Hybrid Vigor 



175 



factors C and B. The hybrid is more \i^'()r{)us than 

 either because it combines all four dominant factors. 

 The attractiveness of this scheme Hes in the fact that it 

 escapes the objections that were made to the older domi- 

 nance hypothesis. 



Fig. 27. — Diagram to aid in visualising Jones's explanation of 

 hybrid vigor by dominance of linked factors. 



I. 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 race homozygous for 

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



As a matter of fact, it would be theoretically possible under 

 this scheme to isolate just such a homozygous race. If crossing 

 over took place during gamete formation by the F,. .1 and C might 

 come to lie on the same chromosome. When a gamete containing 

 such a chromosome mated with another gamete of the same sort, 

 a race would thereby be established which was homozygous with 

 respect to -1 and C. If a similar performance took place (cither 

 simultaneously or in some subsequent generation) in the other 

 chromosome pair, the race would also achieve homozygosity with 

 respect to B and D, and would thereafter breed true for all four 

 factors. 



It must be evident, however, that there would be but a remote 

 chance of realizing this theoretical possibility. in;ismuch as there 

 actually must be many more than two chromosome pairs involved, 



