Comparative genetics of albinism.

Albinism in laboratory mammals is equivalent to human tyrosinase-negative oculocutaneous albinism, and thus the result of recessive mutation in the structural locus for tyrosinase (TYR), which prevents melanin biosynthesis. In the mouse, eight mutant alleles are now known at this locus, with differing effects on eye colour and on the degree of reduction in eumelanin and phaeomelanin pigmentation. Three of these alleles, namely chinchilla, himalayan (acromelanistic) and albino (c) itself, have also been recognized in a number of other species but only albino has been identified in man so far. The himalayan allele (equivalent to Siamese in the cat) is of particular interest because it converts tyrosinase into a thermolabile form, with greater production of melanin in colder areas of the body. The optic track misrouting found in human albinos also occurs in albino alleles in other mammals, which may also show reduced activity and stress responses. The TYR locus is on human chromosome 11, which now has at least 11 loci with homologues on mouse 7. However, their order is markedly different in the two species. For instance, c and Hbb (beta-globin), which are closely linked in mouse, rabbit, cat etc., are far apart on human 11q and 11p respectively. Moreover, some loci (e.g., Fes and Mod-2) which are close to c in the mouse appear to be on human chromosomes other than 11. This extensive chromosomal restructuring in mammalian evolution means that the effects of human albino deletions may differ greatly from those studied in the mouse, which are associated with defects of kidney, liver and thymus. Tyrosinase-positive albinos or near-albinos are known at a number of loci in mice and other mammals. They are the result of the absence or inhibition of melanocytes in the affected areas, so that no melanin is produced. In general they are associated with pathological pleiotropisms which may lead to anaemia, inner ear defects, megacolon, neurological effects, skeletal defects, microphthalmia, osteopetrosis, spina bifida, sterility and so on. Homologies between these and human loci affecting pigmentation are now being discovered.