|MadSci Network: Genetics|
Hi Karsten, Welcome to the wonderful world of human hair color genetics. While hair color is generally cited in basic biology classes as an example of how Mendelian genetics applies to easily observed human phenotypes, the sordid truth is that except in special cases, hair color is not a simple matter of dominant dark alleles and recessive light alleles. Hair color is not a discrete trait, but rather a trait with a continuous distribution. That is, between platinum blonde and black there are a million subtle shades of natural hair coloration. Thus it is likely that there are many alleles at many genes which affect pigmentation, and the sum of the activities of these alleles (as measured by pigment production) produce a given hair color. In addition to the spectrum of coloration in a population, pigmentation also changes over the course of an individual's lifespan. An example of this is eye color: although most European babies are born with blue eyes, many will darken to various shades of brown before six months of age. The pigment in the iris is the same pigment which produces hair color, and it is also frequently observed that European infants who are born with blonde or reddish hair will develop significantly darker hair color within the first year of life. Hair pigmentation also changes in later life, with a significant fraction of adults developing grey hair as they age, although this tends to be a phenomenon of single follicles, not the entire individual. However, a more pronounced change is frequently observed in some European populations, where blonde children will slowly darken to brown hair by adulthood. I can't point to any scientific studies of this phenomenon, but I have observed this phenomenon several times in my own family (we are Scandinavian) and the families of friends from Northern Italy. My pet hypothesis is that this natural correlation between youth and light hair might account for the popularity of bleached blonde hair among American women, but this is only conjecture. Another interesting possibility is that simple pigmentation is not the whole story. Hair texture also tends to coarsen with age, representing thicker individual hairs, and thicker hair strands are likely to alter hair color even if the level of pigmentation is constant. Thus the change in hair color with age might not reflect a difference on pigment production, but an alteration of the physical structure of each hair. Again, I'd bet that the correlation between fine hair and youth explains the frequency of female models with long, straight, flowing tresses in U.S. shampoo commercials. Returning to your original question, hair color is simply not controlled by simple Mendelian genetics at a single gene except in very rare cases (recessive albinism or dominant red hair). Thus, hair color over the life of an individual probably represents the interaction between environmental and genetic factors, which might include sun and chlorine exposure. The observation you report in some children of mixed Maori/European descent simply suggests that the tendency toward darker hair color with age exists in both populations, but is more obvious in individuals of intermediate pigmentation. Chris My apologies for not including references: human hair color genetics has been focused almost exclusively on the simple (rare) Mendelian hair colors, and I was not able to find any significant body of work on hair color vs. age. Thus, my response is based on anecdote, conjecture and experience.
Try the links in the MadSci Library for more information on Genetics.