Ask a Geneticist

by Ky Sha, Stanford University

How did I get green eyes? I understand that blue is recessive and that in theory all children of two blue-eye parents should have blue eyes. But, what about green? My parents are blue-eyed and have 2 green-eyed children and 1 blue-eyed child. And yes, he is my father. My high school Bio teacher embarrassed me by announcing to the entire class that he couldn't be when we studied eye color. So, how did I end up with green eyes? Thanks!

Is there any possible way for two blue-eyed parents to have a green or brown-eyed child?

Is it possible for a green-eyed person and a blue-eyed person to have a brown-eyed child?

One day my father's first cousin and I were discussing our Swedish background. This led to discussion of eye color. She said, "You know, both my mother and father had blue eyes." I looked into her eyes and they were dark brown! I thought I knew enough genetics to know that blue + blue yields ONLY blue eyes. I called my father that night (a physician) and he said he had never thought about it, but now that I mentioned it, my great-grandparents used to tell the story that her mother had thought she had been given the wrong baby at the hospital in Chicago (1927) Her sister had blonde hair and blue eyes and used to tease her about being adopted (she wasn't). She had bright red hair and brown eyes. She must have been the wrong baby!? Can blue eyes + blue eyes = brown eyes? I would never tell her what I suspect, but I am curious for my own sake. I suspect that in downtown Chicago in 1927, she was actually an "O'Brien" instead of a Peterson.

-Curious adults from California, Pennsylvania, Kentucky, and the United Kingdom

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.

Genetic Complementation

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.

Recombination

Again, an example can show how this might work. Imagine dad has blue eyes because of a mutation at the front end of one copy of his eye color gene and a different mutation at the back end of the other copy of the gene. Each gene has a single mutation but at different places in the gene (see Figure 2).

Now imagine that when his sperm is being made, the middle part of the eye color gene is switched between the two genes resulting in one brown eye gene and one blue eye gene with two mutations. Now dad can produce a brown-eyed child. (Again, the same argument works for a green eye gene as well.)

Another way to get brown eyes from blue-eyed parents is for something in the environment to affect the eye color gene. Even though there are well-documented cases in which this happens, the reasons for it are pretty poorly understood.

There are cases, for example, of certain drugs changing a person's eye color—the environment clearly has changed what happened to eye color in this case. Another possibility is that a gene is on or off for some reversible reason instead of an irreversible change in DNA. In this case, something in the environment reverses the change, turning the eye color gene back on or off.

Well, I hope this helped answer your question. As you can see it is all pretty complicated. It would be great if high school biology classes would stop confusing us by teaching eye color as a single trait.

Ask a Geneticist

More Information