A Quantitative Histological Study of the Pigment

HERE is considerable reason to believe that the coat-color genes of mamT mals provide particularly favorable material for the study of the physiology of gene action, as the chain of reactions between gene and final character must be short relative to that in most internal and embryologically determined characteristics (E. S. RUSSELL 1946). Therefore a histological analysis of the pigments found in the hair of the house mouse has been undertaken in an attempt to find as much as possible about the end product of the reaction chain initiated by the action of the pigment genes. The first stage of this study was measurement of variations in the pigment granule attributes which were found to contribute materially to the coat-color differences observed among 36 mutant types tested. Seven such variable attributes were recognized and measured separately for each type: number of pigment granules (1) per medullary cell and ( 2 ) per unit volume of cortex, ( 3 ) tendency to distal arrangement of pigment within the medullary cells, and the (4) size, (5) shape, (6) clumping, and (7) color of the individual granules (E. S. RUSSELL 1946). After this strictly analytical approach an estimate of the total volume of pigment in each genotype was prepared by combining certain of these measurements with new determinations of individual granule volume (E. S. RUSSELL 1948). While many facts about the causes of color differences could be determined simply by studying separately the condition of each of the variable attributes in all of the genotypes, it was felt that just as combining information on several separate attributes gave a measure of total pigment volume valuable to further interpretation of gene action, knowledge of other interrelationships among the attributes could provide background necessary for an analysis of the basic action of the genes involved. In this paper each attribute will be further discussed, and all possible interdependences among them presented in a search for key characters whose variations determine all the rest. Tables and figures will be given of the variations pertinent to each topic discussed, but for a complete picture of all measurements of each genotype and of the methods used, the reader is referred to the earlier papers of the series. FOUND I N THE COAT-COLOR MUTANTS OF THE HOUSE