Racial differences in genetic load?

Another point of confusion in Kanazawa's deleted post:
There are many biological and genetic differences between the races. However, such race differences usually exist in equal measure for both men and women. For example, because they have existed much longer in human evolutionary history, Africans have more mutations in their genomes than other races. And the mutation loads significantly decrease physical attractiveness (because physical attractiveness is a measure of genetic and developmental health). But since both black women and black men have higher mutation loads, it cannot explain why only black women are less physically attractive, while black men are, if anything, more attractive.
Actually, the ancestors of present-day non-Africans existed for the same amount of time as the ancestors of present-day Africans. Any racial differences in the number of deleterious variants per genome would need to be explained in terms of population differences in mutation rate, or strength of purifying selection, or some other relevant population genetics parameter.

A few years ago, an academic geneticist affirmative-action beneficiary made the opposite claim: that due to a population bottleneck, Europeans harbor more deleterious variation than Africans. John Hawks pointed out the impossibility of that study's conclusion around the time it was published:
What is amazing to me is that these same geneticists embrace hypotheses of population history that cannot possibly have happened. The other geneticists quoted in the article, Carlos Bustamante and his graduate student Kirk Lohmueller, wrote a paper earlier this spring arguing that deleterious mutations have reached high frequency in Europeans (moreso than Africans) because of a bottleneck during European history. The press reported this work as "Whites genetically weaker than blacks, study finds." The hypothesis in the paper is that protein-coding sites otherwise conserved in most mammals may differ among humans because of relaxed selection in a bottleneck.

Here's why they're wrong: their bottleneck is impossible. They propose that the European population was a small, isolated population of 5,700 effective individuals from 214,000 years ago up to the Last Glacial Maximum. I suppose I should take some encouragement that they believe Neandertals were European ancestors (because otherwise, where exactly would this small, isolated population of Europeans have lived). But it's still quite impossible -- it implies no gene flow between Africans and Europeans across that entire span. You see, that is the only way that genetic drift can lead to this kind of result -- large differences in frequencies between continents for hundreds of deleterious alleles. It takes a bottleneck of exceptional length, along with complete isolation.

In what has become a troubling trend, these details were hidden away in the online supplementary information of the paper.
Empirically, there seem to be no major racial differences in genetic load. Compared to whites, blacks have much higher rates of spontaneous abortion and infant mortality -- superficially consistent with higher genetic load among blacks -- and an early attempt in Brazil to assess racial differences in genetic load by examining the effects of inbreeding did conclude that blacks have more lethal equivalents. However, a subsequent Brazilian study, with better controls, found no racial differences in inbreeding load:
A new investigation using sib and cousin controls, made in the same Brazilian area where whites and nonwhites (mulattoes and Negroes) had before shown inbreeding loads of different magnitudes, failed to support these findings. Our present data show that the number of lethons is the same in both ethnic groups. A review of all investigations on Brazilian whites and nonwhites suggests that the number of lethons may be accepted as roughly 2 in both ethnic groups.
Similarly, "An analysis of consanguineous marriages in Nigeria" finds:
The total number of heterozygous deleterious genes carried per person in this population was estimated to be 8.71 +/- 3.92 (SD), which is not too different from estimates for several other racial groups.


Anonymous said...

blacks have much higher rates of spontaneous abortion and infant mortality

I'm not questioning this but do you have a link to the source?

Anonymous said...

Are there differences in infant mortality rates between racial and ethnic groups?
In 2005, there was a more than threefold difference in infant mortality rates by race and ethnicity, from a high of 13.63 for non-Hispanic black women to a low of 4.42 for Cuban women.

n/a said...

"Race has been found to be a risk factor of spontaneous abortion in some studies (1, 11). As
shown in table 3, among young pregnant women, black women suffered a more than a doubled
risk of spontaneous abortion compared with whites, Asians, and Hispanics."

"Black, white, and other race/ethnicity women underwent 3666 (4.6%), 68,607 (83.5%), and 8036 (11.9%) IVF cycles, respectively. Spontaneous abortions were more common among black women. The live-birth rate was 26.3% (95% confidence interval [CI], 25.9%-26.7%) among white women compared with 18.7% (95% CI, 17.5%-20.1%) among black women (rate ratio, 1.41)."

I've also seen CDC statistics supporting this for the US overall.