequencies of genotypes, nonrandom mating causes deviations from the Hardy Weinberg expectations. It therefore sets the stage for the action of natural selection.
Basically, there are three models of nonrandom mating positive assortative mating, negative assortative mating and inbreeding.

• Positive Assortative Mating: This occur when individuals of like phenotype mate more often than expected by random mating predictions. Because individuals with like phenotypes are also similar to some degree in genotypes as well, the result of positive assortative mating increases the amount of " 'homozygosity in the population and reduces heterozygosity (P² and Q² greater than expected; 2pq less than expected). Ex. Stature and IQ or Eye color.
• Negative Assortative Mating: This involves mating with an individual who is phenotypically dissimilar. Theoretically, if this occurs more than expected by random mating predictions, it should increase the amount of heterozygosity in the population while correspondingly reducing homozygosity.

Inbreeding  

• Inbreeding, also called consanguinity, occurs when relatives mate more often than expected. Such mating will increase the amount of homozygosity, .since relatives who share close ancestors will more than likely also share similar genes. Inbreeding occurs less frequently than predicted under random mating because all societies have some form of incest taboo, especially nuclear family. Inbreeding reduces the variability among the offspring thereby reducing the reproductive fitness.
• In some societies, due to the small number of potential mates available, inbreeding among fairly close relations such as cousins is actively encouraged or is unavoidable. Ex. Pitcairn Island and Tahitian wives. There are some areas, on the other hand, where inbreeding is avoidable but it is still actively encouraged. In some parts of Japan among certain social classes, first cousin marriages make up almost 10% of all marriages, and in Andhra Pradesh of India, among certain castes, uncle/niece, marriages also make up about 10% of marriages.
• However such considerable inbreeding is an exception rather than a rule among human populations and most groups actively work very hard at maintaining exogamy. As a general rule, then, most human populations do not inbreed if they can at all help it. Inbreeding has important medical consequences in addition to Its effect on genetic equilibrium. When relatives mate, their offspring have increased probability of inheriting an allele in homozygous dose.
• Any departure from random mating naturally leads to complications. In the relationships between allele frequencies and genotype frequencies. If a heterozygote Aa is self-fertilized, it will produce three kinds of progeny AA, Aa and aa, in the ratio of 1:2:1. At this stage, the frequency of the heterozygote is 0.5. If self-fertilization is continued for another generation, the homozygotes will breed true but the heterozygotes will again segregate, reducing their frequency to 0.25. With every succeeding generation of self-fertilization, The. frequency of the heterozygotes drops off by 50%, reaching 0.008 by generation seven and 0.001 by generation ten. At this stage, the population is 99.9% homozygous.
• The process of repeated self-fertilization illustrates the general effects of a form of nonrandom mating called Inbreeding. Inbreeding occurs when the mates are genetically related. Self fertilization is the extreme example, but other examples such as the mating of full siblings, first cousins, parents and offspring, and half siblings have the same effect, namely, to increase the frequency of homozygotes and decrease the frequency of heterozygotes. As a result, the Hardy Wienberg method does not hold.