560 RADIATION BIOLOGY 



dose. A comparison was made of the results obtained for these two kinds 

 of aberrations when the irradiation had been appUed in an oxygen 

 atmosphere and when inert gas had been used. It was found that 

 changing the time-intensity relation of a given dose, either with oxygen 

 or with inert gas, did not change the ratio of chromatid to isochromatid 

 breaks, even though (as might be expected from the effectively denser and 

 larger tails of the electron tracks in the presence of oxygen) there was a 

 higher ratio of the isochromatid to single-chromatid breaks when oxygen 

 had been used. Moreover, the ratio of observed two-hit to one-hit aber- 

 rations was altered to about the same extent, for a dose of the same general 

 effectiveness, by a given amount of alteration of the time-intensity rela- 

 tion, regardless of which atmosphere had been present at irradiation. 

 All this indicated that the joining properties of the fragments and the 

 timing of restitutional versus other unions had not been affected by the 

 atmospheres present during irradiation. The conclusion thus became 

 more secure that the effect of oxygen had been on the production of the 

 primary breaks. 



Opposed to this conclusion is a recent paper by Schwartz (1952), in 

 which he does find a difference in the ratios of two types of aberrations in 

 maize endosperm according to whether X irradiation of the pollen was 

 carried out in an oxygen or nitrogen atmosphere. The two types, which 

 were studied by the effects on marker characters, were (1) mosaic losses 

 caused by the formation of isochromosomal or other dicentrics with 

 resultant breakage-fusion-bridge cycles and (2) stable whole-chromosome 

 deletions. The mosaic type appeared to be much more affected than the 

 stable type by the atmosphere present during irradiation. Since the 

 mosaic type is thought to represent mainly single-hit cases in which junc- 

 tion was delayed until it could occur between sister ends, whereas the stable 

 type is thought to consist mainly of two-break interstitial deletions in 

 which the two distal pieces joined each other, Schwartz argues that the 

 oxygen present during irradiation must have affected the broken ends in 

 such a manner as to reduce their likelihood of joining rather than affected 

 the frequency of primary breaks. 



This argument, however, involves many assumptions since the actual 

 chromosomal structure (or the frequencies of different kinds of structures) 

 of neither of these two phenotypic classes of aberrations is well known. 

 So, for example, many of the mosaics may have been two-break rings or 

 dicentrics, involving only one chromatid, and most of the stable class may 

 have been one-break deletions in which healing of the broken end had 

 occurred. The evidence offered on these points, derived from the relative 

 frequencies of certain types observed in sporophytic tissues, is not 

 germane since these frequencies are known to deviate widely from the 

 relative frequencies in the endosperm. Moreover, the same amount of 

 irradiation was given in conjunction with both gases so that there was a 



