WRIGHT 



sity of only the dark color? with 

 effects visible wherever such colors 

 develop in skin, fur and eyes. 



This classification differs slightly 

 from one previously advanced by the 

 writer ^^ in that classes 2 and 3 of the 

 earlier paper are brought more closely 

 together as classes 2a and 2b. 



As an illustration of the classes of 

 factors, consider a guinea-pig which 

 is like a solid black except for the fol- 

 lowing factors: Sw Cd Cd A Abb. 2w 

 represents unanalyzed hereditary fac- 

 tors of class la which determine a pat- 

 tern of white regardless of anything 

 else. Factor A of class 2a puts a yellow 

 band in each colored hair. Factor Cd 

 of class lb makes the yellow a dilute 

 yellow instead of red, and makes the 

 dark parts of the hair lighter than 

 otherwise and perhaps slightly dilutes 

 eye color. Finally factor b of class 2b 

 further modifies the sepia in the coat 

 and eye producing light brown but 

 does not affect the yellow. The animal 

 is a brown-eyed light brown agouti 

 with yellowing ticking and a white 

 pattern in the coat. 



It is often difficult for one who is 

 not working in the inheritance of coat 

 color to understand just what color is 

 supposed to be determined by a given 

 array of factors. Probably this can be 

 done most easily by considering the 

 factors in the order just given. First are 

 the factors of class la which determine 

 patterns of color and white. No fac- 

 tors considered later can change these 

 white areas. Next to be considered is 

 the pattern of dark and yellow colors 

 visible in the colored areas. These are 

 determined by class 2a. Finally the 

 kind of yellow in the vellow areas may 

 be seen by noting the factors of class 

 lb and the kind of dark color in the 

 remaining areas by a simultaneous 



87 



consideration of classes lb and 2b. Eye 

 color is generally determined wholly 

 by these last two classes, but occa- 

 sionally extreme white patterns of 

 class la invade the eye. 



DISCUSSION OF CLASSES OF FACTORS ^* 



Class 1^.— White patterns are very 

 common in mammals and most of 

 them are obviously determined by fac- 

 tors of this class even where the mode 

 of inheritance has not yet been thor- 

 oughly analyzed. The factors which 

 determine the white face of the red 

 Hereford cattle are a good example. 

 The same white face appears in the 

 black cattle from the cross of Here- 

 ford with Aberdeen-Angus. Evidently 

 the factors involved strike at color in 

 general regardless of its quality. The 

 same is true of the different types of 

 white patterns found in roan and 

 in white Shorthorns, in the black and 

 white Holsteins, and in Dutch belted 

 cattle. The common white patterns of 

 horses, dogs and cats are similarly in- 

 dependent of the ground colors of the 

 animals. This seems to be true of the 

 belt in Hampshire hogs, but is not so 

 certain in other white patterns in hogs 

 which may correspond to extreme 

 dilution of yellow patterns. One or 

 more recessive Mendelian factors have 



15 Wright, S., 1916. Carn. Inst. Wash. 

 Piibl, 241, part II. 



1^ Most of the statements in regard to 

 color inheritance are based on well-known 

 investigations. A very detailed review of the 

 literature to 1913 is given by Lang, 1914 

 (Experimentelle Vererbungslebre, pp. 467- 

 888). A discussion of our present knowledge 

 on the subject with extensive bibliog- 

 raphy is given by Castle, 1916 (Genetics 

 and Eugenics). The writer may say that all 

 of the statements in regard to guinea-pigs 

 can be based on his own experience, and 

 he has also had the opportunity of becoming 

 directly acquainted with the mode of in- 

 heritance of most of the color varieties 

 among rats, mice and rabbits as an assistant 

 in Professor Castle's laboratory for three 

 years. Only a few references are cited in 

 this paper. 



