Wright: Color Inheritance in Mammals 



229 



whether enzyme II is present or not. 

 The interj^radin*^ colors must be looked 

 upon as resulting; from reductions of 

 various kinds in the activity of the 

 enzymes. We have recognized two 

 distinct types of intergrades between 

 black and red, the coarsely granular 

 type of bays and sooty yellows, and 

 the finer type of chocolate browns. 

 A mere reduction in quantity of enzyme 

 II may perha]3S be thought to result in 

 an intermingling of black and red 

 granules as one or the other of the 

 respective enzymes chances to pre- 

 dominate, while a uniform reduction in 

 potency in some other way not involving 

 a reduction in quantity may give the 

 chocolates and the pale browns of the 

 pink-eyed rodents, in which there seems 

 to be little more tendency for red to win 

 in competition than in intense blacks. 

 Similar relations with respect to enzyme 

 I may account for the differences be- 

 tween the maltese and sepia types of 

 dilution of black and the correlated light 

 red and cream types of dilution of red. 

 Of course, any such definite assignment 

 of physiological effects to factors is at 

 present to be taken mainly as a means 

 of visualizing their action in our igno- 

 rance of the real physiology. In order 

 to make wholly clear the relations sup- 

 posed to hold between the different col- 

 ors on the hypothesis, the accompanying 

 diagram (Fig. 9) is given. Full quantity 

 is rejDresented by two symbols, reduced 

 quantity by a single symbol, reduced 

 potency by a symbol of small size, and 

 complete inhibition or impotency by 

 absence of symbol. Variations in en- 

 zyme I are given horizontally, variations 

 of enzyme II vertically. 



The classification of color factors 

 which it is desired to present is based 

 ]3rimarily on the difference between 

 factors which act as if on enzyme I , and 

 those which act as if on enzyme II. A 

 secondary classification is based on the 

 mode of action. There is a wide 

 difference between factors which pro- 

 duce no effect in ]3arts of the coat though 

 with maximum effect in other parts, i.e.. 

 bring out a pattern, and factors which 

 produce the same effect throughout fur, 



skin and eyes. In the former case 

 increase in the array of factors causes 

 extension of the pattern; in the latter 

 whatever pattern may be present tends 

 to be stationary on increase in the array 

 of factors. There is instead a further 

 general change in intensity. The 

 coarsely granular types of intergrades 

 should perhaps form a third subclass in 

 each main class, but for the present they 

 are most conveniently put with the pat- 

 tern factors. They can be considered as 

 determining a fine ])attern within the 

 individual hairs. 



CLASSIFICATION OF COLOR FACTORS 



1. Factors which affect distribution 

 and intensity of color, largely irrespec- 

 tive of the kind of color. (Act as if on 

 enzyme I). 



(a) Factors which affect the distribu- 

 tion of color in contrast with white. 



(h) 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 differentiation from 

 yellow to black — effects, of course, vis- 

 ible only in colored areas. (Act as if 

 on enzyme II.) 



(a) Factors which affect the distribu- 

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

 etc.) in contrast with a yellow. 



(6) Factors which affect the intensity 

 of only the .dark colors 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 follow- 

 ing factors: 2w Cd Cd AAbb. :Ew repre- 

 sents unanalyzed hereditary factors of 

 class la which determine a pattern 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 



15 Wright, S., 1916. Carn. Inst. Wash. Pub., 241, part II. 



