GENE RECOMBINATION AND HETEROSIS 309 



(that is values in all classes) from heterosis for decrease of a character to 

 heterosis for increase of a character, depending upon the character and geno- 

 type (Fi hybrid). 



The most logical conclusion from these figures is that dominance and het- 

 osis to a considerable extent are different degrees of expression of the same 

 physiological genetic phenomena. This hypothesis is greatly strengthened by 

 findings of Powers (1941) that whether a character shows dominance or het- 

 erosis in some cases is dependent upon the environment and in other cases 

 upon the genotype. As pointed out previously, gene recombination in rela- 

 tion to heterosis is no different from combining any two desirable characters 

 by recombination of genes. A study of the component characters of the main 

 characters given in Table 19.7 offers further evidence in support of this 

 contention. 



Yield of ripe fruit as determined by weight of fruit in grams is dependent 

 upon number of fruits that ripen and weight per fruit. The first of these 

 component characters, depending upon the Fi hybrid being considered, varies 

 from partial dominance of fewer number of ripe fruits to heterosis for an in- 

 creased number of ripe fruits. The second component character varies from 

 no dominance to heterosis for smaller weight per fruit. They combine mul- 

 tiplicatively, and in many cases result in heterosis for yield of fruit (Table 

 19.7 and Powers, 1944). Here again, then, is a case involving combination 

 of characters to produce heterosis. To retain some of the benefits of heterosis 

 in inbred lines would involve recombination of the genes differentiating the 

 two component characters. 



In turn the number of fruit that ripens is dependent to a large extent at 

 Cheyenne, Wyoming, on earliness of maturity, number of days from seeding 

 to first fruit ripe (Powers, 1945). Earliness of maturity varies from partial 

 dominance of fewer days from seeding to first fruit ripe to heterosis for the 

 shorter period. The component characters of earliness of maturity are period 

 from seeding to first bloom, period from first bloom to first fruit set, and pe- 

 riod from first fruit set to first fruit ripe. Number of days from seeding to 

 first bloom varies from no dominance to heterosis for the shorter period. 

 Number of days from first bloom to first fruit set varies from complete domi- 

 nance of the longer period to heterosis of the shorter period. Number of days 

 from first fruit set to first fruit ripe varies from partial dominance of the longer 

 period to complete dominance of the shorter period. 



Weight per fruit is dependent upon weight per locule and number of 

 locules per fruit. Weight per locule varies from partial dominance for less 

 weight per locule to heterosis for greater weight per locule. Number of 

 locules varies from no dominance to heterosis for fewer locules. On the 

 arithmetic scale these two component characters combine multiplicatively 

 so that weight per fruit varies from no dominance to heterosis for less weight 

 per fruit. 



