304 MUTATION AND PLANT BREEDING 



mutations in corn which have been subjected to sufficiently 

 critical genetic analysis none has been of this type. 



2. What is the frequency of total mutations? What proportion of the 

 total mutants are favorable? Doctor Gaul has reported some of 

 the information now available in answer to these questions, but 

 it is evident that much more information is required before a 

 definite answer can be given for any crop species. 



3. Are more efficient methods of treatment possible so that the total 

 percentage of mutations can be increased? Doctor Gaul has dealt 

 with this problem, as have others in this symposium. It is obvious 

 that this is still a fertile field of research. 



4. Can Ave look forward to directed mutations? There is already 

 some evidence that, by appropriate treatments with different muta- 

 genes, the relative frequencies of "mutations" versus gross chro- 

 mosomal changes and of various kinds of "mutations" can be 

 altered. Even if some day true directed mutations become possible, 

 we might then ask how useful this will be to the plant breeder. 



5. Are the results of mutation, recombinations, etc., in viruses and 

 bacteria completely applicable to the higher plants? There have 

 been suggestions at this symposium and elsewhere (9) that differ- 

 ences in chromosomal organization and gene action might exist 

 which would limit the validity of extrapolation of results from 

 the lower to higher organisms. 



Another consideration, not directly related to mutations but 

 certainly involved in the extent of genetic variability, is that of link- 

 age and recombination. Numerous experiments have shown some- 

 thing of the magnitude of the genetic variability, particularly for 

 characters determined by polygenes, that is "locked up" in chromo- 

 some blocks by linkage and that is slowly released in succeeding 

 generations by recombination. Anderson (1) has written about the 

 hinderance to recombination imposed by linkage and refers to the 

 cohesiveness of the germplasm. It has already been suggested in this 

 discussion that sudden (or gradual) changes that appear to result from 

 mutation may actually be caused by recombination. One might 

 indeed question whether the results of the selection. experiment in 

 Drosophila, referred to by Doctor Gaul, might have been due to 

 increased genetic recombination rather than to mutations as was 

 suggested. 



