Scientists used to think that inheritance of eye color followed simple, Mendelian rules, which held that there was a gene for brown eyes that was dominant and one for blue eyes, which was recessive.
It turns out that things are a bit more complicated than that. “It’s not that there is a brown pigment and a blue pigment and a green pigment. There is just one pigment, melanin, and how much you have determines the color of your eyes,” said Dr. Bonnie Henderson, director of the comprehensive ophthalmology service at Massachusetts Eye and Ear Infirmary.
People who are albinos, for instance, have no melanin at all, and their eyes look pink because of blood vessels in the back of the eye.
People with blue eyes have very little melanin, while people with brown eyes have a lot. People with green eyes are somewhere in between.
So far, scientists have discovered six genes for eye color, and there are undoubtedly more, said Nicholas Katsanis, an assistant professor of ophthalmology at the Institute of Genetic Medicine at Johns Hopkins Medical Institutes. Each of these genes contributes in a different way to the “life cycle” of melanin -? the amount produced and recycled within eye cells.
“Imagine a hat with different colored marbles inside,” Katsanis said. “You put your hand in, get a bunch of colors. Eye color is like that. It’s a combination of colors that is quasi-random but is controlled by the six genes you inherit from your mother and the six you get from your father.”