CHAPTER 17 Skin Colour, Its Genetics, Variation and Adaptation: A Review

A.K. Kalla

INTRODUCTION due to the accumulation of a red intermediary metabolite, namely, DOPACHROME. Human skin colour is one of the most conspicuous human polytypic variations and has THE MELANIN PIGMENTATION attracted probably more scholarly attention than any other aspect of human variability. It was the The melanin pigment granules are mostly very first character used for racial classification found in the basal layer or the stratum germinativum and has served as a primary feature in most inside the melanocytes. The stratum basale systems of racial classifications. consists of columnar cells, the carotenocytes, In the late nineteenth century, process of with about 10% of the cells comprising melano- melanization in plants was discovered to be cytes. This is the germinal layer of the skin which dependent on the enzyme tyrosinase. Sub- gives rise to the outer layer of cells and the sequently the same pathway was found to be melanin granules that pigment them. The stratum involved, by German histochemist Bruno Bloch, spinosum consists of several layers of irregular in pigmentation of human skin in vitro. By 1950 polyhedral cells, carotenocytes, flattened on their the melanocytes, the cells where these reactions edges. Thus it can easily be inferred that the effect took place, were identified and it was further found of haemoglobin of the blood flowing in the that these cells originate embryonically in the underlying papillary layer of the dermis on the neural crest. skin colour will be masked by the dark brown Edwards and Duntley (1939a) found that melanin pigment present in the overlying basal variation in human skin colour is mainly due to layer of epidermis and particularly so in the darker the presence of four pigments, namely, Melanin, populations. Haemoglobin, Carotene and Melanoid. In addition The metabolic pathway involved in melanin to this, effect produced by the scattering of light synthesis is extremely complicated involving from the skin surface is also involved in giving a several intermediate steps. It starts with the amino particular skin colour to the person. G.A. Harrison acid tyrosine oxidized by the copper containing (1957) demonstrated that there appears to be no enzyme tyrosinase (TYR) to dihydroxy- significant difference between races in haemo- phenylalanine (DOPA) and then to dopaquinone. globin content provided there are no pathological A in the gene for enzyme tyrosinase and/ or malnutritive conditions. He further (TYR) that produces a protein with decreased demonstrated that there appears to be constancy functionality will result in a reduced production in the vascular supply under conditions of of melanin and under extreme decrease in the constant temperature and activity. He also found functionality this produces a genetic form of that the contributions of scattering of light and . Dopaquinone undergoes a series of carotene appear to be respectively constant and non-enzymatic reactions and rearrangements in any case negligible. Further the melanoids are forming the different molecules that are co- also found to be negligible even in dark skinned polymerized to make one of the two types of people particularly at the parts of the body melanin: Eumelanin, which is the dark brown/ protected from the sun. purple/ black compound found in skin/ hair, and No wonder that Edwards and Duntley (1939b) Phaeomelanin, which is yellow to reddish brown had suggested that “the coloured races owe their pigment present in red hair and rarely in human colours only to variations found in melanin skin. Both forms of melanin combine with other content”. No pigment other than the one present proteins to form the melanosome that is distributed in the Whites, excepting an unusual rare red from the melanocytes to the surrounding cells. pigment among the New Guinea indigents (Walsh, There i