172 Outline of Genetics 



number of heterozygous sets in the hybrid. This dia- 

 gram served the purpose of explaining heterozygosis, 

 but it win now be discarded because it does not represent 

 the most important result when two races are crossed. 

 The important result is represented in the following 

 diagram : 



Parents F« 



mbbuDDEEFF ] AaBbCcDdEeFf =...or^ hybrid vigor 



tifcCDDei } ^^imoDdeefj = less hybrid vigor 



The thought is that in each of these two cases the hybrid 

 is more vigorous than either parent, not because it con- 

 tains more heterozygous sets, but because it contains 

 more dominant factors, which means more ''desirable" 

 characters. For example, in the first case each parent 

 contains three factors, the small letters representing 

 merely the absence of factors. The Fi generation, there- 

 fore, contains six factors, and for this reason is more 

 vigorous than either parent. It is stated in the diagram 

 that in the first case there is ''more hybrid vigor" and 

 in the second case "less hybrid vigor," simply because 

 hybrid vigor is a relative term. It represents merely 

 how much more vigorous the hybrid is than either parent. 

 In the first case the parents have three factors and the 

 hybrid six, the increase being three, which measures the 

 amount of hybrid vigor. In the second case each parent 

 has two and the hybrid four; the increase, therefore, 

 is only two, and for this reason there is less hybrid vigor 

 in the second case than in the first. 



Assuming that the majority of dominant factors are 

 desirable, and that desirable factors make for general 



