giovedì 9 gennaio 2014

Pigmentazione chiara della cute.

January 09, 2014

SLC24A5 light skin pigmentation allele origin

From the paper:
Adjustment for undercounting is substantial, increasing the estimated age for the combined samples to 12.4 (95% confidence interval 7.6−19.2) kya. If mutation rates in recent humans are lower than predicted from the human-chimpanzee divergence (Scally and Durbin 2012), true ages will be even older. Our adjusted dates overlap those previously reported (Beleza et al. 2012) and are also consistent with the lower limit for the origin of A111T set by the finding that the Alpine “iceman” dated to 5.3 kya was homozygous for this variant (Keller et al. 2012).


Taking the 12.4ky estimate and multiplying by two (for the slower autosomal mutation rate) yields an estimate of 25ky, so it seems that this allele did not accompany the earliest modern human colonists of West Eurasia but emerged in some region and spread from there. It will be interesting to see (through ancient DNA) by what processes of migration, admixture, and selection this transpired.

G3 doi: 10.1534/g3.113.007484

Molecular Phylogeography of a Human Autosomal Skin Color Locus Under Natural Selection 

Victor A. Canfield et al.

Divergent natural selection caused by differences in solar exposure has resulted in distinctive variations in skin color between human populations. The derived light skin color allele of the SLC24A5 gene, A111T, predominates in populations of Western Eurasian ancestry. To gain insight into when and where this mutation arose, we defined common haplotypes in the genomic region around SLC24A5 across diverse human populations and deduced phylogenetic relationships between them. Virtually all chromosomes carrying the A111T allele share a single 78-kb haplotype that we call C11, indicating that all instances of this mutation in human populations share a common origin. The C11 haplotype was most likely created by a crossover between two haplotypes, followed by the A111T mutation. The two parental precursor haplotypes are found from East Asia to the Americas but are nearly absent in Africa. The distributions of C11 and its parental haplotypes make it most likely that these two last steps occurred between the Middle East and the Indian subcontinent, with the A111T mutation occurring after the split between the ancestors of Europeans and East Asians.