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CHAPTER 21 



duced. The frequency of these will increase 

 linearly with neutron dose, and be independ- 

 ent of the dose rate. The same tracic of 

 ionizations may, as in the case of fast neu- 

 trons, break two different chromosomes; 

 this would be possible when these chromo- 

 somes are closely packed together, as they are 

 in the sperm head. The fact that the total 

 frequency of reciprocal translocations in- 

 creases linearly with fast neutron dose when 

 sperm are treated is evidence both of this 

 and of the view, already presented in Chapter 

 20, that proximity of broken ends favors their 

 union. For such a result would be obtained 

 only if both breaks were produced by the 

 same track, and only if the broken ends 

 capable of exchange union were restricted to 

 those located near each other, broken ends 

 produced in the course of different tracks 

 being too far apart. 



When, however, ordinary X rays are in- 

 volved, the clusters are smaller, and the track 

 of ionizations is shorter. In this case, two 

 breaks will be produced by the same track less 

 frequently, and if they do occur, they will 

 usually be in the same chromosome at places 

 quite close together. For example, the two 

 breaks may occur within submicroscopic dis- 

 tances in successive gyres of a coiled chromo- 

 some, so that only minute to small structural 

 changes will be produced by such breaks. In 

 the case of X rays, two breaks occurring mi- 

 croscopic distances apart in the same nucleus 

 are usually the result of the action of two 

 clusters, each one derived from an independ- 

 ently arising track, so that one break has no 

 dependence upon the other for its produc- 

 tion. For this reason. X-ray-induced gross 

 rearrangements can be dose dependent. For 

 when a low enough dose is given, some nuclei 

 contain only one track and only one break, 

 and no gross two-break rearrangements can 

 occur in them. But when the dose is raised 

 high enough so that these nuclei are traversed 

 by two separate tracks, the two breaks re- 

 quired for such rearrangements can be pro- 



duced in the same nucleus. The higher the 

 dose of X rays, then, the greater is the ef- 

 ficiency in the utilization of breaks to pro- 

 duce gross rearrangements. Accordingly, 

 for doses causing some cells to have two inde- 

 pendently produced breaks, and for doses 

 higher than this, the frequency of these muta- 

 tions increases, not in direct proportion to the 

 dose, but faster than this. There is, however, 

 a small proportion of single X-ray tracks that 

 will, in the treatment of sperm, for example, 

 cause two breaks, each in a different chromo- 

 some. Therefore, for doses of X rays that 

 produce fewer than two tracks per sperm, 

 gross rearrangement frequency will increase 

 linearly with dose. So, there is actually no 

 dose of X rays to sperm which would not have 

 some chance of producing a gross rearrange- 

 ment, which means that no dose is safe in 

 this respect. 



X-ray-induced rearrangements, produced 

 by two or more independently arisen breaks, 

 may also be dose-rate-dependent. When a 

 suitably large dose is given over a short inter- 

 val, the ends produced by separate breaks are 

 present simultaneously and can undergo 

 cross-union. But when the same dose of this 

 radiation is given more slowly, the pieces of 

 the first break may restitute before those of 

 the second are produced, in this way elimi- 

 nating the opportunity for cross-union. In 

 this event, the same dose, given in a pro- 

 tracted manner, would produce fewer gross 

 rearrangements than it would when given in 

 a concentrated manner. While this dose-rate- 

 dependence for X rays holds true for most 

 cells, it does not obtain, however, for mature 

 sperm of animals, probably including man. 

 In these cells the broken pieces cannot join 

 each other and are, therefore, accumulated. 

 For this reason, it makes no difference how 

 quickly or slowly the dose is given to the 

 chromosomes in a sperm head, for the breaks 

 remain unjoined until after fertilization when 

 the sperm head swells. 



We have already mentioned that the spatial 



