What determines eye color?
Eye color is determined mainly by how much melanin pigment is in the iris and how light scatters in it. Many genes are involved, not just one.
The iris has a front layer that can contain varying amounts of melanin, the same pigment that colors skin and hair. Brown eyes have a lot of melanin. Blue eyes have very little, and they look blue because light scatters in the iris, similar to the way the sky looks blue. Green and hazel eyes have moderate amounts.
Scientists once taught that eye color was controlled by a single gene, with brown dominant over blue. We now know that at least 16 genes play a role, especially two called OCA2 and HERC2. That is why two blue-eyed parents can occasionally have a child with brown or green eyes.
How common is each color?
Brown is by far the most common eye color worldwide, found in about 70 to 80 percent of people. Blue eyes are most common in Northern Europe. Green is one of the rarest colors, seen in about 2 percent of people. Many babies of European descent are born with blue eyes that darken during the first year.
Did You Know?
Some people have two different colored eyes, a condition called heterochromia. It can be present from birth or, more rarely, caused by injury or illness.
This answer is general information, not medical advice. For a personal health question, talk to a doctor.