« What a country! | Gene Expression Front Page | Dye hard »
September 20, 2004

Mixed race & inbred?

Below, Joe Hertzingler asked how Jews could be inbred since their source population engaged in interpopulation mixing, that is, Levantine males + indigenous females (let's assume this is so). This seems to come up, and part of the problem is that we forget that populations, "races," are just the sum of their parts and the hybrid populations aren't really that special aside from having various alleles shuffled in a novel fashion (because two individuals from populations A & B do not often mate for whatever reason, geographical or sociological). Here is Sewall Wright's inbreeding coefficient:

(source)

Fa is the inbreeding coefficient of a given common ancestor while the n are the steps (generations) you take from each parent to get back to that common ancestor. The implications from the equation are obvious, if your parents don't have common ancestors (at least recently), you aren't going to be inbred, no matter what yokels they happen to be (so, people whose parents in the previous generation are from two different subpopulations aren't going to be inbred since two subpopulations are assumed to not have been interbreeding and so sharing ancestors). If your parents share several common ancestors recently, you are going to be inbred (you sum multiple times, and 1/2n is larger). On the other hand, if you share common ancestors far back, pretty soon 1/2n goes to triviality. A good thing, since we all share common ancestors.

Inbreeding is basically a measure of how many genes you have pairs of alleles that are "identical by descent," that is, your mother and father both passed on to you the same allele from a common ancestor (you can also define it in relation to homozygosity & heterozygosity, convenient if you want to calculate the level of inbreeding in a population and have experimental data on hand). The problem with having identical alleles is that sometimes you can get defective copies from both your parents. We all have silent lethal recessives in our genetic background, but if your parents aren't related, you are less likely to encounter problems since the lethals shouldn't overlap (that is, you'll get a "good copy" from one parent. Two individuals can have 10 lethal alleles each, but the individual who has several of these alleles one the same genes is screwed while the other who does not have the alleles so concentrated is safe to live another day).

Now, back to my point, a concrete example, if a white Korean war vet and his Asian wife decide to go live in the backwoods of Montana and their mixed-race kids start breeding with each other, the grandchildren will be inbred. Quite obviously they share common ancestors recently (Fz would be .25 for full sib matings). This of course is a bizarre example, but we have a real one that is less extreme, the Pitcairn Islanders, the descendents of British mutineers and Tahitian women. Here you do have inbreeding problems. Since they had so few common ancestors to start out with, after two hundred years they all share them (Fletcher Christian is great-great-great...grandfather to them all!). So here you have the case of inbred mongrels.

Addendum: The power of genetic drift really kicks in for small populations. Over time allele frequencies can shift toward fixation (that is, allele A can crowd out allele B to the point where the gene becomes monomorphic). Monomorphic populations better have a good complement of genes, because they have no "back up" allele to pick up the slack in the case that one of the pair is defective (that is, they're both the same functionality).

Related: Steve Sailer's article about race as an inbred family might be of interest.

Posted by razib at 02:09 AM