A NEW HYPOTHESIS FOR 'CHROMATID' CHANGES 



this is so (Catcheside et al^), the conventional explanation being that there 

 is a certain likelihood that any break may remain open until metaphase. 

 The quantitative aspects of the exchange hypothesis given above, and 

 especially the question of whether it can account for all changes convention- 

 ally scored as chromatid breaks, will be discussed in detail in another paper. 



SURVEY 



It must be borne in mind very clearly that the hypothesis which has been 

 proposed above does not rest on any completely critical evidence. It is, 

 like the breakage-and-reunion hypothesis, a scheme for the interpretation of 

 visible changes in metaphase chromosome structure, and through which they 

 may be related to the quantitative data. Although these changes are 

 observed at metaphase they are, of course, only the consequence of much 

 earlier alterations in chromosome development. Obviously, a knowledge of 

 this development is essential for a detailed understanding of the nature of 

 its aberrations. At present this knowledge is almost entirely lacking, and 

 it therefore seems unlikely that the examination of the proposed exchanges 

 at the time they reach metaphase could alone throw much light on the 

 process of their formation (Loveless^). 



It is for this reason that the new hypothesis, as it now stands, only attempts 

 to account for the observed aberrations in terms of a chromatid exchange, 

 as being the least biological entity resolvable by the technique employed— 

 that of metaphase analysis. 



Now if the exchange and the breakage-and-reunion hypotheses alone are 

 considered, then it seems to the author that three main possibilities may be 

 distinguished. Thus, the exchange hypothesis might be totally incorrect. 

 Or it might be only partially correct : this could mean that the breakage- 

 and-reunion interpretation was not essentially incorrect, but had been 

 inaccurate insofar as it had failed to recognize that certain ' breaks ' were 

 actually intrachsinges ; or it could mean that aberrations happening to 

 resemble one another in some respects were arising by the two quite different 

 mechanisms. Or, lastly, the new hypothesis might be entirely correct : in 

 this case all the ' chromatid ' group of aberrations would be correctly inter- 

 preted as exchanges (complete or incomplete) which occurred either between 

 points on different chromosomes or else between points close together on 

 the same chromosome. 



In the author's opinion the items of evidence numbered 7 to 5 on pages i 



247-249 suggest rather strongly that the new hypothesis is at least partially I 



correct. However, it seems impossible at present to be more definite than | 



this, although it should be pointed out that the observations as a whole 

 provide no critical objections to its being wholly correct. It must therefore 

 be emphasized that the object of the ensuing discussion is to consider briefly 

 what would follow if the new hypothesis were correct, and not what neces- 

 sarily follows from its having been proved correct. 



If all defined ' chromatid ' aberrations were chromatid exchanges then 

 would it be possible to say anything at all about the exchange process ? 



The conventional explanation would be that each exchange is due to 

 two chromatid breaks and reunions. The complete acceptance of the new 

 hypothesis would implicitly question this view because all the ' breaks ' seen 



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