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Scientist: All Blue-Eyed People Are Related

If you've got blue eyes, shake the hand of the nearest person who shares your azure irises: He or she may be a distant cousin.

Danish researchers have concluded that all blue-eyed people share a common ancestor, presumably someone who lived 6,000 to 10,000 years ago.

"Originally, we all had brown eyes," Professor Hans Eiberg of the University of Copenhagen said in a press release. "But a genetic mutation affecting the OCA2 gene in our chromosomes resulted in the creation of a 'switch,' which literally 'turned off' the ability to produce brown eyes."

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That "switch" — a simple change from "A," or adenine, to "G," or guanine, in the DNA — actually sits next to the OCA2 gene, which regulates the pigmentation of our eyes, hair and skin, and hence has only a limiting effect on it.

If the mutation had completely deactivated OCA2, all blue-eyed people would be albinos.

Eiberg and his team analyzed 155 individuals in a large Danish family, plus several blue-eyed people born in Turkey and Jordan.

All blue-eyed subjects had the mutation, and there was very little variation on the genes neighboring it on the chromosome, indicating that the mutation first arose relatively recently.

In contrast, most mammals share the "normal" form of the gene. The six-letter sequence is exactly the same among mice, horses, cows, rats, dogs, cats, monkeys, chimpanzees and humans with brown eyes. (No word on what gives Siberian huskies and Siamese cats blue eyes.)

Eiberg figures the mutation took place on the northern of the Black Sea, but that's an educated guess, assuming the first blue-eyed humans were among the proto-Indo-Europeans who subsequently spread agriculture into western Europe and later rode horses into Iran and India.

Ironically, neither the first person to have the mutation, nor his or her children, would have had blue eyes themselves.

Blue eyes are a recessive trait, and the gene must be inherited from both parents. (Green eyes involve a related but different gene, one that is recessive to brown but dominant to blue.)

It wasn't until the original mutant's grandchildren or great-grandchildren hooked up — cousin marriage is the norm through most of human history — that the first blue-eyed person appeared. He or she must have looked pretty odd for the Neolithic era.

Eiberg stresses that the genetic variation, as the press release puts it, is "neither a positive nor a negative mutation."

That's a bit disingenuous, as the mutation also produces greater instance of blond hair (sexually selected for even today) and fair skin, which confers a survival advantage by stimulating greater production of vitamin D in sun-starved northern European countries — exactly where blue eyes are still most prevalent.

• Click here for the University of Copenhagen press release, and here for the full journal article.