296 MUTATION AND PLANT BREEDING 



The title of this section of the program is "The Evaluation of 

 Mutations in Plant Breeding". During the discussion so far, there 

 has been consideration of the role of so-called "mutation breeding" 

 and some concern regarding the relative efficacy of "mutation breed- 

 ing" versus conventional cross breeding. Perhaps a consideration of 

 the essential phases involved in plant breeding will aid in answering 

 these questions. The three essential phases are: 



1. Finding or developing populations in which there is adequate 

 and appropriate genetic variation, 



2. Effective selection, including critical evaluation of the selections, 



3. Appropriate use of the selections, either directly as the varieties 

 of commerce or indirectly in building such varieties. 



In connection with the discussions in this symposium the third 

 phase is not a matter of concern, but the first two are and therefore 

 warrant some further consideration. 



Consideration of the first phase is based on the premise that all 

 genetic variability is dependent, basically, on differences in function 

 at corresponding loci of homologous chromosomes; in other words, 

 in the classical genetic terminology, that two or more alleles exist 

 of one or more genes. The premise is also held that these differences 

 at various gene loci have resulted from mutation. Therefore, all ge- 

 netic variation must have had its origin in mutation. If these premises 

 are valid, and this discussant can see no obvious alternative, the 

 question posed by the title of this program can be given a definitive 

 answer — mutation is a necessary prerequisite to successful plant 

 breeding for, without mutation, there could be no genetic variation 

 and without genetic variation plant breeding would be impossible. 



The words adequate and appropriate were used to describe the 

 required genetic variation. By appropriate genetic variation, it is 

 meant that for success in the plant breeding endeavor there must be 

 in the population with which the breeder is working the genes which 

 will, in proper combination, condition the expression of the charac- 

 teristics sought in the improved variety. By selection, the breeder can 

 do no more than extract from the population lines or new popula- 

 tions which have combinations of the alleles that existed in the popu- 

 lation. Genetic advance, that is the success of the breeding effort, is a 

 function of the selection differential times the heritability. Both of 

 these factors are determined in part by the extent of genetic vari- 



