Blue eyes correlate with lighter skin, OCA2 & HERC2
The story about HERC2 & OCA2 is getting a lot of press; that is, the genetics behind how people have blue eyes. But see this in ScienceNow:
There are still large questions, though. Why did blue eyes persist? Scientists say it is difficult to see how eye color would have an environmental advantage, as skin color does. Some theories suggest that women may have played a role in driving the selection. Perhaps, Kayser says, “the females thought it more exciting to have a male with blue eyes.”
I already posted this before: the SNPs which are used to predict blue eyes also track skin color variation. In other words, pleiotropy. This shouldn’t be a surprise, OCA2 is a pigmentation locus which in many cases doesn’t exhibit tissue specific expression patterns; its name derives from the fact that some forms of albinism are associated with mutants on it. In any case, some concrete data about skin color and the OCA2 SNPs can be found in a previous paper from a research group behind one of the current publications, A Three-Single-Nucleotide Polymorphism Haplotype in Intron 1 of OCA2 Explains Most Human Eye-Color Variation. Look at table 1, and you find these data:
| Genotype | Fair skin | Medium skin | Olive skin |
| Blue/Blue | 46.5 | 46.1 | 7.4 |
| Blue/Brown | 31.3 | 52.2 | 16.6 |
| Brown/Brown | 25.6 | 37.9 | 37.0 |
Let’s do something with the numbers. Give fair skin a trait value of 1, medium skin 2, and olive skin 3. Then generate an average value for each genotype by weighting appropriately, and divide by the number associated with heterozygotes. This is what I get:
| Genotype | Average Value |
| Blue/Blue | 0.866 |
| Blue/Brown | 1 |
| Brown/Brown | 1.14 |
Looks additive for skin color, doesn’t it? Since blue eyes as a trait seems to exhibit strong recessivity HERC2/OCA2 derived variants are unlikely to have initially been selected for that phenotype. It could be something besides skin color, but that is the most plausible abduction at this point from where I stand (we know that selection was powerful on the locus).
Related: Genome-wide associations, HERC2 and eye color, 1 SNP to rule them & in the darkness bind them?, Why do you have blue eyes?, HERC2 & blue eye color & Danes and OCA2, blue eyes and skin color.
Labels: Genetics, Pigmentation
the females thought it more exciting to have a male with blue eyes
BLue eyes is an abnormality. Its not romantic. Please.
During winter season and you cant do anything, theres nothing else to do but have sex, even with a blue-eyed albino. What choice you have if all the other normal eyed guys are already taken.
:P
If the OCA2 gene had been completely shut down, our hair, eyes and skin would be melanin-less, a condition known as albinism.
http://www.msnbc.msn.com/id/22934464/wid/11915773?GT1=10914
Razib,
The correlation between eye color and skin color may simply be an artefact of geographic origin. Europeans vary clinally for both eye color and skin color along a north-south and west-east gradient, so if the pool of subjects is geographically heterogeneous you will almost certainly get a correlation between eye and skin color. But this doesn’t prove a cause and effect relationship.
In this study, the subjects were white Australians. The authors state that the “Subjects were overwhelmingly (over 95%) of northern European origin (mainly Anglo-Celtic).” Fine. But even within the British Isles, there is significant geographic variation for both eye and skin color. And even if only 4% of the subjects were of Mediterranean origin (Italian, Greek, Maltese, etc.), they would still have made up one third of all ‘olive-skinned’ subjects.
Did Greg Laden ever provide any documentation that blue eyes are found among pygmies?
BLue eyes is an abnormality. Its not romantic. Please.
please shy away from vaue-laden language. some of us like blue eyes fine.
If the OCA2 gene had been completely shut down, our hair, eyes and skin would be melanin-less, a condition known as albinism.
there is plenty of tissue specific expression recall. so a pedantic note.
The correlation between eye color and skin color may simply be an artefact of geographic origin. Europeans vary clinally for both eye color and skin color along a north-south and west-east gradient, so if the pool of subjects is geographically heterogeneous you will almost certainly get a correlation between eye and skin color. But this doesn’t prove a cause and effect relationship.
sure. but do you really think that substructure within the british isles can account for this? and the trait, if not due to substructure, is perfectly additive. i’m just skeptical that a recessive trait would show these selection coefficients, but someone who knows the summary statistics that crop up around this locus could tell me if my issues are unfounded re: recessivity. additionally, we know that OCA2 has skin color effects in other contexts i. and there are various alleles under selection in east asia too from what i recall. finally, the study here is on twins, so that should reduce the subscture effect some since half the sample is related to the other half 0.50 and obviously their geographic ancestry would match (also, previous work by this group found additive action on OCA2, that doesn’t refute your geography argument, but it does suggest that is a reproducible finding, though it was the same sample so that weakens the point).
that being said, it’s an open question easily answered by admixture analysis in african americans. i’ve asked around so perhaps we’ll have an answer soon.
Did Greg Laden ever provide any documentation that blue eyes are found among pygmies?
no. personal communication, he did his field work with the. so either telling the truth, lying, or he’s deluded.
also,
The genetic architecture of normal variation in human pigmentation: an evolutionary perspective and model: A joint effect of MC1R and OCA2 on skin pigmentation in Tibetans has also been demonstrated
DCT, MC1R, LYST, EDA, OCA2 and ATRN also show relatively strong signatures of distinct East Asian selective events.
Signatures of Positive Selection in Genes Associated with Human Skin Pigmentation as Revealed from Analyses of Single Nucleotide Polymorphisms:
In six of these genes we detected, by means of the EHH test, variability patterns that are compatible with the hypothesis of local positive selection in Europeans (OCA2, TYRP1 and KITLG) and in Asians
(OCA2, DCT, KITLG, EGFR and DRD2…
there’s no real eye color variation in east asians, so skin color is the best explanation for sure. but think that these data in east asians alter the posterior probability for the alternative hypotheses of skin color & association vs. skin color & substructure in europeans too. it is theoretically possible for tissue specific expression to be perfect, but most of the other pigmentation related genes turn out additive with large, but not majority, effect of trait value variation. the fact that KITLG polymorphism is strongly predictive of blondism, but also a major effect on skin color, increases my confidence about OCA2 since i assume the same dynamics are at work here (KITLG was studied in an admixed african american population so i don’t think population substructure can be assumed to account for this).
i will post any responses i get to people i’ve emailed ASAP.
ok…before i get an email back, all i can say after skimming the literature is
1) OCA2 is suggested as a secondary effect skin color locus (i.e., smaller than SLC24A5).
2) but i don’t see analysis in an admixed pop. like african americans. so the substructure response stands. looks like the don’t get more narrow than say a regional/national european population.
twin sample irrelevant. they took only from one twin.
Blue eyes is an optical effect due to loss of melanin in the iris. Since the light is not absorbed by melanin, blue light scatters in the lens. Diffraction of blue light is the same reason the sky appears blue. The mutation occured in a single individual around 6 to 10 thousand years ago. This is determined by analysis of mitochodrial DNA. Sperm does not contain mitochodria so the DNA is only inherited from the maternal side an undergoes no gene rearrangements. Mutations in mitochodrial DNA can act as a method for determining approximately when the blue eyed allele appeared. The high prevelance in populations in Europe indicate a strong selection for the blue eyed allele. This is an example that natural selection works on reproductive fitness and not necessarily physical fitness. Obviously, blue eyes was considered attractive since it confers no physical advantage to the individual yet the blue eyed allele was highly selected. Light skin can confer an advantage for production of vitamin D in areas of low sunlight but blue eyes is purely cosmetic.
Obviously, blue eyes was considered attractive since it confers no physical advantage to the individual yet the blue eyed allele was highly selected.
The problem is that the genes involved seem to be expressed in other areas. First of course there is the possibility of some involvement with skin color that razib already mentioned. But there appear to be other differences, see for example this older post: http://www.gnxp.com/MT2/archives/002275.html . In short you shouldn’t look at some phenotypic feature in isolation when you know that it’s coming from a gene that may be expressed elsewhere (see also: MC1R)…