Can you tell me more about the genetics of hair color? My husband and I both have dark blond hair, our son has blond hair, but our daughter was born with dark brown hair.
Each of us has one parent with blond hair and one with dark brown hair. I thought that blond hair was recessive, and thus our children would both have blond hair. Am I wrong?
My internet searches on this topic have only turned up the information that there are several alleles that determine hair color, but nothing about inheritance patterns.
-A curious adult from Oregon
August 17, 2004
Man, I thought the eye questions were tough! There is very little known about hair color inheritance but there are some interesting theories. I am happy to share what I've gleaned from the web. It makes sense to me but I can't necessarily vouch for it.
What is pretty well known is where hair color comes from. Hair color happens because of a kind of pigment called melanin. There are two kinds of melanin, eumelanin and phomelanin.
For the sake of ease (and since it answers your question), we'll focus only on eumelanin. If your hair has a lot of eumelanin, it will be black; 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, either on or off. When the genes are on, they make eumelanin and 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.
To put the answer in more concrete terms, we'll imagine that there are 4 eumelanin genes that determine hair color. Remember you get 4 copies of each gene from your mother and 4 from your father giving you a total of 8.
If one of these hair color genes is on, we'll represent it with H and 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 explain everything in genetics. Hair color is an example of where the end result is determined by how much eumelanin you make from lots of genes. Other examples that do not include dominant and recessive are blood type and curly and straight hair
Disclaimer: As I said at the beginning, this theory explains hair color pretty well but the evidence is, as far as I can tell, pretty slim.