Hybrid Vii^or i6i 



vigor. East of course did not conliniR' to investigate 

 all of these 85 types, but concentrated ui)()n two repre- 

 sentatives. Corn was selected as represent in<( s[)ecies 

 normally cross-fertilized in nature, while tobacco was 

 used to represent those species generally self-fertih'zed 

 in nature. 



East's results with corn need not be discussed in 

 detail, for they confirmed Shull's results in every point. 

 It was found that crosses between plants of aj)pi"()xi- 

 mately the same genotype resulted in little or no hybrid 

 vigor, even in cases where the two parents to the cross 

 had been grown under different environmental conditions 

 (thus correcting Darwin's misconception, see p. 156). 

 It was also observed that some crosses resulted in rela- 

 tively less hybrid vigor than others. From such results 

 East developed a very significant and useful Mendelian 

 interpretation of hybrid vigor. His proposition is that 

 hybrid vigor is proportional to the number of factors in 

 which the parents differ. This situation may be \'isu- 

 alized from the following diagram. 



Parents !• i 



AABBCCDDXAABBCCdd = AABBCCDd = \iii\c hybrid vigor 

 AABBCCDDX AABBccdd = AABBCcDd = more hybrid vigor 

 AABBCCDDX AAbbccdd = .1.1 BhCcDd = still more hybr id vigor 

 AABBCCDDX aabhccdd = AaBbCcDd = mo<.i hybrid vigor 



It is the Fi of course that shows the vigor, but what 

 index can be obtained from the germinal formula of the 

 Fi generation as to the amount of hybrid \ igi)r that it 

 will show? It is evident that this index lies in the fact 

 that hybrid vigor is proportional to the innnber of factors 



