242 HEREDITY AND ENVIRONMENT 



by assuming that the germ cells (ovules and 

 pollen) are pure with respect to each of the 

 contrasting characters, round-wrinkled, yel- 

 low-green, and therefore any combination of 

 these may occur in a germ cell except the com- 

 binations EW and YG. Accordingly there 

 are four possible kinds of germ cells as follows : 



Y G 



V i.e. YH, 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, no two of which 

 are alike, as shown in Fig. 53. 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 ap- 

 pearances and in the following proportions: 

 9RY:3RG:3WY:IWG. Only one individual 

 in each of these four classes is pure (homozy- 

 gous) and continues to breed true in successive 

 generations; in Fig. 53 these are found in 

 the diagonal from the upper left to the lower 



