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 

 F-^ 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 S -.18 8\, and since the formulae L\L and 



\8\8\ 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\8\ like the F^ generation, will behave in the same man- 

 ner when selfed. 



That the ratio wiU be approximately 1|L|L : 2 L\8\ 



1 8 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 8 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 8. The second member of the pair 

 represents the male cell and the chances are likewise l^ 

 that it will be L or 8. 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\8\ . Likewise 



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



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



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



