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 8. When the 

 Fj_ generation is selfed, therefore, it is a matter of 

 chance which of these germ cells meet to form zygotes. 

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



1 L\L : 2 L ti : 1 8\8 , and since the formulae 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-spited, as the case 

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

 mediates, however, having been produced by zygotes 



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



ner when selfed. 



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



1 .6' 8 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 S is the first member of the pair, zygotes 8 L 



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

 these possibilities, the ratio 1\L\L : 2 L S : 1 8 S is 



