Sex-biased gene flow in African Americans but not in American Caucasians

We have previously shown evidence of strong sex-biased genetic blending in the founding and ongoing history of the Brazilian population, with the African and Amerindian contribution being highest from the maternal lineages (as measured by mitochondrial DNA) and the European contribution foremost from paternal lineages (estimated from Y chromosome haplogroups). The same phenomenon has been observed in several other Latin American countries, suggesting that it might constitute a universal characteristic of the Iberian colonization of the Americas. However, it has also recently been detected in the Black population of the United States. We thus wondered if the same could be observed in American Caucasians. To answer that we retrieved 1387 hypervariable I Caucasian sequences from the FBI mitochondrial DNA population database and established their haplogroup and continental geographical source. In sharp contrast with the situation of the Caucasian population of Latin American countries, only 3.1% of the American Caucasian sequences had African and/or Amerindians origin. To explain this discrepancy we propose that the observation of elevated genomic contributions from European males and Amerindian or African females depends not only on the occurrence of directional mating, but also on the “racial” category of the children born from these relations. In this respect, social practices in Latin America and in the United States diverge considerably; in the former socially significant “races” are connected with the physical appearance of the individual, while in the latter descent appears to be the most important factor