I spent a little time doing research on hair color in PubMED and many of the leads seem to end up at the Melanocortin 1 receptor (MC1R). This is what I was curious about, is the "recessive" vs. "dominant" dichotomy illustrated in blonde & brunette inheritance patterns among whites that is taught in introductory high school genetics just an illusion? This is why I ask: most of the children who I have seen who have a non-white black-haired parent and a blonde white parent do not have black hair, rather, their hair tends to be dark brown on average, though sometimes you will see someone with auburn hair. Quite often the children might even be blonde as infants .
My question, is there a clear way to measure the amount of melanin present in hair that can be quantitized? I assume there is-though unfortunately I don't have the time to do any more googling on this topic (so someone can hopefully comment on how this is done). And would one find that the children of two individuals who have extreme phenotypes would have values that show that hair color is a continuous, rather than discrete, trait?  I wonder if in fact some brown-haired Eurasians may have melanin levels somewhere between their blonde white parent and black-haired Asian parent, but are classified with the latter because of the dichotomy that our classification system imposes.
Here is an excerpt from the abstract of one of the articles I found:
Recent work suggests that the MC1R also shows a clear heterozygote effect on skin type, with up to 30% of the population harbouring loss-of-function mutations....In particular, study of the MC1R may provide insights into the lightening of skin colour observed in most European populations. The world wide pattern of MC1R diversity is compatible with functional constraint operating in Africa, whereas the greater allelic diversity seen in non-African populations is consistent with neutral predictions rather than selection....
Here is another abstract snippet
We found variation at five nonsynonymous sites...but at only one synonymous site (A942G)...The Arg163Gln variant is absent in the Africans studied, almost absent in Europeans, and at a low frequency (7%) in Indians, but is at an exceptionally high frequency (70%) in East and Southeast Asians...the nucleotide diversity at the MC1R locus is shown to be several times higher than the average nucleotide diversity in human populations, possibly due to diversifying selection.
Here are some related articles-I haven't had time to sort through all of this. Also, I found that the journal GENETICS seems to offer full access for articles before June of 2003 in their archives.
Why Humans and Their Fur Parted Ways shouts the headline in The New York Times. Turns out our friend, the MC1R gene, shows up again. Dr. Mark Stoneking conjectures that the version of MC1R that produces dark skin among African people must have swept through the human population after we lost our fur. He notes chimpanzees are relatively light-skinned and tan on their faces as they age. Stoneking found that the version of MC1R that causes dark skin became dominant in Africa about 1.2 million years ago, ergo, that is when humans lost their fur. Furthermore, Stoneking hypothesizes that the reason humans lost their fur is to avoid parasitic infestation. He caps his theory off with the assertion that humans have only been clothed for 50,000 years because that is the point of divergence between the human body louse (which resides in clothing) and the chimpanzee louse-which dovetails well with the "cultural explosion" that putatively occurred at that time, The Great Leap that some palaeoanthropologists speak of.
 Bruce Lee would joke to his wife that his son Brandon, who was dark-haired as an adult, was the "first blonde Chinamen" when he was an infant.
 After I re-read this sentence, it seemed stupid, of course hair color is a continuous trait to some extent (ah, weasel words, I know), but our terminology tends to shoe-horn it into discrete classes, sometimes ones that don't map well onto reality ("Hung-Mao" = "Red Haired Folk", in other words, Europoid, but most Europeans don't have red hair). But what I am more interested in is how the genes interact and express the phenotype, and whether those that effect hair color (and other pigmentation), are additive, assuming it is a polygenic (multiple genes effect it) trait. Additionally, I have read that MC1R is pleiotropic, it is a multi-functional spot in the genome. Getting complicated....
I read something about this online not long ago (of intermediate complexity) - my vague memory of it was that there were four sites where mc1r could be expressed and one additional one for the redhead mutation, and each of these five could be hetero or homozygous. The amount of hair pigment depended on the number of functional pigment genes... so if you had a very simple model where a given allele of mc1r either produced eumelanin or didn't, you'd still have anywhere from zero to eight functioning copies of the gene with a light-to-dark coloration spectrum. So blondeness involves having just a few (zero to two? I don't know) functioning mc1r genes. The redheadedness is independent but of course can't be seen if too much eumelanin is present, so for redheadedness you need both blondeness (few functional eumelanin genes) and either one or two phomelanin (red pigment) genes, resulting in either reddish hair or really red hair.
Of course as you point out it's much more complicated due to the profusion of alleles - I think asians have a different set of mc1r alleles than westerners.
One final point: even though hair color is not really mendelian, the greater blondeness of children would support the illusion that it is. If both parents have a small number of eumelanin-producing genes (are blond), the child could end up with more than either and have darker hair in adulthood. But it would be very unlikely that his hair would be darker as a child.
Posted by: bbartlog at August 19, 2003 05:44 PM
bill, your comment was WAY off topic. i suggest you post your anti-Steve Sailer article links on the discussion form, OR, wait until the thread shifts a bit and insert them in at a more appropriate time. since steve sailer is my friend (oh god, i admitted it!), i am not of course going to give you the benefit of the doubt and allow you to hijack other threads constantly....
but as i said, go post your links on the message board, and fix the formatting of your site-i'm using mozilla on XP and your tables looked messed up.
Posted by: razib at August 19, 2003 09:42 PM
Interesting - so we can figure out when humans lost their fur from the diversity of the mc1r gene. Pretty neat! Stoneking's notion that this happened as a way of dealing with parasites is plausible. But if you're a fan of Leakey's 'man the cursorial hunter' idea, then I think another hypothesis presents itself, namely that we would have lost our fur in order to sweat more efficiently.
Posted by: bbartlog at August 20, 2003 05:23 AM