GENE RECOMBINATION AND HETEROSIS 305 



greater than 1.511883 must have genotypes composed of more favorable 

 genes than the Fi, and therefore recombinations of genes to produce plants 

 having a greater weight per locule than the Fi plants have occurred. 



Whether inbred lines retaining this increased weight per locule can be 

 established is dependent upon the number of gene pairs differentiating the 

 parents and linkage relations (Jones, 1917). Close linkage of genes favorable 

 to increase in weight per locule would favor recombination. Whereas close 

 linkage of genes favorable to increase in weight per locule with those not 

 favorable would hinder recombination and hence reduce the chances of ob- 

 taining inbred lines retaining some or all of the advantages attributable to 

 heterosis. 



The data furnish evidence concerning the number of gene pairs differen- 

 tiating weight per locule. From Table 19.5 it can be seen that the plants of 

 the F2 generation falling beyond the value 1.511883 are distributed over four 

 different classes, and those of the Bi to Ponderosa population falling beyond 

 this same value occur in three different classes. The behavior of these plants 

 cannot be explained on the basis of five or more independently inherited 

 pairs of genes, as there are too many of these plants falling beyond the 

 1.511883 class. In addition, the weights per locule of those falling in these 

 classes are greater than can be explained on the basis of chance deviation. 



Further, to account for the plants of the F2 and Bi to Ponderosa popula- 

 tions falling in those classes beyond 1.511883, on the basis of five or more 

 pairs of independently inherited genes differentiating the parents, it would 

 be necessary to assume that 50 per cent or more of the plants were fluctuating 

 around a mean greater than that of the Fi generation. Since the means 

 (Table 19.4) of the F2 and Bi to Ponderosa populations are less than the 

 mean of the Fi, these populations cannot have a greater majority of the 

 plants fluctuating around a mean larger in magnitude than that of the Fi 

 plants. This deduction is confirmed by the frequency distributions of Table 

 19.5, as both of these populations have a greater percentage of their plants 

 in lower classes of the frequency distributions than does the Fi population. 

 Powers, Locke, and Garrett (1950) have shown that the data give a good fit 

 to frequency distributions calculated on the assumption that the parents are 

 differentiated by three pairs of genes. 



Here, proof of recombination of genes to produce plants in the F2 and Bi 

 to Ponderosa populations with greater weight per locule than Fi plants is 

 fairly conclusive. Also, since the number of gene pairs or closely linked blocks 

 of genes is few, it should be possible by selection to establish inbred lines re- 

 taining this advantage. 



MAIN AND COMPONENT CHARACTERS 



The data from the parental and hybrid populations of tomatoes on the 

 main and component characters provide information concerning the rela- 

 tions between gene recombination, dominance, and heterosis. 



