Expert: Dana Krempels, Ph.D. - 9/28/2010

Question
Hi, im wondering about genetics. Say for example I have dark hair, as do both my parents and all my siblings and my partner has red hair, as do both his parents and all his siblings, which is the dominant gene? red or brown. We're working on this at uni and I have no idea how to determine which is dominant or recessive? im a little lost. thanks in advance :)

Answer
Dear Lisa,

Hair color is controlled by more than one gene.  But humans (and other mammals) have two forms of melanin.  Eumelanin is the brown pigment, and phaeomelanin is the reddish form.  They are coded by different genes, and still more genes control how much pigment of each type is deposited in the hair shaft.

A person can express both types of melanin, and the degree to which they express the pigment will determine hair color.  Dark hair with reddish highlights in the sunlight suggest strong deposition of both types of pigment.  Very bright red suggests strong phaeomelanin deposition without much (or any) eumelanin.  Yellow blonde is due to very little eumelanin in the hair shaft, and "strawberry" blonde is due to very little of *both* types of pigment.  And there are multiple ranges of color in between, of course.

So the simple answer is:  there's no simple answer.  Polygenic traits (those controlled by multiple genes) are very difficult to predict, in terms of expression.  But they are responsible for a lot of polymorphism we see in populations.

Here's a good overview, if you're interested in learning more:

http://www.gnxp.com/MT2/archives/001516.html

Hope that helps.

Dana