Sunday, August 03, 2008

Blue eyes

The question of eye color seems to interest quite a few people. Previous discussions (here, here) have been among the most popular.

The actual genetics behind eye color remained somewhat obscure until February of this year. Although it's no longer new news, the findings are worth mentioning.

Humans certainly aren't the only animals having variable eye color. But the surprising thing is that (according to the primary author of the new research), until just a few thousand years ago, blue-eyed humans would have been very rare, at best.

Blue-eyed Humans Have A Single, Common Ancestor (1/30/08)
New research shows that people with blue eyes have a single, common ancestor. A team at the University of Copenhagen have tracked down a genetic mutation which took place 6-10,000 years ago and is the cause of the eye colour of all blue-eyed humans alive on the planet today.

“Originally, we all had brown eyes”, said Professor Eiberg from the Department of Cellular and Molecular Medicine. “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”. The OCA2 gene codes for the so-called P protein, which is involved in the production of melanin, the pigment that gives colour to our hair, eyes and skin. The “switch”, which is located in the gene adjacent to OCA2 does not, however, turn off the gene entirely, but rather limits its action to reducing the production of melanin in the iris – effectively “diluting” brown eyes to blue.

Note what this is saying. It is melanin that produces brown eyes, and melanin production is controlled by the OCA2 gene. A person in whom melanin isn't produced because both copies of OCA2 are faulty will not only not have brown eyes, but will not have brown color anywhere in the skin. However, if there is a certain mutation in both copies of a gene (known as HERC2) adjacent to OCA2 – not in OCA2 itself, as some accounts incorrectly state – production of melanin due to OCA2 will be reduced enough to produce blue eyes, while in skin there is still enough melanin produced to allow some brown coloration.

The reasoning that all people who now have blue eyes descended from a single individual who lived 6-10,000 years ago is based on different evidence, but it's a little more speculative. Another account outlines the argument:

Don't It Make Your Brown Eyes Blue? (2/1/08)
[B]y comparing people with brown or blue eyes, including people from Jordan and Turkey, the researchers were able to pinpoint the exact mutation. It wasn't on the OCA gene but rather on a nearby gene called HERC2. ...

Because blue eye color is found almost exclusively in people of European descent, Eiberg's team speculates that the mutation traces back to the Neolithic expansion, when people in the Black Sea region migrated to northern Europe 6000 to 10,000 years ago.

Two other studies, both appearing in this month's issue of The American Journal of Human Genetics, examined blue eyes in different populations and found the same mutation. The researchers also suggested a common ancestor for all blue-eyed individuals. These teams, however, did not estimate an age for the mutation. Geneticist Richard Sturm of the University of Queensland in Brisbane, Australia, an author of one of the papers says that someday scientists may find additional mutations that cause blue eyes but for now, the signs point to a single change.


Further reading:

One Common Ancestor Behind Blue Eyes – 1/31/08 article at LiveScience

The Family Tree of Blue-Eyed Individuals (2/6/08) – blog article that gives a bit more of the genetic details

Blue eyed people have a single, common ancestor (2/3/08) – another article, which gets some of the details wrong

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