200 RADIATION UIOLUGY 



staining chromatic n'nioii on cither .side of the chromomere and a Hghtly 

 staiiiiiifj; achromatic rc}>ioii (list ally. Each chromosome arm terminates 

 in a sinfile well-delined chromomere. This circum.stance is very favorable 

 for the detection of terminal deiiciencies, especially those in which the 

 break is in the achromatic region. 



Barton found a nnicli higher ratio of deficiencies to translocations with 

 ultraviolet than with X rays. The two translocations found with ultra- 

 violet were deficiency translocations. Mutations also showed a much 

 higher ratio to translocations with ultraviolet. The deficiencies found 

 with both ultraviolet and X-ray treatment included both terminal and 

 interstitial deficiencies. Chromosome breakage was highly localized in 

 the chromatic regions; however, the terminal deficiencies (observed in the 

 ultraviolet series included one in which the break was in the achromatic 

 region. 



The various contrasts in genetic effects which ha\e been mentioned 

 indicate that the chromosome breaks induced by ultraviolet radiation are 

 of a (lualitatively different kind from those induced by X rays. Accumu- 

 lating evideiice from many sources has indicated that the mutations 

 induced by X rays are in many cases, if not in all, extragenic alterations 

 incidental to chromosome breakage. The indication that the chromo- 

 somal effects of the ultraviolet are of a different kind encourages the hope 

 that the induced mutations also may be (lualitatively different. 



This possibility may be investigated effectively only by the critical 

 study of the mutation of specific genes, for there are no general criteria by 

 which mutations induced at miscellaneous loci may be distinguished from 

 the possible effects of known extragenic phenomena. AVith certain 

 selected loci it may be possible to develop special criteria for the identifi- 

 cation of gene mutations and for the recognition of alterations which, in 

 experiments on the general mutation rate, would .simulate gene mutation. 



A comparison of X-ray and ultraviolet mutations of the gene .4 in 

 maize (Stadler and Roman, 1943, 1948) indicates that the mutations 

 induced by the two agents may be qualitatively different. Among about 

 400 alterations affecting A, induced by X rays, and a much smaller num- 

 ber induced by ultraviolet, those most nearly approaching the typical 

 genetic behavior of gene mutations were selected for detailed study and 

 comparison. Among the X-ray alterations, only two were normal plants 

 free from segregating pollen defects. A third haplo-viable X-ray altera- 

 tion, showing segregating for subnormal but not aborted pollen, was 

 included for comparison. Among the ultraviolet alterations, normal 

 plants with normal pollen were more frequent. Three cases with a pheno- 

 type and one with intermediate phenotype (.4") were included in the 

 comparisons. 



The three X-ray mutants were characterized in varying degree by 

 reduced viability in haplopha.se and by reduction in the frequency of 



