20$ MUTATION AND PLANT BREEDING 



changed by means of induced mutations within the framework 

 of the species or even beyond it. 



Genetics of Mutants 



The great majority of these mutations are recessive and segre- 

 gate in a 3:1 ratio; sometimes, however, with a deficiency of reces- 

 sives and occasionally with an excess. In diploid organisms completely 

 dominant mutations have scarcely been found, except some types 

 which are lethal or semilethal in the homozygous condition (cf. 79, 

 110). Therefore, whenever a dominant deviation is met with in 

 mutation experiments with diploids, extreme caution is advisable. 

 For instance, a great deal of the fungi-resistant variants, claimed to be 

 dominant mutants, may probably be a consequence of contamination 

 only. 



There are, however, a few reports of true dominant mutations. 

 Notzel (78), for instance, investigated 40 different barley muta- 

 tions, including types of erectoides, intermedium, macrolepis, and 

 earliness. One of the earliness mutants was dominant, while the 

 other 39 were recessive. Another dominant mutation of earliness 

 has been reported by Scholz (93). Out of 70 erectoides mutants 

 investigated by Hagberg (50), 2 were almost completely dom- 

 inant for ear density and 1 was partially dominant for the same 

 character. However, other characters affected by the same muta- 

 tion may behave as recessive or intermediate. For example, erectoides 

 mutants may be recessive for ear density, but dominant for length of 

 the upper internode of the culm (48, 49). 



Along with the pronounced pleiotropic action of all (or nearly 

 all) mutations, the manifestation of the relative degree of reces- 

 sivity in the different characters concerned can vary considerably. 

 Moreover, superdominance and superrecessivity have been repeat- 

 edly observed for various characters in lethal, sublethal, and vital 

 mutations (47, 49, 112). The degree of dominance may also be 

 greatly influenced by the genetic background (e.g. 45, page 624; 51, 

 72) and/or the environment. The dominant tomato mutant sub- 

 sistens, for instance, which was studied by Endlich (21), is lethal in 

 the homozygous condition. In the heterozygot the mutant allel 

 showed a varying degree of expressivity in different environments. 

 Under low light intensity or high temperature, the mutated char- 

 acter appeared to be recessive or nearly recessive. 



