94 Heredity and Environment 



F 2 generation, viz., dominants and recessives in the ratio of 3 : i 

 (Fig. 30) ; where there are two pairs of contrasting characters 

 in the parents there are nine genotypes (3 2 ) and usually four 

 phenotypes in the F 2 generation in the ratio of (3: i ) 2 = 9 : 3 : 3 : i. 

 Thus when Mendel crossed a variety of peas bearing round and 

 yellow seeds with another variety having wrinkled and green 

 seeds all the offspring of the F^ generation bore round and yellow 

 seeds, round being dominant to wrinkled, and yellow to green. 

 But the plants raised from these seeds, when self-fertilized, 

 yielded seeds of four types, yellow and round (YR), yellow and 

 wrinkled (YW), green and round (GR), and green and wrinkled 

 (GW) in the proportion of 9: 3 : 3 : i as shown in Fig. 31. 



In this case also this ratio may be explained by assuming that 

 the germ cells are pure with respect to each of the contrasting 

 characters, round or wrinkled, yellow or green, and therefore any 

 combination of these may occur in a germ cell except the com- 

 binations RW and YG. Accordingly there are four possible 

 combinations of these characters in both male and female cells 

 as follows : 



Y G 



| X | i.e. YR, YW, GR f GW. 



R W 



Each of these four kinds of male cells may fertilize any one of the 

 same four kinds of female cells, thus giving rise to sixteen com- 

 binations, as shown in Fig. 31. The dominant characters are in 

 this case round and yellow, and only when one of these is absent 

 can its contrasting character, wrinkled or green, develop. Ac- 

 cordingly the sixteen possible combinations yield seeds of four 

 different appearances and in the following proportions : 9 YR : 

 3 GR : 3 YW \ i GW. Only one individual in each of these four 

 classes is pure (homozygous) and continues to breed true in 

 successive generations; in Fig. 31 these are found in the diagonal 

 from the upper left to the lower right corner. All these in- 



