272 



UADIATION HIOLOGY 



that they are quiihtativrly (UlTcicnt l)oth as to their nature and as to their 

 mode of orij^in. \\'h(Mi ultra \iolet is used in combination with X rays as a 

 pretreatment, the frecjuency of terminal deficiencies is no greater than that 



Tabi.k 7-2. Kkequencies of Chromatid Aberrations Induced in the 

 Pollen Tube Chromosomes of Tradescantia by X Rays and 



Ulthavioi.ht 



(RwMiisoii, MM2.) 



expected from the ultraviolet treatment alone (Table 7-3). It would 

 appear that the circumstances which favor the induction of one kind of 

 terminal deficiency actually suppress the appearance of the other kind. 



Table 7-3. Frequencies of X-ray-induced Chromatid Aberrations as 

 Influenced by Pretreatment with Ultraviolet 



(Swanson, 1944.) 



The X-ray-induced aberrations involved in illegitimate fusion, i.e., iso- 

 chromatid deficiencies and translocations, are similarly suppressed. 

 Since a like reduction in frequency of X-ray-induced al)errations is encoun- 

 tered when ultraviolet is employed as a posttreatment, it seems reasonable 

 to assume that the action of ultraviolet is not to prevent the initiation of 

 X-ray-induced breaks, but rather to lessen their probability of realization. 

 Kaufmann and Hollaender (1946) have demonstrated that a combination 

 of the two radiations has a similar depressing effect on gross chromosomal 

 aberrations in Drosophila, whereas Schultz (1951) has shown that the 

 effects of X rays in maize, as judged by the freciuency of gcrmloss seeds, 

 are completely inhibited by a posttreatment with wave length 297 m^t at 

 the same time that the ultraviolet effects remain uninfluenced by X rays. 



