A NEW HYPOTHESIS FOR 'CHROMATID' 



CHANGES 



S. H. Revell 

 Chester Beatty Research Institute, Royal Cancer Hospital, London 



In this paper a new hypothesis is suggested for structural chromosome 

 changes of the 'chromatid' class (Catcheside, Lea and ThodayI). The 

 hypothesis has been evolved during a comparative study of the effects of 

 X-rays and certain chemical agents, and is intended to apply to both. But 

 on this occasion only the radiation side of the matter will be discussed. 



So far as the author can judge, the scheme proposed resembles a ' contact ' 

 hypothesis — that is, in the ' contact first ' versus ' breakage first ' sense — and 

 he is well aware that the revival of anything like the rejected view in this 

 controversy may be regarded as conflicting with evidence pointing in the 

 opposite direction. An attempt will be made, however, to show how this 

 apparent conflict might be resolved. 



It should be emphasized that the hypothesis is at present only advanced 

 to explain what are usually called 'post-split' or 'chromatid' aberrations. 

 However, an important part of the general theory for chromosome changes 

 has been derived from a study of changes of this particular category (largely 

 in Tradescantia pollen grains; Sax^-^ ; Catcheside et al'^'^). Their inci- 

 dence has also been used as an index in testing the relative efficiencies of 

 different types of ionizing radiation, and in testing radiation sensitivity 

 under different conditions. But the hypothesis is not now concerned directly 

 with the explanation of other important types of aberration. Thus no 

 attempt will be made to explain ' chromosome ' changes in the restricted 

 sense ('pre-split' changes), or changes resulting from anaphase breakage, or 

 changes inducible in Drosophila reproductive cells, although this should not 

 be taken as implying that there is necessarily an essential distinction between 

 these other aberrations and those under consideration. It should also be 

 borne in mind that this short paper allows only a very cursory discussion of 

 most of the problems which the proposed hypothesis raises : some of these 

 will be dealt with in more detail in other publications. 



CRITICAL ACCOUNT OF THE HYPOTHESIS 



Viciafaba root tip cells have been used throughout the work, wliich, follow- 

 ing on the observations of Ford\ was originally undertaken with the ob- 

 ject of comparing the distribution of visible abeiTations induced by various 

 chemical agents with those induced by X-rays. But, as this work went on, 

 the orthodox interpretation was found increasingly unsatisfactory, not only 

 for the chemically induced changes but for the X-ray ones as well. This 

 was firstly because it did not seem to account plausibly for all the chromo- 

 some changes that were occurring (this will be amplified below), and 



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