310 LEROY POWERS 



From the above, as was found true for yield per plant, the heterosis noted 

 for earliness of maturity results from the combination of component charac- 

 ters which in certain Fi hybrids may themselves exhibit heterosis. The same 

 is true for weight per fruit. In other words, the study of genetics of heterosis 

 has been somewhat simplified by breaking the main characters down into 

 their component characters. Also, as before, the study shows that gene re- 

 combination to retain some or all of the increase of the Fi hybrid over the 

 parents is dependent upon the same physiological genetic phenomena as are 

 involved in attempting to combine two or more desirable characters into a 

 single inbred line. 



RECOVERING INBRED LINES RETAINING ADVANTAGES 

 ATTRIBUTABLE TO HETEROSIS 



The physiological genetic phenomena that hinder or aid, by the recombi- 

 nation of genes, the recovery of inbred lines retaining some or all of the 

 advantages attributable to heterosis are the same as those emphasized by 

 Jones (1917) and East (1936). These are the number of gene pairs differen- 

 tiating the parents, linkage relations of the genes, pleiotropy, and the inter- 

 action of the genes as determined by the measurement of end products, both 

 interallelic and intraallelic. This genetic information can be obtained only 

 by rather detailed genetic studies. With the quantitative characters such 

 studies are expensive and time consuming. Hence, very few such studies have 

 been made with tomato hybrids. Powers, Locke, and Garrett (1950) and 

 Powers (1950b) have made a gene analysis for some of the main characters and 

 their more obvious components. Even though the gene analysis for number 

 of days from seeding to first fruit ripe has been completed for only one of 

 the four crosses to be considered, this character and weight per locule will be 

 treated as component characters of yield of ripe fruit per plant in the section 

 dealing with number of pairs of genes differentiating the parents. 



Number of Gene Pairs Differentiating Parents 



In considering the bearing that number of gene pairs differentiating the 

 parents has upon gene recombination and heterosis, just two characters will 

 be considered: weight per locule and number of days from seeding to first 

 fruit ripe. That both of these characters have an effect upon yield of ripe fruit 

 should be kept in mind during the analyses and discussions which follow. 

 Also, other component characters listed in Table 19.7 could be studied. How- 

 ever, the additional information gained would not justify the time and space 

 required, as the fundamental principles involved can be brought out from 

 an analysis and discussion of the data for the two characters chosen. The 

 number of gene pairs (effective factors; Mather, 1949) differentiating 

 weight per locule has been determined for all the hybrid populations listed 

 in Table 19.8. For days from seeding to first fruit ripe the number of gene 



