Jim Evans is a Bryson Distinguished Professor of Genetics and Medicine at UNC Chapel Hill. Here, he explains the science behind those baby blues (or browns or greens).
Q: Many of us learned in grade school biology that brown eye color comes from a dominant gene (needing one copy only) and blue from a recessive gene (needing two copies). If that’s the case, how can there be so much variation in eye color in the population, and even within the same family?
It turns out that more than a dozen genes control eye color. So depending on how variations in these genes are mixed and matched by your parents, just about any combination of eye colors can result in a given family. That said, the two genes that appear to be most responsible for determining a person’s eye color are called OCA2 and HERC2.
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Q: How does eye color work? How do we get not just brown and blue, but hazel and green?
Eye color is determined by the interaction of two things: the amount of a dark pigment (called melanin) that one has in his/her iris and the actual molecular structure of the iris, which scatters light. Individuals with a high concentration of melanin will have brown or black eyes.
But blue eyes are a different story – there is actually not a bit of blue pigment in the bluest of eyes. Rather, the blue color is caused by the same kind of light scattering that creates the appearance of a blue sky. It seems rather poetic to me that looking into the eyes can be, in a very real sense, like looking at the sky.
The wide range of eye colors that we see results from complex interactions among genes that encode pigment and various structural components of the eyes. Brown eyes are the most common eye color in humans while blue eyes are rare in mammals (Siamese cats are an exception). In people, blue eyes are rather unusual with only about 2 percent of the world’s population being blue-eyed. Green is the least common eye color.
Q: Why does eye color change after birth, and continue to change with age?
Children are usually born with lighter eyes than they will eventually have by the time they are a few years old, since melanin gradually accumulates after birth. Eye color is more stable later on but sometimes it can shift during puberty, pregnancy or old age, presumably due to changes in the amount of melanin and because of structural changes in the iris.
Q: What causes people to have irises with different colors?
This phenomenon is known as heterochromia and is usually due to an uneven distribution of melanin, the main pigment in the eyes. A number of causes are known, including genetics, trauma (it is said that this is why David Bowie has heterochromia) or certain medications.