4 MUTATION AND PLANT BREEDING 



That there has been some success in the development of artificial 

 methods to increase variability cannot be denied, and perhaps the 

 introduction of various mutagenic agents is the outstanding example 

 of this. However, I think it is safe to say that these agents have not 

 had a significant impact on the development of improved techniques 

 of selection, but are now viewed by the breeder primarily as a means 

 of enhancing variability, with the added hope that an occasional 

 spectacular, useful variant will appear. Rather, it should be antici- 

 pated that the use of these agents, along with other techniques 

 available to the geneticist and plant breeder, in the interests of a 

 better understanding of the nature and action of the genetic elements, 

 stands to add far more to the development of improved breeding 

 techniques than their direct use to enhance, through increased 

 variability, the prospects of success in a conventional selection 

 program. 



But the traits of particular concern to the plant breeder are most 

 often quantitative and are usually controlled by numbers of genes 

 whose individual genetic analysis appears particularly unrewarding. 

 Unless we hold with the idea that genes governing quantitative traits 

 are unique in their actions, a premise that I consider indefensible, the 

 reasonable alternative is to deal individually with genes having 

 so-called qualitative effects. 



For some time it has been apparent that extragenic events make 

 up a considerable portion of the occurrences we call mutations, 

 and while we were at one time accustomed to think of variability in 

 nature as due to the reshuffling of genetic entities whose differences 

 were ascribable ultimately to qualitative changes that we preferred 

 to think of as gene mutations, a great deal of painstaking work has 

 led to increased emphasis on extragenic events as more immediate 

 sources of variability in a population. As increasing numbers of muta- 

 tions are resolved as extragenic events, the classical gene mutation 

 appears more and more to be an elusive phenomenon and some, 

 perhaps, would even doubt its validity as a biological concept. 



If, in fact, the propagation of coded genetic information from 

 mother to daughter cells, from parent to offspring, is a far more 

 accurate process than had earlier been considered, and we suppose 

 that the intramolecular alteration we think of as gene mutation may 

 range in frequency for individual loci from 10 _,i or 10~ 7 to lower 



