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Amerijoe
06-27-2016, 12:10 PM
http://phys.org/news/2016-06-paper-current-gain-loss-heavy.html

ThirdTerm
06-29-2016, 01:55 AM
As for vitamin D production, while reduced exposure to UV rays may have played a role in the evolution of the extremely light skin found in residents of the northernmost areas of Europe and Asia, Elias and Williams propose that the moderate pigment reduction seen in Central European and Asian populations did not evolve to support additional vitamin D production. In support of this proposition, they cite studies by other researchers showing that even darkly pigmented individuals, though better protected from UV light, are still quite efficient at producing vitamin D.


It's unclear if Vitamin D deficiency has something to do with light skin alleles observed in Europeans and East Asians as often claimed by evolutionary scientists. The pigmentation variation in Europeans and East Asians is attributed to specific SNPs in the region of OCA2 and HERC2 (Donnelly et al. 2012).



The new paper also proposes that pigmentation was lost in central European and Asian populations because a pigmented skin barrier, which is metabolically expensive to produce, became less important.


The light skin allele of rs1800414 at OCA2 is found almost exclusively in East and Southeast Asia (up to 76%). The three-SNP haplotype system at OCA2 BEH1 (rs4778138, rs4778241, and rs7495174) is associated with blue eyes. There is no known biological advantage to having blue eyes and light skin mutations such as OCA2 and HERC2 may have occurred randomly to some of our ancestors as a form of albinism. Mutations in OCA2 are associated with albinism.

http://s33.postimg.org/ewo4e7x3j/snp21.png



Mutations in OCA2 are known to cause oculocutaneous albinism type 2. However, the gene is also known to play a role in variation in normal pigmentation. In European populations, it is primarily associated with blue irises. Several sites in and around OCA2 have been reported to be the functional variant or to be tightly linked to the functional variant leading to blue eyes. These sites include a three-SNP haplotype (rs4778138, rs4778241, rs7495174) and four individual SNPs, rs1129038, rs12913832, rs916977, and rs1667394 (Duffy et al. 2007; Sturm et al. 2008; Kayser et al. 2008; Sulem et al. 2007; Mengel-From et al. 2010; Walsh et al. 2010). Four of the SNPs (rs1129038, rs12913832, rs916977, rs1667394) are actually located in introns of the Hect Domain and RCC1-like Domain 2 (HERC2 [MIM 605837]), which are located 10 Kb upstream of OCA2. These are thought either to be located in or near an upstream regulatory region of OCA2 or to be in linkage disequilibrium (LD) with functional elements in HERC2 and affect a possible HERC2 regulation of OCA2. The actual function of HERC2 is unknown but it shows homology to known E3 ubiquitin-protein ligases. One of the HERC2 SNPs (rs1667394) has been associated with blond hair in Europeans (Sulem et al. 2007). Specific polymorphisms and the haplotypes are illustrated in Fig. 1; all 21 SNPs studied are listed in Table 2. The derived allele of another SNP at OCA2, rs1800407, has been associated with green/hazel eyes in Europeans (Branicki et al. 2009). Rs1800407 is an arginine to glutamine missense mutation (Arg419Gln) found in exon 13 of the OCA2 gene. Sturm et al. (2008 ) concluded that the derived allele of rs1800407 increased the penetrance of the blue eye phenotype associated with the derived allele of rs12913832.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3325407/