Consanguineous marriage and its genetic outcomes

Marriage has long been the predominant institution within which procreation occurs and genes are transmitted. Therefore a key initial step in the investigation of genetic differences at individual and population levels is to examine past and present marriage preferences in different societies. In terms of populations genetics theory, infinite population size and random mating are important preconditions of the Hardy-Weinberg principle, which describes the intergenerational transmission of genes. It is, however, doubtful whether either of these assumptions has ever existed in human populations. For example, estimates of the effective population size of the Out-of-Africa migration some 60,000-70,000 years ago (Behar et al., 2008) range from approximately 10,000 (Harpending et al., 1998) to as few as 700 individuals (Zhivotovsky et al., 2003). Further, since the early human groups were itinerant hunter-gatherers, random mating would have been impossible and close kin mating almost inevitable. 
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\nEven in countries with large immigrant communities, such as the United States, Canada, and Australia, recent arrivals typically marry within their own ethnic and/or religious communities during the first and second post migration generations. This practice, described as positive assortative mating, provides strong social advantages. However, it has important genetic implications, since it is probable that couples from the same national, ethnic, or religious sub-community will have a significant proportion of their genes in common. Therefore, as will be further discussed, their progeny are more likely to be homozygous (or, more correctly, autozygous) for a detrimental recessive disorder (Bittles, 2002).
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\nThe probability of genetic drift, i.e. chance fluctuations in gene frequencies in successive generations, is greatest in communities with restricted numbers of potential mating couples. In human populations this situation can arise in a variety of ways; e.g., through founder effect, when a subgroup of a population establishes a new breeding colony; via a demographic bottleneck following a major disease- or disaster-related mortality (Auton et al., 2009); and in populations subdivided into multiple, strictly endomagous communities (Bittles, 2009a).
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\nVirtually all traditional human societies are divided into long-established communities, with limited intercommunity marriage (Bittles, 2008), and genome based association studies in industrialized Western societies have demonstrated the on-going existence of similar, if less pronounced, subdivisions leading to significant population stratification (McEvoy et al., 2009). A higher probability of homozygosity results, with a consequent increased likelihood of recessive gene expression. Since a recessive founder or de novo mutation can rapidly increase in frequency within a small community by chance alone, the birth of an affected child may occur whether the parents are known to be consanguineous or consider themselves nonrelatives (Zlotogora et al., 2006).

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