30 PHYSICAL BASIS OF HEEEDITY 



generations, but as soon as a generation hatches under 

 favorable conditions they are as abnormal as though all 

 their ancestors had been of this sort. Thus it is evident 

 that no fundamental importance is to be attached to domi- 

 nance of characters. On the other hand, it is equally 

 obvious that it would be entirely unwarranted to suppose 

 that incompleteness of dominance is due to failure of 

 segregation of the genes that stand for the characters. 



While the problem of segregation can be studied to 

 greatest advantage where the characters of a pair are 

 sharply separated, yet even where the pair does not 

 possess this advantage, the cleanness of the segrega- 

 tion process can be just as definitely, though more 

 laboriously, demonstrated. 



In cases where there is an overlap between the hetero- 

 zygous type and one of the parental types it may, simply 

 as a matter of convenience, be advantageous to call that 

 character that gives the more continuous F2 group the 

 dominant, thus leaving the smaller more sharply defined 

 group as the recessive. For example, the F2 group from 

 black by wild-type Drosophila may be represented by 

 such a scheme (Fig. 6) as the following: 



Fig. 6. — Relation of black body color to wild type as shown by the classes of F2 flies. 

 The heavy outline includes the mutant class, the lighter line the wild type, «nd the dotted 

 line the heterozygous class. 



Here the heterozygous flies are typically intermediates, 

 but their variability overlaps that of the wild type to 

 such an extent that separation of the intermediate from 

 the wild type is practically impossible. On the other hand, 

 there is no difficulty in making a complete separation 

 between the heterozygous class and the homozygous black. 



