GENE RECOMBINATION AND HETEROSIS 319 



weight per locule, and the Fj hybrid did show heterosis for increased weight. 

 Also, as has been shown in the F2 and Bi and P2 populations some individuals 

 were obtained having greater weight per locule than the Fi plants and this 

 greater weight per locule proved to be due to recombination of favorable 

 genes. 



Also, the interallelic interactions of the genes as determined by the inter- 

 relations of the component characters are of interest because of the informa- 

 tion they provide concerning recombination of genes and heterosis. The 

 effects of the Bb series of genes, the ^'^ series, and the Rr series, respectively, 

 were found to be cumulative. On an average the S genes would be e.xpected 

 to shorten the period from first bloom to first fruit set less in the presence 

 of the R genes than in the presence of the r genes — if the physiological reac- 

 tions affecting these two component characters that were instigated by the 

 environment were the same as those instigated by the 6'5 and Rr gene series. 

 That such was the case seems probable from the results of Goldschmidt's 

 work (1938) with phenocopies. In fact it seems almost axiomatic that this was 

 the case, because the second order interaction (Ss gene series X Rr gene 

 series X environment) was such that, on an average, when the ^5 series 

 responded to a given environment by shortening the period from first bloom 

 to first fruit set the Rr series in the same plant tended to produce a longer 

 period from first fruit set to first fruit ripe. Then the effects of these two 

 series of genes were less than additive as regards the dependent character pe- 

 riod from seeding to first fruit ripe 



About the same situation existed in respect to the Lclc series and the Ww 

 series of genes in that greater number of locules, on an average, was ac- 

 companied by less weight per locule. This type of interallelic interaction 

 would tend to decrease the possibility of obtaining inbred lines combining 

 desirable characters. This would be particularly true of the interallelic inter- 

 action between the Ss and Rr gene series, because a shorter period from first 

 bloom to first fruit set tended to be accompanied by a longer period from 

 first fruit set to first fruit ripe 



The data do not furnish any evidence concerning that type of intraallelic 

 interaction postulated by East's (1936) physiological hypothesis, other than 

 to say that no cases of overdominance were found. This would indicate that 

 probably overdominance does not play a predominant part in the produc- 

 tion of heterosis in the tomato hybrids studied. 



