gaul: induced mutants in seed-propagated species 209 



In contrast to diploids, dominance or semidominance seems to 

 be a fairly common phenomenon for many characters of vital muta- 

 tions in polyploids like wheat (70). Here, the phenotypic change is 

 probably more often caused by chromosome mutations instead of 

 point mutations and polyploids are more tolerant to chromosomal 

 aberrations as compared with diploids. 



There is no essential difference between the alleles available 

 from the world collections of natural forms and those from induced 

 mutations. This has been adequately demonstrated for some char- 

 acters in barley. Thus, some erectoides mutants (50, 78), macrolepis 

 mutants (78), naked-kernel mutants (92), smooth-awned mutants, and 

 a mutant with waxy stalks and leaves (Scholz as cited by Stubbe, 

 117), have been shown in crosses to be allelic with corresponding 

 forms of the world collections. In tomatoes several loci of induced 

 mutants of Lycopersicon pirn pinellijolium have been proved identi- 

 cal with those of the corresponding esculentum mutants (117). 



Identity of the locus does not mean that the alleles are iden- 

 tical. The occurrence of multiple alleles or of pseudoalleles can 

 usually be studied only with mutants which do not differ other- 

 wise in their gene content. From a careful morphological study 

 of 70 erectoides mutants in barley, including the various pleiotropic 

 effects, Hagberg (50) arrived at the conclusion that it seems to be 

 impossible to copy an individual mutation. These 70 mutants were 

 scattered over 22 separate loci. Concerning the phenotype belong- 

 ing to an individual erectoides allele there are important differ- 

 ences between the loci, but there is also a remarkable variation 

 within a locus. This finding is in general accord with the evidence 

 accumulated from microorganisms and Drosophila in the last 10 to 

 15 years which has shown that genes have a complex nature. So 

 far as plant breeding is concerned, these results imply that the 

 induction of mutations does not simply reproduce the natural 

 variability, but may expand it to a large extent. Through muta- 

 tions of a given character, not only may loci which are not yet 

 known be discovered, but also new alleles within the loci may be 

 created. 



Phenotypic Classification of Mutants 



The alteration of characters induced by mutations may be 

 large or small. There are transitions all the way from macro-muta- 



