TACTICS IN PIGMENT-CELL RESEARCH 325 



resemble the black-and-tan pattern resulting from the agouti allele, a 1 -. The darken- 

 ing of the fur of these animals is attributable to the cumulative action of a number of 

 factors which are independent of the agouti locus. 300, 1067 Because of this extreme 

 variation in the amount of eumelanin and phaeomelanin which can and does occur in 

 heterogeneous stocks of agouti-locus genotypes, it should be recognized that in such 

 studies as those concerned with possible enzymatic differences between these genotypes, 

 it is extremely important to control the background on which each of these agouti 

 alleles expresses itself. 



Studies on nongenetic aspects of mammalian pigmentation. — Inbred strains are almost 

 obligatory for determining the significance of nongenetic factors in producing coat- 

 color variations. In the guinea pig Wright and Chase 1454 showed that white spotting 

 depends primarily on a recessive factor, s, with many minor genetic factors having 

 additive effects. Because of the availability of highly inbred strains of white spotted 

 guinea pigs Wright 1420 was able to demonstrate that, in addition, nongenetic factors 

 also play a part in determining this character. Variation in the amount of white 

 spotting even within highly inbred lines of these animals was found to be related to the 

 age of the dam, the amount of white spotting of her offspring increasing with her age. 

 A similar situation is also found in mice. In the C57BR/cd strain a white patch is 

 present on the ventral surface in a majority of the animals. The extent of this non- 

 pigmented area is extremely variable, ranging in size up to 10 per cent of the ventrum. 

 Although, as Murray and Green 925 demonstrated, the occurrence and extent of these 

 markings appear to have a hereditary basis ; the total effect of heredity is relatively 

 slight, most of the variation being nongenetic in origin. Here, unlike the guinea pig, 

 however, the age of the female apparently has only a very slight influence on the amount 

 of white spotting in her offspring so that most of the nongenetic variation has still to be 

 accounted for. 



DEVELOPMENT OF COISOGENIC COLOR LINES 



While the establishment of inbred strains of color stocks made available an un- 

 limited supply of genetically uniform animals of specific coat-color phenotypes, it still 

 had to be recognized that the differences between the coat-color patterns of these 

 stocks might be attributable not only to the major color factors with which these stocks 

 differed but also to the different genetic backgrounds on which these major factors 

 were incorporated in each of the established strains. Although with some of the major 

 genes concerned with pigmentation these variations are almost certainly negligible, 

 with others, as noted above, they are certainly not. For this reason the only genetic 

 material in which the effects of different alleles and different genetic loci can effectively 

 be compared are coisogenic strains — strains which differ from each other in respect of 

 a single locus concerned with melanin formation or, as in the case of white spotting, 

 with its absence. 



Production of coisogenic lines. — The production of such color stocks is, indeed, a very 



