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August 07, 2019 – Blonde hair is a rare human phenotype found almost exclusively in Europe and Oceania. Human pigmentation varies considerably within and among populations and is a function of both variation in exposure to ultraviolet radiation (UVR) and the type and quantity of melanin produced in melanocytes and keratinocytes. Common belief would hold blonde people to be either located in Europe or being of European (Caucasian) descent when encountered in other parts of the world.
There are some remarkable variations to this belief, however. Thus, genome wide association (GWA) studies in European populations have yielded numerous insights into a genetic basis of pigmentation variation within Europeans, meaning that even among Europeans, blonde does not mean the same thing in every case, and the relevance of these associations outside of Europe remains largely unexplored. This becomes really interesting when you encounter, for example, “black” people with naturally blonde hair. Black meaning in this context “dark skinned”, suggesting an African ancestry of the person concerned.
Such is the case for Solomon Islanders, where about 5 to 10% of the population is naturally blonde (by means of hair color) while otherwise dark skinned. Recent research has identified a cystine-to-arginine amino acid change at an otherwise highly conserved residue in tyrosinase-related protein 1 (TYRP1) as the single source of blonde hair in Solomon Islanders. This missense mutation is predicted to impact catalytic activity of the protein and causes blonde hair through a recessive mode of inheritance. TYRP1 is a melanocyte-specific gene product (protein) involved in melanin synthesis. In addition to its role in melanin synthesis, TYRP1 is involved in stabilizing of tyrosinase protein and modulating its catalytic activity. TYRP1 is also involved in maintenance of melanosome structure and affects melanocyte proliferation and melanocyte cell death.
The newly identified mutation in TYRP1 occurs at a frequency of 26% in the Solomon Islands but is absent outside of Oceania and represents the largest genetic effect on a visible human phenotype reported to date. The findings of this research demonstrate that alleles of large effect reach appreciable frequencies in geographically isolated populations and underscore the importance of extending medical genomics to humans worldwide.
Generally, human pigmentation varies considerably within and among populations and is a function of both variation in exposure to ultraviolet radiation (UVR) and the type and quantity of melanin produced in melanocytes and keratinocytes. The focus of the present research was on understanding the genetic basis of blonde hair in the Solomon Islands, a population that breaks from the general trend of darker skin and hair pigmentation near the equator where there is higher UVR. Strikingly, while individuals from the Solomon Islands and other locations in Oceania near the equator have both the darkest skin pigmentation outside of Africa, they also have the highest prevalence of blonde hair (5–10%) outside of Europe.
How come? The earliest inhabitants of Neaw Oceania settled at least 40’000 years before present (YBP) and are thought to have remained in relative isolation on the archipelago for at least 25,000 years. There is clear evidence of at least one subsequent migration from Southeast Asia about 3’000–4’000 YBP, and waves of more recent gene flow from other islands in the Pacific and from Europe. To understand whether sharing of this rare human phenotype is due to globally common genetic variants or arose independently, the genetic mechanism underlying hair color in Solomon Islanders was investigated. Autosomal recessive inheritance of the variant gene (TYRP1) coding for tyrosinase-related protein 1 and skin and hair pigmentation in 1’209 Solomon Islanders was determined and 85 individuals from the extreme 10% tails of the hair pigmentation distribution (43 blonde and 42 dark-haired individuals) selected for genotyping. Estimates of global ancestry proportion showed that individuals from the Solomon Islands are genetically distinct from nearby populations, as has been previously observed. Further, no systematic differences in ancestry between blonde and dark-haired Solomon Islanders were found suggesting that the presence of blonde hair in the Solomon Islands is not due to recent gene flow from other populations.
A intriguing question comes up in this context. How come that individuals who are homozygous for the TYRP1 variant, which leads to diminished melanin production and eventually to the blonde hair phenotype maintain their dark skin complexion? One would expect, if TYRP1 were involved in a sole pathway leading to melanin, that carriers of the mutated version of TYRP1 would also not have high enough levels of melanin leading to dark skin. One would expect an appearance of lighter skin. It would certainly be interesting to see if there are other pathways, possibly independent of TYRP1, involved in melanin synthesis and if the expression of such pathways may be regulated body position- or organ-specific (i.e., skin versus hair).
See here a short sequence on the beauty of dark skin and blonde hair: