COLOR INHERITANCE IN MAMMALS 



II. The Mouse — Better Adapted to Experimental Work than Any Other Mammal- 

 Seven Sets of Mendelian Allelomorphs Identified — Factorial Hypothesis 

 Framed by Cuenot on Basis of His Work with Mice 

 Sewall Wright 

 Bureau of Animal Industry, Washington, D. C. 



IN THE first paper of this series,^ a 

 classification of color factors in 

 mammals was suggested on a hypo- 

 thetical chemical basis. A general dis- 

 cussion was given of the factors belong- 

 ing to each class. The present and 

 following papers attempt to give a more 

 detailed analysis of the present state of 

 our knowledge on the subject in each of 

 the mammals on which work has been 

 done. 



By far the most experimental work 

 has been carried on with four rodents — 

 the mouse, rat, rabbit and guinea-pig, 

 and the factors have been determined 

 with very much more certainty than 

 in the larger animals. A few facts are 

 known in regard to other rodents. 



Among the larger domestic animals 

 it is a curious fact that the amount 

 known is in inverse relation to the 

 difficulty in breeding. The color factors 

 of the horse have been quite thoroughly 

 worked out. Cattle stand next, while 

 in sheep, goats and hogs our information 

 is rather scanty. The reason for this 

 inverse relation is easy to understand. 

 Just because of the slowness of breeding 

 horses, there has been, for the most 

 part, but little attempt to fix the colors 

 in the different breeds. Thus stud 

 book records show all kinds of matings 

 with segregation of the colors in the 

 offspring. The breeds in cattle have 

 been fixed in color to a greater extent 

 than in horses but the Shorthorns 

 and Highlands, at least, continue to pro- 

 duce several colors. In swine, each 

 breed has its characteristic color and 

 nothing can be learned from the records. 

 In European breeds of sheep, all colors 



have been nearly eliminated except 

 white, owing to the greater desirability 

 of this color for wool. Other colors 

 exist as in the tan and black Barbados, 

 but little is known of their inheritance. 

 Dogs and cats are even richer in 

 color varieties than the rodents, but 

 the factors are none too well understood. 

 Finally in man, the problems in color 

 inheritance appear to be of an unusually 

 difficult nature. 



CLASSES OF COLOR FACTORS 



In the previous paper color, factors 

 were classified into four groups defined 

 as follows : 



1. Factors which affect distribution 

 and intensity of color, largeh" irrespec- 

 tive of the kind of color. 



(a) Factors which affect the distribu- 

 tion of color in contrast with white. 



(6) Factors which affect the intensity 

 of color in all colored areas of the skin, 

 fur and eyes. 



2. Factors which affect the distribu- 

 tion and intensity of the differentiation 

 from yellow to black — -effects, of course, 

 visible only in colored areas. 



(a) Factors which affect the distribu- 

 tion of a dark color (black, sepia, brown, 

 etc.) in contrast with a yellow. 



(b) Factors which affect the intensity 

 of only the dark colors with effects 

 visible wherever such colors develop 

 in skin, fur and eyes. 



In a more detailed study of the 

 factors a rough subdivision of classes 

 la and 2a becomes convenient. This 

 gives eight classes which may be defined 

 briefly by the following list of char- 

 acteristic variations. 



1 Wright, S. 1917. Jour. Heredity, 8:224-235. 



373 



