PHENOMENA OF INHERITANCE 333 



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

 of 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 figure 54. 



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-wrinkled, yellow-green, and therefore any 

 combination of these may occur in a germ cell except the combinations 

 RW and YG. Accordingly there are four possible kinds of germ cells 



as follows: X *■ e -> YR > YW > GR > Gw - Each one of tnes e four 



r x x ir 



kinds of pollen may fertilize each one of the same four kinds of ovules 

 giving rise to sixteen combinations, no two of which are alike, as shown 

 in Fig. 54. 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 combina- 

 tions yield seeds of four different appearances and in the following 

 proportions: 9RY: 3RG: 3WY: 1WG. Only one individual in each of 

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

 successive generations; in Fig. 54 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. 



When parents differ in three contrasting characters a much larger 

 number of combinations are possible in the F 2 generation. Thus if a 

 pea with round (R) and yellow (Y) seeds, and with tall (T) stem is 

 crossed with one having wrinkled (IV ) and green (G) seeds, and dwarf 

 (D) stem all the progeny of the F x generation have round and yellow 

 seeds and tall stem, R, Y and T being dominant to W, G and D. But 

 in the F 2 generation there are sixty-four possible combinations (geno- 

 types) of these six characters; but since a recessive character does not 

 develop if its contrasting dominant character is present there are only 

 eight types which come to expression (phenotypes) and in the follow- 

 ing numbers : 27RYT : 9RYD : 9RGT : 3RGD : 9WYT : 3WYD : 3WGT : 

 1WGD. Of these sixty-four genotypes only eight are homozygous and 

 breed true (those lying in the diagonal between upper left and lower 

 right corners in Fig. 55), while only one is pure dominant and one pure 

 recessive (in the upper left and lower right corners of Fig. 55). 



When the parents differ in one character only, the offspring formed 

 by their crossing are called monohybrids, when there are two contrast- 

 ing characters in the parents the offspring are dihybrids, when three> 

 trihybrids, and when the parents differ in more than three characters the 

 offspring are called poly hybrids. There are certainly few cases in which 

 parents actually differ in only a single character, but since each con- 

 trasting character may be dealt with separately, as if it were the only 



