Hybrid Vigor 



175 



either because it combines all four dominant determiners. 

 The attractiveness of this scheme is that it escapes the 

 objections that were made to the older dominance 

 hypothesis. 



1 . The fact that 100 per cent hybrid vigor cannot be 

 fixed is quite in accordance with Jones's scheme, for 

 it is obviously impossible to isolate a homozygous race 

 with all four factors A, B, C, and D. 



2. A simple mathematical demonstration will show 

 that the distribution of F^ individuals is quite what it 



X 



Fig. 38. — Diagram to aid in visualizing Jones's explanation of 

 hybrid vigor by dominance of linked factors. 



should be, represented by a symmetrical curve similar 

 to the curve of probabilities. 



In fact this new theory, "the dominance of linked 

 factors," seems altogether sound. We should reason- 

 ably expect that each chromosome would contain one or 

 more dominant determiners (conducive to vigor) linked 

 with one or more recessives. 



The discussion has left the impression that hybrid 

 vigor is to be explained by dominance (and linkage). In 

 this day of factors and determiners such a hypothesis is 

 quite appropriate. It may be, however, that in the 



