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Two major genes on chromosome 15 affect the quantity and quality of the melanin produced by melanogenesis. HERC2, a large ubiquitin ligase, contains the promoter region for OCA2, the P protein. When a T is replaced with a C in rs12913832 of intron 86, OCA2 transcription is depressed, resulting in a blue-eyed individual. This epistatic relationship demonstrates the significance of introns and how a single-base change greatly affects an aspect of the individual. OCA2 contains regions for the numerous eye colors, but one SNP is a strong predictor for brown/blue eyes. A change in rs1800407 causes a change in the protein, Arg419Gln, and a change from brown to blue eyes. Therefore, the residue change causes a problem with the P protein, and melanin maturation decreases. Aside from these two genes, the genes involved in melanogenesis and other minor genes also contain regions that account for eye color. Having little effect on eye color, many of them deal primarily with hair and skin pigmentation. Lastly, disorders involved in eye color include ocular albinism and heterochromia. Depending on how little pigment the melanocytes produce, albinism causes red or violet eyes. Producing multicolored irises, heterochromia stems from mutations in certain cells of the iris.
https://www.nature.com/articles/jhg2010126
We report that the blue-eye associated alleles at all three haplotypes were found at high frequencies in Europe; however, one is restricted to Europe and surrounding regions, while the other two are found at moderate to high frequencies throughout the world..
Two pigmentation genes called OCA2 and HERC2 on chromosome 15 are primarily responsible for human eye colors. Most East Asians only have the OCA2 gene and they naturally have brown eyes, while the frequency of HERC2 in East Asia is less than 2%. The HERC2 gene acts as a promoter of the OCA2 gene to trigger a change from brown to blue eyes. The frequencies of the HERC2 gene are more than 90% in northern Europe and less than 50% in southern Europe. The Nature paper (White and Rabago-Smith 2011) explains the mechanics behind human eye colors.
Allele frequencies of rs12913832 in human populations surveyed as part of the Human Genome Diversity Project. The SNP, rs12913832, is in the HERC2 gene.
Haplotype name | SNPs included | Blue-eye associated allele |
---|---|---|
BEH1 | rs4778138, rs4778241, rs7495174 | ACA |
BEH2 | rs1129038, rs12913832 | TG |
BEH3 | rs916977, rs1667394 | CA |
So does this mean that these three "haplotypes" evolved from the same source or at entirely different contexts? I'm still not sure if this means that those central Africans with lighter eyes evolved the light eyes independently from everyone else
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