STEPHENS: RESUME OF THE SYMPOSIUM 505 



it should be advantageous to minimize crossing over in the early 

 stages of a breeding program so that the genome of one of the par- 

 ents could be recovered intact save for the particular chromosome or 

 chromosomal segment which carried the property to be transferred. 

 At a later stage enhanced crossing over would permit the "whittling 

 down" of the transferred segment without disrupting the parental 

 combinations in the remainder of the genome. Chemical or other 

 agents which would serve as general crossover enhancers and suppres- 

 sors might be of considerably practical value to the plant breeder. 



"Mutation Breeding" and "Conventional Breeding" Methods 



An evaluation of the comparative merits of the so-called "muta- 

 tion breeding" and "conventional breeding" methods is perhaps 

 premature at this time. A comparison between the novel and rela- 

 tively untested, on the one hand, and the established and relatively 

 conservative, on the other, could be unfair to both. I would pre- 

 fer to agree with MacKey's viewpoint that "plant breeding is noth- 

 ing more than controlled evolution" and that "it seems very likely 

 that mutation work in the future will be more completely inter- 

 woven with the so-called conventional methods, with which it is now 

 too often set in opposition." 



The degree to which evolution can be controlled by the plant 

 breeder is limited by (1) the amount and kind of genetic varia- 

 tion available in the breeding population, and (2) the efficiency 

 with which the most desirable genetic combinations can be accumu- 

 lated through selection. 



Theoretically, the first limitation can be reduced either by 

 inducing mutations, by outcrossing, or by a combination of both 

 techniques. Because the effects of mutagens can be measured more 

 readily in inbred homozygous populations, Ave have little critical 

 information on their effects in cross bred populations where the 

 results of mutagenic activity and hybridization are confounded. It 

 would be interesting to know, in a range of crop plants, the rela- 

 tive increases in genetic variance which could be expected from the 

 irradiation of parental lines as compared with those resulting from 

 normal F 2 segregation. The studies reported by Gregory represent 

 pioneer experiments in an otherwise unexplored field. They also 

 raise the interesting question whether the genotypic response to 



