Discussion of Session III 1 



W. M. MYERS 



University of Minnesota, St. Paul, Minnesota 



In the early papers of this symposium, and particularly in Doctor 

 Rhoades' discussion, reference has been made to different kinds of 

 events which collectively may be referred to as mutation. These may 

 include true changes in the molecular structure of the gene as well as 

 minor chromosome changes such as deletion, duplication, or position 

 effects resulting from inversions. They may also include major chro- 

 mosomal derangements, especially in the polyploids and in vegeta- 

 tively propagated organisms, including losses or duplications of large 

 chromosome segments or entire chromosomes. Finally, recombination 

 by normal crossing-over may result in effects which can be distin- 

 guished from true mutation only by refined genetic and cytological 

 analysis, if at all. These may include sudden "large" changes due to 

 crossing-over between members of a compound locus or, on the other 

 hand, merely increases in genetic variability in characters controlled 

 by polygenes. 



In the experiments on the role of mutations in plant breeding, 

 it is evident that the very old and very broad definition of mutation 

 as a sudden heritable change has been applied. The necessary cyto- 

 genetic tests have not been used in most studies to classify in a more 

 specific fashion than this the kinds of mutations observed. Indeed, it 

 is evident that with presently available knowledge sufficiently critical 

 tests are not possible, in most instances, to distinguish between 

 minute duplications or deficiencies on the one hand and true changes 

 in the physical-chemical structure of the gene on the other. Further- 

 more, for the purely practical assessment of the value of mutations in 

 plant breeding, it is of little or no consequence whether the heritable 

 changes observed were the result of changes in the DNA molecule 

 itself or of some chromosomal aberration. To be sure, there are good 

 reasons why the plant breeder wants to know the basic nature of the 

 genetic change. But it is not inappropriate that in many of the plant 

 breeding studies, no effort is expended in determining it. 



^aper No. 1062, Miscellaneous Journal Series, Minnesota Agricultural Experiment 

 Station. 



295 



