Feb 28, 2008 - Health + Traits

One SNP Makes Your Brown Eyes Blue

Paul Newman has them. So do Brad Pitt and Daniel Craig.

Every president since Richard Nixon has had them, too. (Editor’s note 1/2014: That was true at the time of this publication in 2008, but it is no longer the case. )

It seems blue eyes give a man an edge in Hollywood and Washington — now scientists know why they happen.

Three recently published papers (here, here, and here) report that a single SNP determines eye color specifically whether a person’s eyes will be blue; every blue-eyed person in the world has the same version. The findings also suggest that the blue-eyed version of the SNP can be traced back to a single ancestor that lived about 6,000 to 10,000 years ago.

It’s been known for a while that eye colors like green and hazel (deviations from the brown color found in the majority of people) can be explained by SNPs in a gene called OCA2. The protein made by this gene is involved in the production of melanin, a pigment found in the cells of the iris. This is the same pigment that gives your hair and skin their color. Darker eyes have more melanin than lighter colored eyes.

But none of the known variations in OCA2 could explain blue eyes. The new research seems to have solved the mystery. A SNP near OCA2, but not in it, determines whether a person will have blue eyes.

The SNP, rs12913832, is actually in a gene called HERC2. Scientists think that instead of affecting HERC2, the SNP controls how much protein will be made from the nearby OCA2 gene. Low levels of OCA2 protein, caused by the G version of the SNP, lead to lower levels of melanin, which in turn leads to blue eyes. 23andMe customers can check their genotype at this SNP in the Genome Explorer (now called Browse Raw Data) or in the Gene Journal (now called Health and Traits). (Note: In the Gene Journal you’ll see other SNPs also associated with eye color. The combination of these SNPs with the blue-eyed version of rs12913832 can end up giving a person green eyes instead of blue).

Across all three recent studies, blue eyes could be explained by the G version of rs12913832. This suggests that this version of the SNP can be traced back to a single source. The authors of one of the studies hypothesize that the mutation event that created this version of the SNP happened somewhere around the Black Sea during the Neolithic expansion (6,000-10,000 years ago) as generations of migrating farmers gradually carried agriculture from the Near East to northern Europe, where blue eyes are most often found (and where the participants from all three studies were from). These same researchers found that blue-eyed individuals from the Mediterranean (5 from Turkey and 2 Jordan) also have the G version of rs12913832, suggesting that blue eyes the world over can be explained by variation in this one SNP.

Now that scientists know how people got blue eyes, the next question is, why did they persist? The widespread nature of blue eyes in Europe suggests that it was a trait that was somehow selected for after it arose. But there is no known advantage to having blue eyes. In fact, some eye diseases are more common in people with light eyes.

Perhaps blue eyes persisted for much the same reason they are so prevalent among actors and presidents today — people find them attractive. Some theories posit that the frequency of the blue-eyes — and the particular version of the SNP that produces them — increased simply because people with blue eyes were considered more desirable mates.

It seems blue eyes might have given a man an edge back in the Stone Age, too.

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