JOHNSON: DISCUSSION OF SESSION IV 407 



the advantages to be gained by first defining the needs for improve- 

 ment, then carefully surveying the parental material available which 

 together may contain the desired genes for recombination, followed 

 by hybridization to create the pool of variability upon which subse- 

 quent selection and evaluation can be practiced. To me, one of the 

 most serious shortcomings of mutation breeding: for characters whose 

 expression is conditioned by many genes is the transfer from the 

 philosophy of "creating planned variability" to creating" "chance 

 variability". At best, the desired recombination of many genes has a 

 relatively low probability, but it is advantageous to know something 

 about the odds as based on previous experiments. 



During the progress of this symposium examples have been cited 

 of gains made by following the mutation breeding route. Other 

 examples have been given by Burton and Caldecott showing lack of 

 significant progress. These demonstrations through experimentation 

 that gains or lack of gains have been made neither prove nor disprove 

 the value of mutation breeding. The critical test is the comparative 

 progress that could have been made with similar expenditure of time 

 and resources to achieve a particular goal. Modern plant breeding 

 approaches with the customary limitations of resources at research 

 institutions require that progress be equated in terms of gains per 

 unit of investment. 



Throughout the reports in the literature on mutation breeding 

 for improvement in quantitative characters there seems to be an urge 

 on the part of the breeder to achieve progress faster than he has a 

 right to expect. As previously stated, presently used mutagenic agents 

 are not selective. It would be expected that as many (or more) undesir- 

 able chromosome alterations would be derived as favorable ones. 

 The evolutionary development of favorable internal and relational 

 balance in our present economic plants has been a slow one. Evidence 

 presented in the literature 1 has shown that after many generations 

 following hybridization in barley important measurable changes 

 have occurred in such hybrid-derived populations. Is it not likely that 

 much more potential gains could be achieved if populations derived 

 from induced mutation were allowed to be subjected to a consider- 

 able period of time for "restoration" to chromosome balance before 



^Jal, B. S., Suneson, C. A., and Romage, R. T. Genetic shift during 30 generations 

 of natural selection in barley. Agron. Jour., 51: 555-557. 1959. 



