Phenomena of Inheritance 93 



(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 x 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 (ovules and pollen) 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 combinations RW and YG. Accordingly there are four 

 possible combinations of these characters in the pollen and four 

 in the ovules as follows : 



Y G 



. \ X I i.e. YR, YW, GR, GW. 

 R W 



Each of these four kinds of pollen may fertilize any of the four 

 kinds of ovules, thus giving rise to sixteen combinations, 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. Accordingly the sixteen 

 possible combinations yield seeds of four different appearances 

 and in the following proportions: 9 YR:$ GR:^ YW: I GW. 

 Only one individual in each of these four classes is pure (homo- 

 zygous) 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 other individuals are heterozygous 

 and show Mendelian splitting in the next generation. 



Trihybrids. When parents differ in three contrasting char- 



