Ol' blue eyes.
Scientists now know the molecular basis of Frank Sinatra's nickname.

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Don't It Make Your Brown Eyes Blue?

Blue-eyed? Thank a genetic switch that turns off your body's ability to make brown pigment in your peepers. Researchers have finally located the mutation that causes blue eyes, and the findings suggest that all blue-eyed humans share a single common ancestor born 6000 to 10,000 years ago.

Researchers have implicated the OCA2 gene in several eye colors. The gene is involved in the production of melanin, a pigment that gives hair and skin their hues. It also codes for brown eyes and can lead to green or hazel eyes when mutated. Despite years of searching, however, scientists have not found a mutation for blue eyes on the gene.

It turns out they were looking in the wrong place. Trying to narrow the site of the mutation, gene mapper Hans Eiberg of the University of Copenhagen and colleagues examined members of a large Danish family, an approach that allowed them to follow DNA as it passed from one generation to another. Then, by 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.

The mutation works like a switch that regulates the OCA gene, the team reports in the January issue of Human Genetics, turning off the production of brown eye color and allowing blue eyes to shine through. 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.

Sturm says that it's not uncommon for one gene to regulate another, but it is difficult to locate the mutation in the controlling gene. One of the most cited examples is the mutation involved in lactose tolerance, which is also caused by regulation from outside the gene. Sturm says that such regulating genes may contribute more to genetic diversity than previously thought.

The findings also have applications in forensics. Geneticist Manfred Kayser of the Erasmus University Medical Center in Rotterdam, the Netherlands, an author of the other paper that appears in The American Journal of Human Genetics, says if police fully understood mutations behind eye color, for example, then they could use them to determine the eye color of a suspect based solely on DNA evidence.

Geneticist Tony Frudakis of DNAPrint Genomics Inc., a Sarasota, Florida, company that develops genetic-testing products, is shocked that the mutation happened just once. Although there are about 10 ways to get someone with red hair, the scientists found only one way to get someone with blue eyes. "I would have thought blue eyes arose several times independently," Frudakis says.

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."

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