CALDECOTT AND NORTH: RADIO-SENSITIVITY OF SEEDS 391 



tions and chromosomal aberrations tend to be independently induced 

 events (15), seed from fertile heads will carry as many mutations as 

 seed from semi-sterile heads. While seed selected in this way may 

 incidentally carry an inversion it would likely be of little consequence 

 to the breeder and could be easily eliminated if the need was 

 indicated. 



In suggesting the use of recurrent irradiation in diploid species 

 for inducing maximum genetic variability for both quantitative and 

 qualitative traits, it is recognized that gene mutations of both a dele- 

 terious and beneficial nature will be induced in the same cells and 

 that the method does not provide for their separation between gen- 

 erations. From this it follows that the immediate products from 

 recurrent irradiation of a diploid could not likely be used as com- 

 mercial varieties. However, after screening for mutants of agronomic 

 value in a population that had been subjected to recurrent irradi- 

 ation, it would be easy to produce hybrids between the original 

 progenitor and the irradiated material to place the character, or char- 

 acters, of value in what was an otherwise desirable oenetic back- 

 ground. In so far as the author is aware, this particular approach 

 has received little consideration by plant breeders. 



In polyploids 



When attempting the induction of detectable mutations in the 

 self-fertilizing polyploids among the small grains, a special set of 

 circumstances confronts the geneticist. For every pair of genes that 

 influence the expression of a character in the diploid, both quantita- 

 tive and qualitative, there presumably usually exist two and three 

 times as many genes in the tetraploid and hexaploid, respectively. 



If considering mutation only for qualitative characters, theoreti- 

 cal assumptions suggest that continued re-irradiation of the seed 

 generation of the polyploids, in which the genes governing the same 

 order of biochemical function are in the homozygous dominant con- 

 dition on homeologous chromosomes, will ultimately result in the 

 appearance of a high frequency of chlorophyll mutations in the popu- 

 lation. When such mutations appear, F 2 segregation for the mutant 

 trait in crosses with the original parental type should be 15:1 in the 

 case of the tetraploid and 63:1 in the case of the hexaploid. Among 

 the normal F 2 phenotypes from these crosses there should exist geno- 



