Research on human hair pigmentation has relied primarily on subjective qualitative descriptions, such as ‘black’ or ‘brown’, rather than objective quantitative methods. Although verbal descriptions have facilitated the identification of genetic loci associated with variation in European populations, the categories of hair color used describe most non-European populations as ‘uniformly dark-haired’. Previous research has detected some variation among dark-haired populations using narrow-band reflectance spectrophotometry, suggesting that different methods may reveal more fine-scale variation. Here, using chemical methods to assess the absorbance of melanin and specific degradation products of eumelanin and pheomelanin, we demonstrate an appreciable level of variation within perceived hair color categories among human metapopulations. Samples of African hair categorized as ‘black’ have more eumelanin degradation products and higher spectrophotometric absorbance than Asian and European ‘black’ hair. Variation within and across these hair color categories suggests that a lot of information may be lost by relying solely on perceived hair color. These chemical methods may represent a way of capturing salient, but imperceptible, phenotypic variation that is relevant to the genetic architecture of human hair pigmentation. Moreover, understanding variation quantitatively is essential for exploring the extent to which genetic drift and selection pressures have acted on these traits. The feasibility of applying these methods, or variants thereof, on a larger scale by future genetic association studies is discussed in our work.
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