Pigmentation, melanocortins and red hair.

Abstract

`Do freckles and red hair help Irishmen catch leprechauns?'1 There are at least two reasons for being interested in the biology of hair and skin colour. First, variation in skin and hair colour is perhaps the most polymorphic of all visible differences between humans, and has historically been of profound social importance. We get used to seeing little people—they are called children—whereas even the most liberal parent shows concern when children start verbalizing their classification of human skin colours. It is likely that the influence of differences in pigmentation rivals or exceeds that of infectious diseases on human history. Second, variation in pigmentation is the most important risk factor for the major forms of skin cancer, both melanoma, and basal and squamous cell carcinoma.2,3 If one were to collect a random sample of people from around the world, their constitutional risk of developing skin cancer would vary over 100-fold, and most of this would be attributable to differences in skin colour.4,5 The interaction of skin colour and ultraviolet radiation provides a timely reminder of the difficulties of viewing nature and nurture as anything but contingent: to an epidemiologist, ultraviolet radiation is the major determinant of skin cancer, whereas for a geneticist, pigment, predominantly under genetic control, is the major determinant. Both views are of course correct. Of course in between the two reasons given above lie a host of interesting biological questions. Can we explain the variation in human pigmentation solely in terms of protecting against ultraviolet radiation, or alternatively do we imagine that natural selection is only part of the explanation. Why are the Scandinavians taller and blond and the Irish more often red-haired? Is this all natural selection at work, or is there an element of assortive mating depending on choice …