332 BIOCHEMICAL GENETICS 



favorable for melanocyte differentiation. It was with this idea in mind that the 

 experiments described below were undertaken by Markert 851 and Markert and 

 Silvers. 100 



In the mouse there are at least 14 different loci associated with white spotting. 499 

 Some of these genes, for example, IV, W v , W j (dominant spotting) and Mi wh (white), 

 when homozygous, produce animals which are completely white with melanin pigment 

 occurring only in the cells of the retina. Indeed, the fact that the retina is pigmented 

 in these otherwise all white mice indicates that the genetic capacity for tyrosinase 

 synthesis certainly must exist. It is pertinent that retinal melanocytes originate from 

 the outer wall of the optic cup instead of from the neural crest. Since the hair follicles 

 of these white animals contain no demonstrable clear cells, they are essentially one 

 large spot and are, therefore, extremely useful subjects for investigating the etiology of 

 white spotting. 



To test the hypothesis that these animals are white because of an environmental 

 effect which prevents a step in the differentiation of neural-crest cells into melanocytes, 

 various embryonic tissues containing neural-crest derivatives, from both potentially 

 pigmented and potentially black-eyed white animals, were implanted intraocularly 

 into albino hosts. While such implants from potentially pigmented genotypes usually 

 gave rise to numerous melanocytes that migrated over the inner surface of the host 

 eye, 853 in no case did this occur with implants derived from potentially black-eyed 

 white animals. Thus, if these animals are white because of an arrest in the differen- 

 tiation of melanoblasts, then this genetic suppression may be the result of a locus 

 which acts autonomously within the melanoblasts themselves rather than through the 

 environment in which these cells occur. 



While such a basis for white spotting is attractive in explaining the all-white 

 phenotypes which are produced when such genes as those of the W series are homo- 

 zygous, it does not adequately explain the localized white spotting produced in the 

 otherwise pigmented animals heterozygous for these factors. Further investigations 

 are required to elucidate the etiology of white spotting, which may very well result 

 from a multiplicity of causes inasmuch as there are so many different loci associated 

 with this character. 



OTHER INVESTIGATIONS UTILIZING INBRED AND COISOGENIC COLOR STOCKS OF 

 MICE 



Other studies concerned with genetic aspects of pigment function in which inbred 

 and coisogenic strains of color stocks have proved invaluable include : those concerned 

 with the capacity of hair bulbs of different genotypes to bring about the oxidative 

 blackening of dopa 1119 ; those concerned with the ability of homogenates prepared 

 from skin of different genotypes to oxidize tyrosine, tryptophane, or other amino 

 acids; 395 autoradiographic studies of the incorporation of C 14 -labeled tyrosine into 

 melanin; 381 - 852 and microscopic analyses of the size, shape, number, and arrangement 



