7 70 PHYSIOLOGIC GENETICS 



on an incompletely recessive gene s. There are unanalyzed modifiers, 2 (Ms), that 

 can shift the median percentage of white in inbred strains with ss from about 10 per 

 cent to 98 per cent. In all cases females average a little whiter than males. The 

 same female produces whiter young on the average as she becomes older. The pattern 

 has some orderliness — with the strongest tendency to color near eyes and ears, strongest 

 tendency to white on feet, nose, and midline of belly; but there is always an enormous 

 amount of variability that can only be attributed to accidents of development. An 

 inbred strain may range from a trace of white to black-eyed self white and yet show by 

 the absence of correlation between parent and offspring that there is no genetic varia- 

 bility. In such a strain there is indeed no correlation between points one third of the 

 length of the animal apart with respect to presence or absence of white. 



Gene s is not mutable germinally in inbred strains. Spotting is probably due to 

 interaction of two modifiable patterns: migration of melanocytes from the neural 

 crest into the skin, and differentiation of the skin. 117a - 1352a The points of most 

 interest here are interaction effects with certain other processes indicating that the 

 spotting process is not merely one which leads to presence or absence of pigment cells. 

 It is a process in which the pertinent cells in the areas where color is present fall into 

 different states in a spotting pattern which affects certain color processes but not others. 

 Those affected most conspicuously are the tortoise-shell pattern due to e p (5, 6 in 

 figure 22) and the pattern of dinginess in browns of genotype EbbCCPPFF (28 in figure 

 22). The pattern of silvering due to sisi (2 in figure 22) and the intensity of pale sepia 

 or pale brown (22 in figure 22) are affected similarly but much less frequently. 

 There are very rare cases of mosaics with respect to other loci which suggest somatic 

 mutation but which are usually related to white spotting in a way that suggests that 

 gene s has something to do with them. 1456 



Tortoise shells of genotype SSe v e p are predominantly eumelanic but usually show 

 scattered yellow hairs and less frequently more or less yellow in blotches. With 

 sse p e p or even Sse p e p , the amount of yellow is increased and there is a strong tendency 

 to segregation of yellow and eumelanin into a few large areas each often with scattering 

 admixture of the other color. 199a ' 627, 1419 These areas are often separated in whole or 

 part by white streaks. Sometimes a streak between eumelanic areas is white at one 

 end, yellow at the other, indicating that the determination of yellow is related to the 

 process that leads in more extreme cases to white by absence of the pigment cells. 

 Similarly the orderly pattern of dinginess, found in the presence of SS, is broken up 

 in the presence of ss into a coarse mosaic of light and dark dingy areas, often separated 

 in whole or part by white streaks. In this case, it seems to be the determination of 

 the darker areas that is most closely related to the determination of white. 1439 



COLOR FACTORS AND ACTIVITY OF TYROSINASE 



There has been considerable research on differences in enzymatic activity in 

 pigment cells of different genotypes. There are definite differences which may be 



