A reader emailed me asking about the genetics of hair color. Since I've discussed the topic before I simply pointed them to the query of the topic on my other blog. But, I thought it might be good to directly answer two specific questions:
Why do many people have much lighter hair during childhood?
The same reason that European babies are often with blue eyes or dark skinned babies are born with light skin, we get darker as we age (with the partial exception of females during puberty). The darkening of hair is simply a symptom of a general trend toward increased melanin as we age (or as women go through pregnancies). Proximately in an individual's life history it probably has hormonal upstream causes, e.g., the increased level of testosterone vs. estrogen levels in females after menopause (and pregnancies), or the elevation testosterone in males after puberty. Our ancestral state was probably "pink," but after we lost our fur humans developed dark skin (a "consensus sequence" is found among tropical peoples). Light skinned peoples are a response to the relaxation of this particular selection pressure, but the darkness is something that we develop (some of us more than others) over our lifetime as genes express proteins which lead to the phenotype.
Also discuss the dependency and independency between hair and eye color, and hair and skin color.
Skin color and hair color are partly dependent. For example, the MC1R locus is loss of function, along with others, in red-haired individuals. These individuals tend to be very fair, and unable to tan. In Europeans blonde hair is also loss of function, but to a lesser extent, and so one may have dark skinned people with blonde hair. If you look at pictures of mixed-race Brazilians you will see individuals with brownish skin and dark blonde hair. But, I have never seen any individual with brown skin and bright ash blonde hair, suggesting to me that the full expression of blondness can only be exhibited by individuals who also carry a genetic configuration which renders impossible brown skin (or, conversely the loci responsible for a particular melanin production level in the skin guarantees a minimum level in the hair).1
There is one important exception to this though: some populations of Australian Aboriginals, and perhaps Melanesians, exhibit blonde hair via a different genetic mechanism. These tribes tend to be concentrated in the center and west of the the nation. The late Australian anthropologist Joseph Birdsell studied these individuals and it seems that the pattern of phenotypic expression was dominant, insofar as hybrids between blonde Australians and brunette Europeans also produced blonde offspring. This suggests that this feature is not a convential loss of function, but might be a dominant gain of function mutation which blocks the production of melanin. These individuals have brown-black skin, so this is unrelated to complexion.
1 - The link between eye color and skin color seems weaker (mostly on the OCA2 locus). I have met nearly black skinned African Americans with blue eyes. Never have I met black skinned African Americans with blonde hair.
While the mechanisms are somewhat-to-largely independent, is there any evidence for linkage disequilibrium among the genes which lead to skin, eye, and hair color? Are any of them located on the same chromosomes? Clearly there are local populations where certain combinations tend to be linked (e.g., the "classic" red-haired, fair-skinned, green-eyed Irish) but I'm wondering if there is any underlying structural genetic reason or if it is just due to population history (e.g., founder-effects or population bottlenecks)? I could look this up but it's so much easier just to ask someone already familiar with the details.
but I'm wondering if there is any underlying structural genetic reason or if it is just due to population history (e.g., founder-effects or population bottlenecks)?
red-hair is due in part to a strong structural linkage, specifically, loss of function on MC1R seems necessary, and dark skinned people are never like this. blondism is far more weakly linked to MC1R and seems distributed across more loci so the physical association is far weaker, but there is still an overlap. blue/grey eyes seems to be on chromosome 15 and it seems pretty unlinked to skin color (it is mostly a one-locus trait).
Although you can be black, red-haired and Jamaican:
http://www.nature.com/jid/journal/v121/n1/full/5601853a.html
Unfortunately, my genetics knowledge is rather perplexed by that article :)
"The same reason that European babies are often with blue eyes or dark skinned babies are born with light skin, we get darker as we age (with the partial exception of females during puberty). The darkening of hair is simply a symptom of a general trend toward increased melanin as we age (or as women go through pregnancies)."
Neat, I had never heard that. It does explain something I had been wondering about, namely how my light-auburn-haired mother turned into a dark-brunette over the years.
So I posted the following over at Coturnix's Blog Around The Clock under some pictures of a pair of cats there, one of which did, and the other did not, have a tuxedo coat pattern. Coturnix referred me to this blog, where this post -genetics of hair color- seemed most germane.
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Here's a biology question. You can find all sorts of information on cat coat colors in genetic texts and online. Tabby vs. plain, white with/without deafness, calico and tortoiseshell, agouti, yellow vs. gray, Persian, etc., etc. Everything but how the tuxedo pattern comes about. Do embryonic melanocytes stop short of the ends of the limbs, belly/chest, chin/lip when they migrate away from the neural crest during development? If so, why? Why do cats often have a black tip to the tail but rarely a white one, when dogs often have both? Do melanocytes somehow pile up at the end of the neural crest to make a black tip on tails? Other domestic animals, have the tuxedo pattern and countershaded wild mammals have a sort of tuxedo pattern. Why do some mammals, e.g. the ratel (Mellivora capensis), have a black belly? What gives here?
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Oh, re human hair color: sadly, none of this matters to me anymore, as mine's almost all grey now.
I'm doing a project on genetics in hair color so if anyone could give me some good information or any good sites please email me with them. thanks
We are debating a claim by an Armenian friend of ours that Armenians are blonde as children and have black or dark brown hair as adults. This is interesting since the population is Middle Eastern in origin. I asked my friend if perhaps her family was an exception and had European ancestors. She claims that they do not. I have many other Armenian friends and have been to many festivals and large gatherings. I have not seen blonde Armenian children there. Have you heard of this? Don't people have to have the expression of a recessive blonde jean to exhibit light blonde hair even as children, with the exception of some populations of Australian Aboriginals?
Have you heard of this? Don't people have to have the expression of a recessive blonde jean to exhibit light blonde hair even as children, with the exception of some populations of Australian Aboriginals?
the genetics of this is more complex than recessive/dominant. i have not heard of blonde armenians, but blondism is not a purely european trait. many middle eastern peoples exhibit it at low frequencies (e.g., iranians, kurds, lebanese, etc.). some of this (lebanese) might be european introductions, but the iranian and cases in afghanistan are almost certainly (or from an ancient radiation of the trait).
Look into the DNA Project for Armania. For example, the English genetics are not what I thought. There was ice over England until 16,000 to 9,000 years ago. They were nomadic people-gentics from spain,france, northern Italy Greece and Iraq. She wouldn't necessarily know the patterns of ancient migration of Armania.