Expert: Dana Krempels, Ph.D. - 6/20/2007

Question
I am not asking for exact color, just light or dark.  My parents and all my 4 of my siblings have blue eyes.  My husband and his dad have dark brown eyes and his mom and sister have blue. I have blonde hair, fair skinned everyone else in my family have light brown hair.  My husband has dark brown hair as do his sister and his dad, but his mom has blonde.  I would love my baby no matter what color hair or eyes, just curious because I love kids with light eyes and dark hair.

Answer
Dear Jennifer,

Eye and hair color in humans are both complex traits, each controlled by more than one gene locus.  

Let's do eyes first.

The best known eye color gene locus codes for "brown" or "blue," but other genes contribute other pigments (e.g, yellow pigment overlying a "blue" iris will create green), and other genes will control *how much* of each type of pigment is deposited in the iris.  The combination of these genes results in a range of human eye colors that one scientists cataloged at 149 different possibilities!

If you have blue eyes, though, your side of the genetics equation is simple:  You will donate one blue eyed allele (form of the blue/brown gene locus) to your baby.  If you don't have any greenish tinge to your eyes, then you probably lack the pigment deposition genes for melanin or other pigments (carotenoids), so blue is blue.

If your husband has brown eyes, but his mother and sister have blue, that means that he is heterozygous for the blue/brown gene:  he has one copy of each.  That means he has a 50% chance of donating the brown version to a baby, which would mean the baby will have brown eyes, and a 50% chance of donating the blue version, which would mean the baby will have blue eyes.

If his family members also have some of the yellow pigment genes "hidden" behind the dark brown, the baby may also develop green, hazel, or any variation as s/he matures.

Hair color, like eye color, is controlled by at least two different genes for pigment (one for the brown pigment, melanin, and one for a reddish carotenoid pigment) plus others that control the amount of pigment in the hair shaft, making hair range from dark brown (almost black) to light brown to blonde to reddish brown/blonde, pale red, and dark red.  Whew!

If you have blonde hair with no reddish tinge, then you will probably donate only those recessive alleles to your baby, and those will code for blonde.  But since your husband has dark brown hair and his mother is blonde (if natural blonde, anyway), then he again has a 50% chance of donating the dark hair version to the baby (which is dominant, and the baby would have dark brown hair) and 50% chance of donating the blonde allele he got from his mom, in which case the baby would have blonde from both you and dad--and be blonde!

So there's pretty much a 50:50 chance of having either blue or brown eyes, and a 50:50 chance of dark or blonde hair, given the genetic background you've provided.  

I can't see the subtle variations in hair color of everyone involved, and that could complicate matters.  But this is about as simple an answer as one can give with the information at hand.

In any case, I hope the baby is healthy, no matter what color!  :)

I hope this helps.

Dana