GENE RECOMBINATION AND HETEROSIS 317 



cent of crossing over it is doubtful whether it is possible for the plant breeder 

 or geneticist to isolate individuals from the F2 population carrying twelve 

 plus genes. 



The data in Table 19.9 emphasize that even with the probably over- 

 simplified genetic situation depicted it is not possible to recover in a single 

 individual all of the genes favorable to the production of a desirable charac- 

 ter for which the F2 population is segregating, unless the favorable genes are 

 located on the same chromosome and immediately adjacent to each other 

 without unfavorable genes intervening. If any of the favorable genes are 

 adjacent to each other without unfavorable genes intervening, then decided 

 advances can be made by selection in the F2 populations up to a certain point. 

 Beyond that point further selection in the F2 will have no effect, and selec- 

 tion in advanced generations does not offer much promise. The most difficult 

 situation is that in which the linkage relation is such that the favorable and 

 unfavorable genes alternate on the chromosome and the number of such 

 linkage groups is at a minimum for the number of gene pairs involved. 



For the sake of clarity of illustration only three linkage relations were 

 shown. However, it is apparent that undoubtedly in the material available 

 to plant breeders and geneticists, the possible different kinds of linkage rela- 

 tions are almost innumerable. Some will aid the investigator in obtaining the 

 desired recombination of genes and others will be a decided hindrance. In 

 the cases of undesirable linkage relations it will be almost impossible for the 

 breeder to obtain individuals possessing recombinations of genes making 

 that individual equal to or superior to the Fi for the character exhibiting 

 heterosis. On the other hand, desirable linkage relations may make it pos- 

 sible to obtain the recombination of genes sought even though a large num- 

 ber of gene pairs differentiates the parents used in hybridization. 



Pleiotropy, and Interallelic and Intraallelic Interactions 



Powers, Locke, and Garrett (1950) have made a rather detailed genetic 

 study of eight quantitative characters in hybrid and parental populations 

 involving the Porter and Ponderosa varieties of Lycopersicon esculentum 

 Mill. The characters studied and the indicated number of major gene pairs 

 differentiating the parents are as listed immediately below. 



Character Gene Symbols 



Percentage of flowers that set fruit FifiFifiFifiFifi 



Period from seeding to first fruit ripe: 



Seeding to first bloom BihiB-ibiBibz 



First bloom to first fruit set SiS\S2Si,SiSi 



First fruit set to first fruit ripe RxfiRiTi 



Weight per fruit: 



Number of locules LcilciLcildLcilci 



Weight per locule WiWiWiW'zWzWz 



With most quantitative characters it is difficult to distinguish between 

 pleiotropy and linkage. It seems highly probable that linkage instead of plei- 



