324 BIOCHEMICAL GENETICS 



apparently only be greatly influenced through the local environment in which this cell 

 occurred, also commended these cells as attractive subjects for experimental studies in 

 a wide variety of biological fields. 



Although there is a great array of pigmentation patterns among different species 

 of mammals, all of these patterns owe their origin to factors fundamentally similar. 

 They are based on regional or local variations in the distributions of two basic pigment 

 types, eumelanin (black, brown) and phaeomelanin (yellow, red). They include 

 phenotypes in which pigment is completely or partially absent. Familiar examples are 

 white spotting involving localized regions of the body and silvering or mottling due to 

 variations in the pigmentation of individual follicles. Furthermore, it is probably true 

 to say that almost all the diverse patterns of pigmentation found in mammals are 

 phenotypically represented within certain domesticated species, particularly in dogs. 

 Despite its presentation of a wide assortment of coat-color patterns which have, to a 

 certain extent, been analyzed genetically, 797 the dog is an unfavorable species for 

 critical studies of genie action because of the difficulty evident in attempts to breed it 

 and inbreed it on a large scale. Attention was, therefore, devoted to the small labora- 

 tory rodents as convenient subjects with which to investigate these problems. 



Since the effect(s) of a particular genetic locus can only be recognized on the basis 

 of variations (alleles) from wild type produced by mutations, the development of suitable 

 laboratory animals has depended on the recognition and description of coat-color 

 mutants and their preservation. 



DEVELOPMENT OF INBRED COLOR STOCKS 



A thorough, systematic investigation of the genetics of mammalian pigmentation, 

 however, requires more than maintaining coat-color mutants. It requires the produc- 

 tion of different stocks of animals each of which can be defined with respect to its coat- 

 color genotype. Furthermore, since it has been shown that the expression of many 

 genes concerned with melanin formation, or suppression of its production, may be 

 drastically modified by other genes (modifiers) , it is obligatory to produce inbred color 

 stocks in order to control genetically the phenotypic variations that are almost un- 

 avoidable in noninbred material. For example, in the mouse the genetics of white 

 spotting differs from the inheritance of most color patterns in that it is much more 

 quantitative in character. Success in studying this quantitative character, therefore, 

 depends on developing genetically homogeneous lines. 



The relative amounts of eumelanin and phaeomelanin present in the hair shafts, 

 determined by the agouti series of alleles of the mouse, can also be greatly influenced 

 by the cumulative expression of other genes with which these alleles are associated. 

 For this reason there exist certain modified strains of genotypically yellow mice, usually 

 described as being of sable color, which contain a variable amount of black pigmenta- 

 tion down the middle of the back or even the entire dorsum. Some of these animals 

 contain so much dark pigment that, although genetically yellow, phenotypically they 



