CALDECOTT AND NORTH: RADIO-SENSITIVITY OF SEEDS 395 



The previously mentioned work of Wallace and Vetukhiv (28) 

 with Drosophila, which has demonstrated that flies heterozygous for 

 one or a few loci may be better adapted to their environment than 

 when they are either homozygous dominant or recessive for these 

 loci, is information that could be of import to the plant breeder 

 using diploid species if he could devise some method of obtaining 

 lines that would breed true for the heterozygous condition. There 

 seems to be only one possible method for achieving this end, within 

 the confines of the diploid, and that is to obtain a duplication of the 

 first type mentioned so that the normal and mutant alleles are located 

 on the same chromosome. This would necessitate irradiatino Fi seeds 

 that were heterozygous for the allele under consideration. 



One might well ask three questions concerning the feasibility of 

 using the method: (A) What is the probability of obtaining a trans- 

 location at dissimilar points on homologous chromosomes? (B) What 

 is the probability that the duplicated segment of the interchange will 

 involve the gene in question? (C) What is the likelihood of recog- 

 nizing the duplication once it is produced? 



The answer to the first question is that translocations between 

 opposite arms of homologous chromosomes occur with a frequency 

 that is apparently due to chance (19). It can be assumed, therefore, 

 that translocations between corresponding arms would occur with 

 the same frequency. From this it follows that the lower the number of 

 chromosomes in the species under study, the more readily will such 

 interchanges arise. Actually, in diploid barley and maize, cytogenetic 

 studies have shown that translocations between opposite arms of 

 homeologous chromosomes are a relatively common occurrence 

 (4, 6, 20). 



The second question cannot be answered except to point out 

 that, based on purely physical assumptions, a gene in the median 

 position of an arm should be involved in duplications more commonly 

 than one close to the centromere or on the distal end of an arm. 



The answer to the third question is simply that it depends on 

 whether or not the heterozygous state produces a recognizable pheno- 

 type. If it does, plants carrying a duplication involving a dominant 

 and recessive allele should be readily detected because they will 

 either not segregate for the character in question or give aberrant 

 se»reoation ratios. 



