Re: What accounts for children born blonde and going brown in hair color?

Thanks for the follow up question Karin,

Now I am glad that I gave myself a bit of space in my previous explanation. It turns out that straight Mendelian inheritance can’t always explain the genetic phenomena that we see in living organisms. Oftentimes, you have genes for which the expression changes over time, or in response to specific stimuli. I think that that turns out to be the case for the examples you cited.

I didn’t mention this in the previous posting, but the expression of both eumelanin and phomelanin is controlled by melanocyte-stimulating hormone. This hormone binds to a receptor, called the melanocortin-stimulating hormone receptor, and tells melanocytes to produce more pigment. The gene for this receptor is called MC1R. It turns out that there are a large number of alleles for the MC1R gene, and some of these alleles probably modulate the degree to which the receptor responds to the hormone.

Now in all the cases you described, the hair color darkened, which is consistent with increased levels of eumelanin production. Although I am not positive, my quick answer would be that these sorts of changes are responses to changes in the levels of the melanocyte-stimulating hormone. As you suggest, these hormonal changes might be correlated with the other hormonal changes that occur at puberty. However, I know that some people’s hair color changes in the months right after birth, but there are hormonal changes going on then too.

For more Information on the MC1R gene, take a look at these references:

Valverde P, Healy E, Jackson I, Rees JL, Thody AJ. (1995) Variants of the melanocyte-stimulating hormone receptor gene are associated with red hair and fair skin in humans. Nature Genetics. 11 : 328-30.

Rana BK, Hewett-Emmett D, Jin L, Chang BH, Sambuughin N, Lin M, Watkins S, Bamshad M, Jorde LB, Ramsay M, Jenkins T, Li WH. (1999) High polymorphism at the human melanocortin 1 receptor locus. Genetics. 151 :1547-57.