466 



GENETICS 



with many plants and animals. For example, when a gray mouse 

 is crossed with a white mouse all the offspring are gray, which 

 shows that gray coat-color is dominant to white coat-color in mice 

 (Fig. 245 j. In the F2 generation gray and white mice occur in the 

 ratio of 3 : 1 . Of the gray mice, which constitute three-fourths of 



GRAY MOUSE 



WHITE MOUSE 



GRAY MICE 



/ 



1 HOMOZYGOUS 

 GRAY MOUSE 



INTERCROSSED 



A 



2 HETEROZYGOUS GRAY MICE 



N 



1 HOMOZYGOUS 

 WHITE MOUSE 



INTERCROSSED 

 I 



HOMOZYGOUS GRAY MICE 



/^ 



— V 



INTERCROSSED 



INTERCROSSED 

 I 



HOMOZYGOUS WHITE MICE 



~N 



1 HOMOZYGOUS 

 GRAY MOUSE 



i 2 HETEROZYGOUS GRAY MICE 



1 HOMOZYGOUS 

 WHITE MOUSE 



Fig. 245. — Results of mono-hybridization of mice which differ with respect to 

 color of coat. Gray coat-color is dominant to white coat-color (c/. Fig. 252). 



this generation, some are found to breed true for grayness, while 

 others reproduce both gray and white offspring. Complete 

 analysis reveals that 25 per cent of the F2 generation are gray and 

 will breed true for gray coat-color, 50 per cent appear gray but will 

 not breed true, and 25 per cent are white and reproduce only 

 white individuals. 



Up to this point the examples used have been cases of inheri- 

 tance with dominance. Dominance and recessiveness are not, how- 



