Expert: John Locke - 5/26/2007

Question
My husband is a fraternal twin, he has blue eyes and his brother brown,along with the rest of his family.I have grey green eyes.My son of 14 months has brown eyes, is this possible?

Answer
Thanks for using AllExperts, Heather.

I can answer your question in two ways: yes, it is possible in the sense that it has happened; your son has brown eyes, so it is a genetic possibility for your son to have brown eyes along with the other eye colors in your family. Second, I can describe how it would happen, or what makes it happen,, which I think is the more informative answer.

Human eye color is controlled by three genes, which are specific sections of DNA inside every cell, human or otherwise. The DNA that controls eye color tells the cells of the eye to make specific pigments in the iris, or the colored part of the eye. These pigments are what we judge as eye color; several different colors of pigments can be present in one eye. More on this below.

We inherit half of our DNA from each parent, so there are two copies of each of these three genes, for six total copies (2 copies x 3 distinct genes). Each gene, separately, can be one of three different types (different types of genes are called alleles): blue, brown, or green. These are considered "pure" colors, in the sense that any allele can only be brown, blue, or green: there are no alleles for hazel eyes, gray eyes, violet eyes, etc. If all six alleles are of the same color--brown, blue, or green--then that person will have pure brown, blue, or green eyes, respectively. Any other combination, however, will result in a mixed eye color: hazel is a combination of brown and green, for instance. There are some more complicating factors (there are other, poorly understood genes that contribute to darker or lighter eye color that are really beyond the scope of this discussion), but those are the basics--and they're enough to predict most human eye colors.

The specific number of each type of allele affects the resulting eye color. To take an example, blue-green eyes result from a mixture of those two colors, obviously. If there are, say, four blue alleles and two green alleles, then the eye color will be closer to blue than green. If there are five green alleles and one blue allele, then the eye color will be very close to pure green--but not quite.

In your case, your son has almost certainly inherited a mix of alleles for brown, green, and blue eye color; the combination likely contains more brown and green pigments than blue ones, with the result being that your son appears to have brown eyes. I will direct you to a website that works out eye color probabilities: it gives a nice visual way to see the different probabilities for eye colors and how the different genes combine to produce eye color. Enjoy!

http://museum.thetech.org/ugenetics/eyeCalc/eyecalculator.html