Can you tell me more about the genetics of hair color?

Man, I thought the eye questions were tough! As it turns out, there is actually very little known about hair color inheritance, but there are some interesting theories. I am happy to share what I've learned from my research on the web.

What is pretty well known is where hair color comes from. The color in hair comes from a kind of pigment called melanin. There are two kinds of melanin, eumelanin and pheomelanin.

For simplicity’s sake, we'll focus only on eumelanin. If your hair has a lot of eumelanin, it will be black; only a little eumelanin and your hair will be blonde.

The amount of eumelanin in your hair is determined by lots of genes. Let's imagine (although the real case is probably more complicated) that there are two possibilities for each of these genes: they can be either on or off. When the genes are on, they make eumelanin. When they are off, they don't make anything.

One other thing you need to know is that eumelanin genes work in an additive way instead of in a dominant and recessive way. In other words, the more eumelanin genes that are on, the darker your hair will be.

Given these assumptions, the answer to your question is that your son inherited few of the "on" eumelanin genes while your daughter inherited a lot of them.

To give a simplified example, let’s imagine that there are 4 eumelanin genes that determine hair color. In this case you’ll get 4 copies of eumelanin genes from your mother and 4 from your father, giving you a total of 8 copies.

If one of these hair color genes is on, we'll represent it with H. If it is off, we'll represent it with h. Using this system, someone with very black hair would be HHHHHHHH and a blonde person would be hhhhhhhh.

You said both you and you husband had dark blonde hair. If we imagine that both of you are HHHhhhhh, then it is easy to imagine how your son and daughter's hair color came about.

Remember, your kids will get 4 copies of the gene from each of you and there is no dominance per se -- they add up to give a hair color. If each of you contributed only h's, then you would get a blonde haired kid like your son with a genotype of hhhhhhhh. Your daughter got more H's than h's (perhaps HHHHHhhh) and so has dark hair.

 Hope this answered your question. As you can see, dominant and recessive doesn't necessarily explain everything in genetics. Hair color is an example of where the end result is determined by lots of genes.

(Disclaimer: As I said at the beginning, this theory is an example that could explain hair color inheritance, but it’s hypothetical. We still don’t have good evidence for how the genetics of hair color work.)