No. 594] 



COLOE MUTATIONS IN MICE 



337 



among the mice raised from the gray-bellied agouti stock, 

 there was not even an approach to the white-bellied agouti 

 type. 



The dilute brown animals which were used belonged to 

 a race descended from a single pair of dilute brown mice 

 which had been tested by suitable matings in order to 

 establish their homozygosity. These animals in common 

 with other "non-agouti" varieties of mice lack all visible 

 traces of the agouti pattern and are with the exception 

 of a slightly lighter ventral than dorsal surface uniformly 

 pigmented throughout. 



The cross between these two races, then, was one be- 

 tween a gray-bellied agouti and a "non-agouti" race. 

 Both races used as parents were therefore color varieties 

 which were hypostatic to the white-bellied agouti type 

 and which for this reason could not carry that pattern as 

 a recessive. 



Four females of the dilute brown race were crossed 

 with a gray-bellied agouti male 131 and produced 26 gray- 

 bellied agouti young as shown in Table I. 



TABLE I 

 Gray-Bellied Agouti 



$blx' D c?131 6 



$ b2 X c? 131 8 



$ b3 X c? 131 6 



$b4X<5'131 6 



26 



The F, gray-bellied agoutis were more intensely pig- 

 mented than their gray-bellied agouti parents. This 

 deepening of color has frequently been observed in cross- 

 ing wild agouti varieties with tame races and is probably 

 due to modifying factors introduced by the particular 

 tame race used or else to a general acceleration of pig- 

 ment production due to heterozygosis. 



The F x animals were crossed inter se and 285 F 2 young 

 were raised. Of these 14 died at too early an age to have 

 their color recorded. 



