34 



THE INHERITANCE OF COLOR IN MICE. 



The relation of the restriction factor It to the factors P and D has been 

 considered brief!}' under the head of those factors. The occurrence of dilute 

 yellow mice, clearly distinguishable from "creams" or "light yellows," was 

 reported by the writer (1911) in the dark-eyed color series. Since that time a 

 few crosses have been made with pink-eyed dilute yellows and they have been 

 found to conform in their general behavior with other yellow varieties. They 

 apparently can not be obtained in a homozygous state, and when crossed inter 

 se give pink-eyed dilute yellow and pink-eyed dilute non-yellow, as 2 to 1. 



Dilute yellow animals, whether clear, sooty, or sable, are in appearance 

 more heavily pigmented than are the lightest creams; but they, as the intense 

 yellows, are found in shadesof yellowthat vary between cream and deep orange. 

 Any and all dilute yellows have, however, the characteristic streaky, washed- 

 out look of the dilute black or dilute brown forms. In crosses, dilute yellows 

 bred inter se have given: 



Observed 



Expected, 2 to 1 . . 

 Expected, 3 to 1 . . 



Dilute 

 yellow. 



Dilute 

 non-yellow. 



100 



102.67 



115.5 



54 

 51.33 



38.5 



It will, then, be seen that dilute yellows, like intense yellow T s, give a 2 to 1 

 ratio, the homozygous type being apparently unable to develop. 



Dilute yellows crossed with dilute non-yellows have given the following: 



8. Sooty Yellows and Sables. 



It is well-known that varieties of yellow mice exist which have a varying 

 amount of dark pigment (brown or black) in the hairs on their dorsal and lateral 

 surfaces; this produces types of yellow known as "sooty yellows" or "sables." 

 The amount of dark pigment varies from a few dark hairs, scattered along the 

 mid-dorsal line, to a condition in which the whole dorsal and nearly all the 

 lateral surfaces are dark, leaving yellow confined to the more ventral portions 

 of the sides and to the belly. A series of arbitrary grades indicating the degree 

 of sootiness has been constructed within the past year. By this means it is 

 hoped to get more accurate ideas as to the nature of sooty yellows. The grades 

 are based upon skins of "sooty" animals and these skins are used as the types 

 by which the living animals are graded. Several experiments have been at- 

 tempted with the sooty yellows, but all these are still in a preliminary state. 

 Higher numbers of individuals from experiments now in progress may tend to 

 change the conclusions to be drawn from work with this variety. 



