Friday, November 20, 2009

Latin America is not panmixia   posted by Razib @ 11/20/2009 03:35:00 PM

A new provisional paper, Ancestry-related assortative mating in latino populations. Here are the 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.

The correlations are to the left. An interesting point is that the correlations of total genome content seem too high to be explained by assortative mating for salient physical features (skin color, hair form, etc.) alone. From the text:
Another possibility involves physical characteristics, such as skin pigment, hair texture, eye color, and other physical features. Certainly, these traits are correlated with ancestry and are likely to be factors in mate selection. However, the spouse correlation for these traits must be high and the correlation of these traits with ancestry must also be high to explain the observed ancestry correlations....


If the spouse trait correlation is 0.6 (a reasonably high value), then for a spouse ancestry correlation of 0.3 (Puerto Ricans), the trait-ancestry correlation is 0.7; for a spouse ancestry correlation of 0.4 (Mexicans), the trait-ancestry correlation is 0.8. Previous studies on assortative mating in Latin American groups have retrieved correlation coefficients of 0.29 to 0.46 for education level, 0.48 for skin reflectance, 0.07 to 0.18 for eye and hair color, and 0.16 to 0.24 for different anthropometric measurements

As noted above, they controlled for SES and geography, and the correlation remains. Looking at the correlations within the genomes of these individuals they also inferred that assortative mating in the past was actually greater than it is today (they also have a historical citation which suggests this). I wonder of the correlation of ancestry is due to sorting by many traits which are subtle and nuanced, and relatively difficult to capture in surveys of the coarse salient traits are used to categorize phenotypic races. Looking at many traits, as opposed to a few, and one would have a better sense of total genome content. When it comes to mating one might look to a range of traits which in other circumstances are not noted, or fall below the threshold of reflective awareness. I'm assuming there might be something here which is Gestalt and subconscious. Kind of like the various studies which attempt to correlate mate preferences by HLA polymorphism.

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