Genetics of eye color

http://genetics.thetech.org/ask/ask29

July 2, 2004

These are excellent questions. People are often very confused by eye color genetics because reality seems to fly in the face of the simple genetics we are taught in school.

First, the answer is yes to both questions: two blue-eyed parents can produce green or brown-eyed children. Eye color is not the simple decision between the brown (or green) and blue versions of a single gene. There are many genes involved and eye color ranges from brown to hazel to green to blue to.

How does eye color work? Eye color comes from a combination of two black and yellow pigments called melanin in the iris of your eye. If you have no melanin in the front part of your iris, you have blue eyes. An increasing proportion of the yellow melanin, in combination with the black melanin, results in shades of colors between brown and blue, including green and hazel.

What we are taught in high school biology is generally true, brown eye genes are dominant over green eye genes which are both dominant over blue eye genes. However, because many genes are required to make each of the yellow and black pigments, there is a way called genetic compensation to get brown or green eyes from blue-eyed parents.

The best way to illustrate how this might happen is with an example. Let's say there is a genetic pathway made up of four genes (cleverly named A, B, C, and D) that are needed to make brown eyes. A mutation in both copies of any one of these genes results in blue eyes (these mutations are denoted with lower case letters, a, b, c, and d).

Now let's say that dad has blue eyes because of a mutation in both his copies of gene A and mom because of a mutation in both her copies of gene D. As I am sure you know, we have two copies of each gene, one from our mom and one from our dad. If either parent gives you a brown version of a gene, it will be dominant over the blue copy.

Let's suppose that mom gives you a brown copy of gene A and dad gives you a brown copy of gene D. What color eyes would you have? Brown. (The same argument works for green eyes as well.)

Another common genetic process that could be responsible for brown eyes from blue-eyed parents is called recombination. When eggs and sperm are made, only one of a pair of chromosomes gets put into an egg or sperm. Before this happens, there is a bunch of DNA swapping that goes on between the pair of chromosomes. Sometimes when the DNA is swapped or recombined, DNA mutations get fixed.

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[See link above for other mechanisms that can produce unexpected eye colors.]