« Nuclear Space Ships | Gene Expression Front Page | $Everyone Loves a Jewish Girl$ »
January 14, 2004

Brazil & racial admixture

The article that indicates that there has been some disassociation between geographic ancestry and racial phenotype in Brazil is out as full text for free online: Color and genomic ancestry in Brazilians. Here is the part that made it into the papers:

...our data indicate that, in Brazil as a whole, Color is a weak individual predictor of African ancestry.

If we consider some peculiarities of Brazilian history and social structure, we can construct a model to explain why Color should indeed be a poor predictor of African ancestry at an individual level. Most Africans have black skin, genetically determined by a very small number of genes that were evolutionarily selected (5, 6). Thus, if we have a social race identification system based primarily on phenotype, such as occurs in Brazil, we classify individuals on the basis of the presence of certain alleles at a small number of genes that have impact on the physical appearance, while ignoring all of the rest of the genome. Assortative mating based on Color, which has been shown by demographic studies to occur in Brazil (22, 33), will produce strong associations among the individual components of Color...On the other hand, we expect that any initial admixture association between Color and the PSAs will inevitably decay over time. It is easy to see how this combination of social forces could produce a population with distinct Color groups and yet with similar levels of African ancestry. Let us take as an example, the historically common Brazilian mating of a white European male with a black African slave woman: the children with more physical African features would be considered black, whereas those with more European features would be considered white, even though they would have exactly the same proportion of African and European alleles. In the next generation, the light-skinned individuals would assortatively tend to marry other whites and conversely the darker individuals would marry blacks. The long-term tendency would then be for this pattern to produce a white group and a black group, which would, nonetheless, have a similar proportion of African ancestry.

Our study makes clear the hazards of equating color or race with geographical ancestry and using interchangeably terms such as white, Caucasian, and European on one hand, and black, Negro, or African on the other, as is often done in scientific and medical literature (34, 35).

I think all goes well until the last paragraph. Note that the article is emphasizing the Brazilian context, until the conclusion, where it simply drops any Brazilian qualifier-and makes it appear that this result is generalizable. But the fact is that a paper showing the strong relationship between self-reported racial origin and genetic ancestry by geographic region had come out just before the Brazil paper (they even thank one of the authors of the latter paper in the acknowledgements for a sample!)....

Posted by razib at 12:47 PM