CHAPTER XIX 

 HYBRIDIZATION 



The usual purpose of the plant breeder who resorts to hybridiza- 

 tion is to secure new and better combinations of characters among the 

 progenies resulting from his crosses. Improvement of a given species 

 may consist merely in the elimination of undesirable characters or of the 

 production of entirely new combinations of characters already existing 

 within the species. In this work the application of the Mendelian 

 principle of segregation and recombination is of the greatest prac- 

 tical value. By concentrating his attention on only a few important 

 characters at a time the breeder can sometimes secure the desired 

 combinations in F 2 . But at the same time one who is informed in 

 regard to modern genetical principles will be prepared for possible 

 disappointment in meeting an early realization of his aim. Moreover, 

 he will understand how to select in F 2 and later generations for further 

 testing. He will realize that a specific character difference in his parental 

 forms may be conditioned by more than one factor difference; also that 

 some specific factors display considerable variability in expression; and 

 that linkage, crossing-over, multiple factors and multiple allelomorphs 

 may play a role in conditioning or preventing the particular character 

 combination for which he is striving. Furthermore, the ideal sometimes 

 demanded of the breeder involves character complexes which include all 

 the functions of the plant. As has been shown already the comparative 

 difficulty between different cases of this sort depends directly upon the 

 number of chromosomes possessed by the species in question. Finally, 

 demands are sometimes made for the "creation" of characters which are 

 unknown in available phenotypes and for which there is no genotypic 

 representation within the species. In such cases recourse may be had 

 perhaps to species hybridization. But those who are familiar with the 

 results of species hybridization will be prepared for complete disappoint- 

 ment from the first. It is not the motive of these remarks to discourage 

 intending hybridizers, but merely to warn against the anticipation of 

 success in all cases simply because of the generality of Mendelian princi- 

 ples. Hybridization, even of varieties, in order to be generally successful 

 must be intelligently performed and in the long run the experimentalist 

 who is the most thoroughly informed concerning his plants will stand the 

 best chance of securing the improved forms he desires. Each species 



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