GENE RECOMBINATION AND HETEROSIS 



301 



growth in one character and inhibit it in another, and hence is more in ac- 

 cord with modern physiological genetic concepts. 



The article by Powers (1936) furnishes additional information j)ertaining 

 to gene recombination and heterosis. If genes other than Vv are responsible 

 for the heterosis noted, then F2 plants having a genotype identical to the Fi 

 generation should give a somewhat lower yield than the Fi. Since the Fi 

 plants were not grown in a randomized experiment with the F2 plants, the 

 comparison must be made through the Bl parent. As compared through the 

 Bl parent an actual reduction of one gram in yield of seed per plant was 

 found (Powers, 1936). This reduction could be due to genes carried on chro- 

 mosome 1, as are V and v, or to genes carried on other chromosomes. In 

 either event, theoretically some of the genes favorable to increased weight 



TABLE 19.3 



COMPARISON BETWEEN PARENTS AND F2 PAREN- 

 TAL GENOT\TES FOR WEIGHT OF SEED 

 PER PLANT IN A BARLEY HYBRID 



* VVBrBr, genotype of Bl parent, two-row normal. 



t vvbrbr, genotype of Brachytic parent, six-row Brachytic. 



of seed per plant that resulted in the heterosis noted in the Fi population 

 must be capable of recombination. 



Even though some of the genes favorable to increased growth can be re- 

 combined, the yield of the lines in which the favorable genes have been com- 

 bined depends upon the nature of the interaction of the genes. The weights 

 of seed per plant of parents and F2 plants of the parental genotypes are given 

 in Table 19.3. From this table it can be seen that the F2 plants of the VVBrBr 

 genotype gave an increased yield of 2.0 grams per plant over the Bl parental 

 plants having the same genotype. However, the F2 plants of the same geno- 

 type as the Brachytic parent gave an increase over this parent of only 0.5 

 grams per plant, which is not statistically significant. The interaction of 1.5 

 grams (Table 19.3) is statistically significant. This means that a preponder- 

 ance of the genes favorable to increased weight of seed per plant must have 

 entered the cross from the Brachytic parent. The balance of the unfavorable 

 genes that entered the cross from the Bl parent did not cause a correspond- 

 ing decrease in weight of seed per plant of the F2 plants possessing the vvbrbr 

 genotype. 



