188 



GENERAL BOTANY 



Gametes (1), pure 

 S or w 



Children, 

 ation 



Gametes 

 S or 



gener- 



#), pure 



flowers would be almost certain to bring about all possible 

 combinations of S and w, when fertilization, or union, of the 

 gametes occurred to produce the F l children. 



Mendel assumed also that when the hybrid children (Sw) 



formed pollen and em- 

 bryo sacs, the deter- 

 Parents miners for S and w 



would again segregate 

 and occur singly in 

 each germ cell. The 

 gametes (2) would 

 therefore carry S and 

 w in the manner in- 

 dicated in Fig. 98. 

 Chance pollination and 

 fertilization would then 

 result in the combina- 

 tions of S and w shown 

 in the (JQ grandchil- 

 dren. Gametes (3), 

 formed by these grand- 

 children might then 

 combine to produce 

 pure and hybrid off- 

 spring in the ratio 

 shown in the F^ great- 

 grandchildren. If, there- 

 fore, we are willing to 

 accept Mendel's idea 

 of the purity of the 

 gametes, we are pre- 

 pared to understand how the offspring of hybrids may occur 

 in mathematical ratios similar to those obtained by Mendel. 



Paired contrasting characters. Mendel also crossed peas in 

 which the parents differed in two pairs of contrasted characters. 

 In Fig. 99 the results of such an experiment are indicated by 

 drawings and letters similar to those employed in Fig. 98. 



Grandchildren, F 2 

 generation 



Gametes (3), pure 

 S or w 



Great-grandchildren, 

 F s generation 



FIG. 98. Diagram illustrating Mendel's law of 

 purity of gametes 



