24 INTROGRESSIVE HYBRIDIZATION 



plains them in the following way: Let us suppose that the 

 differences between two hybridizing entities are conditioned 

 by a single factor. If there is no dominance, the condition 

 for one parent may be written as AA, and that for the other 

 parent as A'A\ and the Fi hybrid will be A A' and inter- 

 mediate. In the F2 these differences will segregate in a ratio 

 of 1 A A : 2 A A' : 1 A'A\ If the differences between the 

 two parents are due to two genes A vs. A' and B vs. B\ then 

 again the hybrid A A' BE' will be intermediate, but this time 

 in the F2 we shall have a much more comphcated segrega- 

 tion. The genotypes and their ratios will be : 



Now for the purposes of illustration, we consider the ex- 

 tremely simple case of a difference between 2 parents that is 

 equally due to 2 pairs of genes, A vs. A' and B vs. B' . Let 

 us suppose (to take an example simpler than any for which 

 we yet have experimental e\ddence) that the difference be- 

 tween the 2 parents lies entirely in leaf length and that this 

 difference is 4 units. If we diagram the short-leaved parent 

 as AABB and the long-leaved one as A'A'B'B', and if, as we 

 have supposed, the length difference is borne equally by the 

 2 gene pairs and is without dominance effects, then the Fi, 

 AA'BB' , will be 2 units larger than the small-leaved parent. 

 An additional unit of leaf length will have been contributed 

 by A' , and another unit by B'. In a similar way we can as- 

 sign length values to the 9 possible genotypes in the F2. 

 They will all go into 5 size classes, i.e., (1) those with no ad- 



