• 9 >1S MUTATION AND PLANT BREEDING 



line, since this method is especially suitable for the transfer of the 

 distinct types of mutation here discussed. We need, however, a more 

 detailed understanding about the combining ability of induced 

 macromutations, since there are many apparent differences between 

 the genetic behavior of natural and induced factors. Thus, we have 

 produced a great many mutations resistant to diseases. Almost all of 

 these mutations are recessive, while dominant mode of inheritance is 

 more common among natural genes of the same kind. If Fisher (23) 

 is right, this difference may depend on genie environment and 

 co-adaptation. 



The combining ability of a mutation has been proved to vary 

 greatly with environment and background genotype (4, 10, 41, 42, 

 105). Only one example, cited by Stubbe (106), will be given here. 

 In the snapdragon, a mutation called eramosa was characterized by a 

 nearly complete loss in the ability to branch and by a very erect 

 growth, desirable properties for an ornamental plant. Unfortunately, 

 however, less pleasant features were also associated with the eramosa 

 mutation, viz., considerable inhibition and deformation of the flow- 

 ers. By recombining the pathological mutant with other genotypes, it 

 was possible to neutralize parts of the pleiotropic complex and to 

 find a genie background where the desirable but not the undesirable 

 characteristics of the eramosa mutation could be established (Vogel, 

 unpublished). Induction of new neutralizing mutations can be con- 

 sidered another variation on the same thema (34, 37, 44). 



In principle, there is no difference in the behavior of macro- and 

 micromutations. They gradually pass into each other, and their 

 effects may be cooperant or opposed. Quantitative variation can 

 seldom definitely be proved to depend on multigenic differences of 

 small magnitude only. The smaller phenotypic contribution of indi- 

 vidual micromutations necessitates, however, recombination or recur- 

 rent induction, since only their additive effects can be phenotypically 

 scored. Recombination, which allows accumulation of valuable and 

 omission of negative modifiers, occurs to great advantage automati- 

 cally in allogamous plants. In autogamous plants, it must, however, 

 be artificially stimulated if the evolutionary so important micro- 

 mutations are to be fully utilized. For this reason and also for the 

 chance to evaluate the induced macromutations in different genie 

 environments, mutation experiments with self-fertilizers should pref- 



