392 MUTATION AND PLANT BREEDING 



types which contain the normal gene in the homozygous dominant 

 condition on one pair of homologs and are homozygous recessive on 

 the other pair or pairs of homeolgous chromosomes. In plants of this 

 genetic constitution crosses with the recessive phenotype should give 

 a 3:1 segregation for the mutant character. In effect, then, it can be 

 said that the polyploid has been "diploidized". It follows that the 

 recurrent re-irradiation of any polyploid that shows bivalent pairing 

 should eventually result in the material used in such studies 

 being indistinguishable, by simple Mendel ian tests, from naturally 

 occurring diploids. 



It might well be asked if there exist any factual data to support 

 these considerations. The answer is simply that the incidence of 

 chlorophyll mutations in hexaploid oats has increased each genera- 

 tion through six successive generations of re-irradiation. Mutant types 

 have been perpetuated, but crosses with the parental type have not 

 been made. 



In addition to the hexaploid oat work, which will be mentioned 

 again in the next section, recurrent irradiation studies are also under- 

 way at the University of Minnesota with diploid and tetraploid oats 

 and tetraploid and hexaploid wheat. The incidence of chlorophyll 

 mutations in the wheat material after irradiation of two successive 

 seed generations supports the general considerations. 



It seems pertinent to ask the following question: If diploidiza- 

 tion of a polyploid is indeed possible, is it likely that it will have any 

 practical significance? The answer, based on theory and existing 

 recurrently irradiated plant material, is yes. To give substance to this 

 assertation it is again necessary to resort to theoretical considerations 

 and the findings of other workers. Heterosis in corn is commonly 

 attributed to differences in alleles which govern expression of the 

 same characters, i.e., heterozygosity at several loci, or to epistasis. 

 Similarly, it has been demonstrated by Wallace and Vetukhiv (28) in 

 Drosophila that genes which are recessive and deleterious when in 

 the homozygous condition may benefit the survival and growth of a 

 fly in which they are in the heterozygous state. 



What has this to do with diploidization of a self-fertilizing poly- 

 ploid, specifically, for ease of presentation, hexaploid wheat? As has 

 been indicated, in the hexaploid often there are presumably six genes, 

 one on each of three pairs of homeologous chromosomes, which 



