A NEW HYPOTHESIS FOR 'CHROMATID' CHANGES 



is not always a one-to-one correspondence between ionization paths and 

 exchanges. The properties of the associations where intra- and /«/f?-changes 

 may occur differ in some way which happens to be critical for the different 

 radiations. Thus, in the case of small w^rachanges, either the two loci 

 involved are always close enough together, or other conditions at the critical 

 region of proximity are always such that one ionizing particle is enough to 

 involve both chromosomes in the exchange. But the properties of inter- 

 chromosomal associations cause them to react differently to the different 

 radiations : the amount of ionization — or the distribution of its effects, or 

 the precise chemical changes induced — along the tracks of recoil protons 

 and a-particles is such that they are singly effective in causing inter- as well 

 as /w^rfl-changes. In the case of X-rays, the passage of a mean of two 

 primary electrons is required for /w/^rchanges, and these passages must 

 occur within a very short time to effect an exchange. If this were correct, 

 then there would be in this respect a primary event — an unstable effect of 

 some kind — corresponding to what is conventionally visualized as breakage, 

 its decay being ' restitution '. A successful exchange initiation, on the other 

 hand, would correspond to ' reunion '. It should be stressed that whatever 

 actual processes it is they represent, these tw^o stages need not be the 

 only ones connecting the release of the dose energy with the final metaphase 

 exchange : it could merely happen that they are the only two stages which 

 are separable by the experimental method of varying the amount of the 

 X-ray dose or its intensity. In similar neutron or a-ray experiments they 

 are not separable at all. 



In distinguishing the new hypothesis from the breakage-and-reunion 

 hypothesis, the present problem is that the data do not reveal anything about 

 the nature of the ' association ', as it has been conveniently called in this 

 paper. Thus, although it seems certain that the two chromosomes are 

 close together (and that they are sometimes even homologously close to- 

 gether), it is not known whether it is this closeness itself which alone confers 

 sensitivity or whether there are specific physiological conditions necessary 

 for exchange induction which exist only at these associations. Now whether 

 this hypothesis is really a ' contact ' one seems to depend definitively on the 

 answer to the question of whether or not the primary event — whatever it 

 is, which corresponds to ' breakage ' — can be induced at loci other than those 

 in associations. The orthodox theory necessarily assumes that this may 

 happen because the premise is that chromatid breaks are the primary events 

 induced and that two go to make an exchange : therefore it must follow 

 that all single breaks which are actually observed as such are primary events 

 which have not participated in mf^rchanges and hence were not in ' contact ' 

 by definition. This is an assumption which in no way depends on the statistical 

 relation between interchanges and X-ray dose. It also follows that, if all parts of 

 the chromosomes are equally liable to breakage, there is about ten times the 

 observed number of breaks which disappear without trace, due to restitution. 

 Both these postulates appear to depend on the initial assumption that the 

 observed discontinuities are breaks and that they are visible examples of the 

 primary units of change. 



On the other hand, in terms of the interpretation presented in this 

 paper, there is no visible evidence that the primary events occur anywhere 



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