SNPwatch: One SNP Makes Your Brown Eyes Blue

SNPwatch gives you the latest news about research linking various traits and conditions to individual genetic variations. These studies are exciting because they offer a glimpse into how genetics may affect our bodies and health; but in most cases, more work is needed before this research can provide information of value to individuals. For that reason it is important to remember that like all information we provide, the studies we describe in SNPwatch are for research and educational purposes only. SNPwatch is not intended to be a substitute for professional medical advice; you should always seek the advice of your physician or other appropriate healthcare professional with any questions you may have regarding diagnosis, cure, treatment or prevention of any disease or other medical condition.

jessejames9.jpgPaul 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 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.

Photo courtesy of Warner Brothers

  • This brown-blue problem is a typical light biology issue that can make headlines but not any deep understanding on what’s goin’ on in our nucleotides.

  • Subho

    I agree with Attila from a scientific point of view but disagree from a commercial point of view. When customers buy a service, they care more about what they get out of it, rather than the underlying intricate science or technology. So, I think the ‘Blue-eye light biology’ will attract as many customers as the ‘susceptibility for say, breast cancer’.

  • ok. but, what about people like me whose eyes change color at certain times of day@ my eyes change from blue-green to blue-gray in the afternoon. is that genetic? my brothers eyes change usually when mine do as well. but my sister has brown hazel eyes.

  • @ Katerina

    Your eyes might appear to change color, but really what’s happening is that you are noticing changes in how light reflects offs the pigments in your irises.

    Eye color gets set when we’re pretty young. Babies are often born with blue-ish eyes that then darken to a final brown color. And there are some medications that can darken a person’s eyes. But other than that, changing eye color is really a kind of optical illusion.

    But, in a way, your eyes that appear to change color are genetic: your genes determine the mix of pigments in your irises, and these are what are determining the colors you see.

    Thanks for writing!

  • Dee

    I am just curious. My son has gray eyes. Are gray eyes a variation of blue or brown? I have brown with green and my husband blue, as does our daughter.

    • Hi Dee,

      That’s a great question, and it turns out that scientists are still figuring out exactly how gray eye colors are produced. (The reasons for brown vs. blue and green vs. blue eyes are better understood.) It has to do with the amount of melanin produced in the different parts of the iris, which is definitely influenced by genetics. And pretty much all eye colors are a variation of that same underlying theme. See here for a nice in-depth explanation of how eye colors work:

  • ilyse

    My Fiancés’ 23 and me results came back saying that he has dark eyes. His eyes are between a Greyish Blue and Green. Nobody would ever say he has dark eyes, not even slightly. Any idea why this happened?

    • 23blog

      Our trait reports on such things as eye color reports on the genetic likelihood to have higher or darker eyes.It’s reporting out the probability for that trait, not whether you have that trait. There is also an accompanying scientific detail that explains who we make that prediction, the genes involved and other influences on eye color. If you don’t have access to a report or are not a customer, you can see a sample of our trait reports here:
      Just scroll down the trait section. How this helps.

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