Recent work points to the OCA2 gene as the key gene in deciding between brown and other color eyes. The green eye gene has not yet been found.
What Is Eye Color?
Eye color is really the result of how much pigment is in the front part of the iris of the eye. Lots of pigment gives brown eyes, some gives green eyes and little or no pigment leads to blue eyes.
The amount of pigment in this part of the eye is determined by at least two genes (and probably more). The dominant gene in eye color is called OCA2.
OCA2 Distinguishes Brown from Blue
OCA2 is like a lot of other genes in that it comes in at least two forms—a working and a nonworking version. As long as someone has at least one working version, they’ll have brown eyes. (Remember we have two copies of most of our genes—one from mom and one from dad.)
This is why brown eyes are said to be dominant over blue. It takes just one good OCA2 gene to end up with brown eyes.
A recent study by Sturm and associates at the Institute for Molecular Bioscience at the University of Queensland in Australia definitively identified OCA2 as the key brown eye gene. The researchers found that people with blue eyes had one of at least three different changes in the OCA2 gene that led to not having brown eyes.
Green Eyes vs. Blue
Scientists still don’t know exactly how green eyes happen. They suspect that a separate gene causes green eyes and have even named this unidentified gene gey (green eye). OCA2 and gey are thought to work together to produce brown, green, and blue eyes.
Having two genes involved makes eye color predictions very tricky. But there are excellent sites that can help expectant parents predict their child’s eye color. Here are two:
There Are More Than Two Genes Involved in Eye Color
Scientists have suspected for a long time that more than two genes are involved in eye color. For example, there are more colors than just brown, green, and blue and OCA2 and gey do not explain these other colors very well.
Also, two blue-eyed parents can sometimes have a brown-eyed child. This shouldn’t be possible but does happen. The best explanation is that there is another gene involved.
Eye color can also be affected by genes that affect pigmentation elsewhere. People with albinism tend to have light eyes. People with Waardenburg syndrome can have part or all of one or both eyes end up blue.
The genetics of eye color is very complicated but is becoming better understood. Scientists continue to make progress but it will be some time before expectant parents can confidently predict the eye color of their future children.
