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Decline in inbreeding over time

Gene ExpressionBy Razib KhanMarch 16, 2009 12:10 PM


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Genetic Future points to a new paper which suggests that inbreeding is declining over time. Their methodology involved surveying different age cohorts and noting the decreased levels of homozygosity among the youth. Dienekes points out that the use of living people might simply be confounding inferred mating patterns with the fact that homozygous individuals have a higher life expectancy. The plausibility of these two hypotheses varies based on how much weight you put on the shifting of mating patterns. In Consanguinity, Inbreeding, and Genetic Drift in Italy the authors used Catholic Church records to show quite clearly that inbreeding declined rapidly with the rise of modern transportation and communication. But that decline varied by region, with the shift toward exogamy occurring first in the north and moving over time toward the south. In Europe as a whole the rate of consanguinity has not changed monotonically over the past 1,000 years. After the Reformation cousin marriage was sanctioned in many Protestant nations whereas before the Catholic Church had strictly proscribed it. The main caveat to this is that religious regulation of marriage was a much greater concern for elites than it was for common people, as the latter often entered into common law relationships which were never formally solemnized in any case. In any case, after the initial spike in cousin marriages in Northern Europe the same variables which drove up the rates of outmarriage in Italy were also instrumental in driving the proportion back down.

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