PHENOMENA OF INHERITANCE 



241 



four types, round and yellow (RY), wrinkled 

 and yellow (WY), round and green (RG), 

 and wrinkled and green (WG) in the propor- 

 tion of 9 : 3 : 3 : 1 as shown in Fig. 53. 



In this case also this ratio may be explained 



YRCGW) 



2T7 



YR 



YR YW GR GW 



YW 



GR 



GW 



F^ 



FIG. 53. Dihybrid Diagram showing results of crossing peas 

 having yellow-round (YR) seeds with others having green- 

 wrinkled (GR) ones. Four types of germ cells are formed 

 by such a hybrid, viz. YR, YW, GR, GW, and the 16 possible 

 combinations (genotypes) of these $ and $ gametes are shown 

 in the small squares. Since recessive characters do not ap- 

 pear when mated with dominant ones these 16 genotypes pro- 

 duce 4 phenotypes in the following relative numbers: 9YR: 

 3YW:3GR:1GW. There is 1 pure dominant (upper left cor- 

 ner), 1 pure recessive (lower right corner), 4 homozygotes in 

 diagonal line between these corners, and 12 heterozygotes. 



