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April 03, 2004

Cousin has perty lips

A year back a study came out that prompted headlines like First cousins face lower risk of having children with genetic conditions than is widely perceived. Then articles came out that implicated cousin marriage in particular cultural pathologies (from a Western Eurocentric persecptive of course). It is true that the additional risk for birth defects for first cousin marriages is only a few percent. But, statistics can be presented in many ways. Here is some data I am getting from a table in Principles of Population Genetics (from Morton, 1961):

Condition % of affected children whose parents were first cousins
Total color blindness 15
Albinism 21
Xeroderma Pigmentosum 23
Ichthyosis Congenita 35
Tay Sachs 40

Please note that these statistics are from 1961 and the United States. The percentages fit the theoretical prediction, which follows from the following equation (for harmful recessive diseases):
K = c(1 + 15q)/(c + 16q - cq)
where c is the proportion of first cousin matings in a population, and q denotes the frequency of the rare recessive allele in the population, and K is the proportion of children who are affected with first cousin marriages. During the period of the study above first cousin marriages were only 1% of American marriages.

Posted by razib at 09:38 AM