Ancestry-related assortative mating in latino populations (Risch et al. 2009)

When different races admix, then in the first few generations there is a spectrum of ancestry proportions, ranging from pure individuals of the constituent races to admixed individuals with varying proportions of ancestry.

If there is random mating, then over several generations all individuals tend to have similar ancestry proportions, determined by the number of founders from the two constituent races. Mix 30,000 Europeans with 70,000 Africans, randomly mate them for 10-20 generations, and pretty soon almost everyone will have 30:70 European/African ancestral proportions with a little variation.

However, if there is assortative mating, then this process takes much longer to complete, as matings of individuals with very different ancestry proportions are rare, and the spectrum of varying individual ancestry is maintained. In the above-mentioned example, if there is perfect assortative mating, then after 10-20 generations you will still have 30% of the population having 100% European genes, and 70% of them having 100% African ones.

Previously, I had argued that the fact that Latin Americans, unlike Central Asian Turkic populations (such as the Uyghurs), have such a wide spectrum of ancestry proportions is due to the more recent admixture in the Americas than in Central Asia (less time for homogenization to take place), and the continued importation of Europeans.

Assortative mating is a third factor that may be behind this phenomenon. A stronger parallel may be found in South Asia, where the two constituents have been co-existing for a much longer time, but under a rigid, formalized regime of assortative mating (the caste system), homogenization has not taken place.

Genome Biology doi:10.1186/gb-2009-10-11-r132

Ancestry-related assortative mating in latino populations

Neil Risch et al.

Abstract

Background

While spouse correlations have been documented for numerous traits, no prior studies have assessed assortative mating for genetic ancestry in admixed populations.

Results

Using 104 ancestry informative markers, we examined spouse correlations in genetic ancestry for Mexican spouse pairs recruited from Mexico City and the San Francisco Bay Area, and Puerto Rican spouse pairs recruited from Puerto Rico and New York City. In the Mexican pairs, we found strong spouse correlations for European and Native American ancestry, but no correlation in African ancestry. In the Puerto Rican pairs, we found significant spouse correlations for African ancestry and European ancestry but not Native American ancestry. Correlations were not attributable to variation in socioeconomic status or geographic heterogeneity. Past evidence of spouse correlation was also seen in the strong evidence of linkage disequilibrium between unlinked markers, which was accounted for in regression analysis by ancestral allele frequency difference at the pair of markers (European versus Native American for Mexicans, European versus African for Puerto Ricans). We also observed an excess of homozygosity at individual markers within the spouses, but this provided weaker evidence, as expected, of spouse correlation. Ancestry variance is predicted to decline in each generation, but less so under assortative mating. We used the current observed variances of ancestry to infer even stronger patterns of spouse ancestry correlation in previous generations.

Conclusions

Assortative mating related to genetic ancestry persists in Latino populations to the current day, and has impacted on the genomic structure in these populations.

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