GENE RECOMBINATION AND HETEROSIS 307 



Powers, Locke, and Clarrett (1950) found the number of major gene j)airs 

 differentiating number of locules to be 3. Since weight per locale was found 

 to be differentiated by 3 pairs of major genes, a comparatively few (prob- 

 ably 6) pairs of major genes differentiate weight per fruit. Hence, the number 

 of major gene pairs responsible for heterosis of weight per locule is no greater 

 than the number of major gene pairs responsible for no dominance of num- 

 ber of locules and partial or complete dominance of weight per fruit on the 

 logarithmic scale. Then, in this study the number of pairs of major genes 

 differentiating the character has no bearing on whether the hybrid popula- 

 tions will show no dominance, partial dominance, complete dominance, or 

 heterosis. 



From these results it follows that in this material recombination of genes 

 to retain the advantages of heterosis is no different than recombination of 

 genes to combine desirable characters. Furthermore, these data furnish 

 rather convincing evidence that dominance and heterosis are different de- 

 grees of expression of the same physiological genetic phenomena, as was 

 postulated by Powers (1941, 1944). 



Main and Component Characters of 45 Hybrids Produced 

 by Crossing 10 Inbred Lines of Tomatoes 



Table 19.7 summarizes the dominance relations of the main and com- 

 ponent characters of 45 hybrids produced by crossing 10 inbred lines of 

 tomatoes. 



The percentage values given in Table 19.7 were calculated from data pre- 

 sented in a previous article (Powers, 1945). The reader is referred to this 

 article for the experimental design, a description of the material, and meth- 

 ods. Here, only the method of compiling the data need be given. All of the 

 values of this table with the exception of those listed under heterosis were 

 calculated from the formula 100[2Fi/(Pi + A)]. The percentages listed 

 under the column headings "heterosis" were calculated from the formula 

 lOO(Fi/Pi) and 10G{Fi/F2), respectively. Fi is the mean of the Fi popula- 

 tion, Fi the mean of the parent with the smaller value, and A the mean of 

 the parent with the larger value. The 11 characters listed in Table 19.7 were 

 originally expressed in the following units of measurement: Spread of plant 

 in inches, yield per plant in grams, number of fruit that ripened per plant, 

 height per plant in inches, weight per locule of the fruit in grams, number of 

 days from first fruit set to first fruit ripe, number of days from first bloom 

 to first fruit set, weight of fruit in grams, number of days from seeding to 

 first bloom, number of locules per fruit, and number of days from seeding 

 to first fruit ripe. 



The odds against any value belonging in an adjacent classification (col- 

 umn) are greater than 19: 1 with the exception of the two values designated 

 with an asterisk. Even for these two values the odds against their deviating 



