C. p. SVVANSON 



affected by the absence of oxygen than isochromatid deletions and chromatid 

 exchanges. If the reduction in frequency of aberrations is expressed as an 

 air/nitrogen ratio, then that for chromatid deletions was about 1 -4 as com- 

 pared to 2-6 for isochromatid deletions and exchanges. These observ'ations 

 have been confirmed (Svvanson and Schwartz"*, 1953), and in addition it 

 was possible to show that the air/nitrogen ratios varied with the stage of cell 

 division. Dominant lethals and translocations in Drosophila also exhibit 

 different air/nitrogen ratios with the same quality of radiation (Baker and 

 Edington^, 1952 ; Baker and Von Halle^, 1953), therefore any hypo- 

 thesis dealing with the relation of oxygen tension to radiation effect must 

 be flexible enough to account for differential reductions among the possible 

 aberration types. 



Table I. Chromatid Aberrations Produced in Tradescantia by Four Qiialities of Radiation in Air 



and in Nitrogen. 150 r a< 8-9 r/minute 



Thirdly, it must also be recognized that the influence of oxygen tension 

 on the frequency of breakage is closely linked to the type of radiation em- 

 ployed. Thoday and Read'' (1949) have demonstrated that the oxygen 

 effect is much less with alpha rays than with X-rays, and that neutrons 

 appear to occupy an intermediate position (Giles, Beatty and Riley^, 

 1952). The ion density of the radiation will therefore be a determining 

 factor in governing the magnitude of the oxygen effect. Additional infor- 

 mation has now been obtained from Tradescantia studies in which the effects 

 from three qualities of X-rays, together with 1-1- to 1 -S-MeV gamma rays 

 from a cobalt-60 facility, have been compared (Sw^anson^). The results 

 of one of four similar experiments are given in Table I. 



If we first consider the frequencies of aberrations obtained in air, it is 

 apparent that the chromatid deletions increase in frequency as the ion 

 density of the radiation decreases but that the reverse holds true for isochro- 

 matid deletions. In fact, this shift in frequency of the two types of deletions 



255 



