Coat color genes in rodents and carnivores.

Abstract

INTRODUCTION A NY comparative review of coat color genes in mammals faces unavoidable difficulties. Only a few species have been studied sufficiently to provide enough information for accurate analysis on which genetic similarities or differences between species can be determined. There are at least four types of evidence that will help to estimate the probability of interspecific homology of genes. These will be referred to in discussion of loci in each group. These types of evidence are as follows: (1) similar linkage relations between two or more genes; (2) similar pleiotropic effects of genes; (3) similarity of multiple allelic series; and (4) similar morphogenesis and function of melanoblasts. The interspecific homology of genes or loci will, however, remain to some degree uncertain unless direct evidence from interspecific crosses is available. It seems proper, therefore, to list under each species the genes and loci recorded in it, and later to discuss under the loci the points of similarity and dissimilarity which comparison between species reveals. There seems to be little question that the basic chemical processes underlying melanin pigment formation are similar in rodents and carnivores. It is undoubtedly more than a coincidence that three, and only three, pigments (yellow, brown, and black) are found in the coats of each group. It is also important to note that in all the species observed yellow pigment formation and distribution may, under certain conditions, be affected by genes, independently of the brownblack pigments. The opposite is also true, and one finds genes which appear to act only on brownblack and not on yellow. To a large degree also the basic origin and morphogenesis of melanoblasts appear to be the same in both groups. There may be in carnivores (at least in the Felidae) a skin pattern involving structural differences which is a phenomenon absent or as yet unrecorded in rodents. This condition has been described in the domestic cat by Pocock (1907), who noted that the black bands in striping and blotching were underlined by thicker skin than were the areas which remained light (tawny) as the pigment developed. The presence of skin areas of this type may be the reason for the so-called ghost patterns which sometimes appear in newborn or young black animals and which disappear during the uniform distribution of dark pigment or by the replacement of the juvenile dark pigment by yellow at a later age. This matter will be further discussed when the color genes of cats and dogs are considered. Over a number of decades, efforts have been made to utilize genetic experimentation to help to elucidate the chemical and morphogenetic factors and processes underlying the formation and distribution of melanin pigment in mammals. In genetic studies, the house mouse has had the longest and most important role. From the obviously primitive efforts of the writer (1913) to classify coat color genes, through the much improved analysis by Wright (1917), and up to the