Monday, March 20, 2006

Bedouin pedigree collapse?   posted by Razib @ 3/20/2006 06:22:00 PM

This article about Bedouin genetic diseases in Israel due to cousin marriages seems like clear cut pedigree collapse wreaking havoc. One of the most annoying things about people not knowing science is that a superficial understanding of a topic can be really problematic. Check out this article from Slate from a few years back:

Now a study by the National Society of Genetic Counselors says that having a child with your first cousin raises the risk of a significant birth defect from about 3-to-4 percent to about 4-to-7 percent. According to the authors, that difference isn't big enough to justify genetic testing of cousin couples, much less bans on cousin marriage.

This prediction equation needs to be kept in mind by everyone:

K = c(1 + 15q)/(c + 16q - cq)

The proportion of first cousin matings being c within the population, and q being the frequency of the recessive allele in question. Though the individual risk in a given mating from a cousin marriage might not be problematic, doubling the number of children within a population with many birth defects is problematic in an era of socialized and quasi-socialized medicine. The frequency of individuals with recessive diseases that are the result of cousin marriages is something to keep in mind when a child with a very rare but expensive-to-treat- ailment is going to be an inevitable public charge.

In the case of the bedouin above, I think this equation is salient:

Inbreeding coefficient = sum over all common ancestors[(1/2)i * (1 + FA)]2

[Interjection: to clear up any confusion, I'm referring to the inbreeding coefficient formulated by Sewall Wright. The primary issue in the context of the issue of pedigree collapse is that when we are thinking of "first cousins" we consider that they share two out of four grandparents in common because of the sibling relationship of two parents, one of each respective individual. The inbreeding coefficient is measuring the path of descent from the common ancestors and taking into account the dilution of ancestry that occurs over the path of descent. FA is important because it is the inbreeding coefficient of the common ancestor. If you have an inbred group, then many common ancestors with their own inbreeding coefficients start to "contribute" to the overall sum, far above and beyond what a calculus that projects 1-2 generations into the past would suggest. Of course if you work back far enough we are all 'inbred,' that is what races and species are, groups that tend to breed disproportionately or exclusively with their own 'kind,' but mutation adds novel noise into the system while natural selection cleans the deleterious dirt out over the generations.]

The Arab bedouin of the Negev are likely a small initial population substructured into clans who have been breeding amongst themselves. Transgenerational normative incest reduces the long term effective population and so increases the power of random geneti c drift in fixing deleterious alleles even in the face of selection. A perfect recipe for families where children born without eyes aren't that abnormal.

This is even not mentioning the likely negative impact of cousin marriage socially. Egalitarians might want to keep in mind that cousin marriage has been cross-culturally justified as a way to keep wealth concentrated in particular families (due to partible inheritance).

Via Diana.

Related: Cousin be perty-part n.