Friday, August 26, 2005
I finally read Heather Norton's review of worldwide MC1R polymorphism. In case you forgot MC1R is a locus that has been implicated in the expression of pigment in human beings, and mutations on it are often correlated with the various phenotypes we see around us (red hair, light skin, etc. etc.). The major take home point of Heather's paper is there is more than meets the eye than MC1R. For example....
1) Heather points out that there is no association (ie; mutation ~ change in phenotype) between eye color and MC1R. Something else is controlling the expression of eye color. This is interesting because Joe Birdsell in Microevolutionary Patterns in Aboriginal Australia states that iris color is often inverted in direction of trend from hair and skin color in many populations as a function of sex (including Australian Aboriginals). In other words, males tend to have lighter eyes and darker hair and skin than females.
2) There are likely gene-gene interactional effects between MC1R and other loci. In other words, there isn't going to be a simple and elegant prediction equation like the ideal gas law (PV=nRT) which maps various additive loci onto a phenotype (different combinations can lead to the same phenotype, for one, and the genetic effect of a locus is dependent on the alleles at the other loci).
3) Europeans are extremely diverse on this locus vis-a-vi other populations. There are 30 alleles which break the 1% threshold within European populations It seems that Heather and her coauthor suggest that relaxation of functional constraint (UV radiation) is the most plausible hypothesis, but the power of the statistical tests doesn't warrant a final conclusion (ie; it could be some sort of diversifying selection).
4) East Asians tend to exhibit very high frequencies of the Arg163gln allele (as high as 80% among the Dai of Yunnan province in China, decreasing to 40% among the Uighers of Xinjiang and less than 10% in South Asia [ie; India]). The authors suggest that this allele might have been subject to strong directional selection in the recent past. They note that "under neutral expectation, the estimated arrival time of the Arg163Gln allele is older than the age of modern humans...." Things that make you go hhhmmm....
Overall, skin color is a messy polygenic trait. It is almost certainly not a nice Fisherian topography, but we have mapped the rugged gene-gene interactions at work so generalizations are hard to come buy that add any value to our knowledge-base. Additionally, some loci like MC1R are likely to have particularly strong effects because they lay at the nexus of various regulatory pathways. Finally there is the hypothesis that Greg Cochran and others have put forward which opines that perhaps some of these alleles (look at how many Europeans carry!) "jumped" from other ancient hominid lineages and were under selection subsequent to the hybridzation event.
Update: Lei has more.
Related: Blonde Australian Aboriginals. Black and strawberry. What controls variation in human skin color? Genetics of Hair Color (again). Genetics of hair color. It's better on fire. An email Heather sent me. Evolutionary speculations.