THE MECHANISM OF HEREDITY 



53 



comes to produce gametes they bear either the one or the 

 other — and never both — of these factors. In other 

 words, the germ cells (both male and female) of the hy- 

 brid are half of them L and half of them S. When the 

 F^ generation is selfed, therefore, it is a matter of 

 chance which of these geiTa cells meet to form zygotes. 

 If a large progeny is produced, there will be a ratio of 



1|L|L| : 2|L|/S'| : IIaS'IiS'I, and since the formulas L\L\ and 



S\S\ are like the zygotic formulae of the long-spiked and 

 short-spiked parents, respectively, the plants that they 

 produce will be long-spiked and short-spiked, as the case 

 may be, and will breed true to that character. The inter- 

 mediates, however, having been produced by zygotes 



L S lilie the F^ generation, will behave in the same man- 



ner when selfed. 



That the ratio will be approximately 1 L\L : 2 L S\ 



1 8 S is plain if one thinks for a moment what the result 



would be if a thousand tickets bearing the letter L and a 

 thousand tickets bearing the letter S were shuffled up in a 

 hat and drawn out in pairs, replacing the pair each time 

 after drawing and recording. Suppose the first member 

 of the pair represents the egg cell; the chances are % 

 that it will be L or S. The second member of the pair 

 represents the male cell and the chances are likewise % 

 that it will be L or ^S'. Therefore, when L is the first mem- 

 ber of the pair, half of the time the zygote formed will be 



L L and half of the time it will be L S , Likewise, 



when 8 is the first member of the pair, zygotes \S\L\ 



and S S| will be formed in equal quantities. Combining 



these possibilities, the ratio 1|-^|L| : 2|L|5'| : 1|<S'|<S'| is 



