Radiation-Induced Structural Chromosome Changes 



183 



X-ray ion track less frequently, and if they 

 do occur, they are usually quite close to- 

 gether. Note, however, that two breaks oc- 

 curring within submicroscopic distances in 

 successive gyres of a coiled chromosome 

 produce structural changes whose size ranges 

 only from minute to small. Nevertheless, a 

 small proportion of single X-ray tracks — in 

 the treatment of sperm, for example — do 

 cause two breaks, each in a different chro- 

 mosome. Therefore, for X-ray doses that 

 produce fewer than two tracks per sperm, 

 gross chromosomal rearrangement frequency 

 increases linearly with dose. So, there is 

 actually no dose of X rays which does not 

 have some chance of producing a gross re- 

 arrangement. In other words, no matter 

 how small a dose of ionizing radiation is 

 received, the possibility of a chromosomal 

 break and a gross chromosomal mutation 

 always exists. 



In the case of X rays or fast electrons, 

 two breaks that occur in the same nucleus 

 usually result from the action of two ion 

 clusters, each derived from a different, inde- 

 pendently arising track, so that each break 

 is induced independently. Fast electron or 

 X-ray-induced, two-break gross rearrange- 

 ments of this origin are dose dependent, for 

 when a small enough dose is given, a nucleus 

 is traversed by only one track, and only 

 one-track — not two-track — gross rearrange- 

 ments can result. But when the dose is large 

 enough for a nucleus to be traversed by two 

 separate tracks, the two breakages required 

 for two-break gross rearrangements can be 

 produced independently. Therefore, the 

 higher the dose of X rays used, the greater 

 the efficiency in producing multi-break gross 

 rearrangements caused by breaks independ- 

 ently induced by separate tracks. Accord- 

 ingly, for doses causing some cells to experi- 

 ence two such independently produced 

 breaks and higher doses, the frequency of 

 these mutations increases more than in di- 

 rect proportion to the amount of dose. One 



example is the exponential rise in the fre- 

 quency of reciprocal translocations obtained 

 after treating sperm in inseminated Dro- 

 sophila females with increasing dosages of 

 fast electrons (Figure 13-3, curve T). 



X-ray-induced rearrangements involving 

 two (or more) breaks induced by separate 

 tracks also depend upon the rate at which 

 a given dose is administered. When a suit- 

 ably large dose is given over a short interval, 



i i i i — i — i — r 



10 14 18 22 26 30 34 38 

 DOSE IN RADS(XIOO) 



figure 13-3. Percentage of mutations, ±2X 

 standard error, recovered from Drosophila 

 sperm exposed to different dosages of 18 mev 

 electrons. The sex-linked recessive lethal fre- 

 quencies (L) are joined by solid lines and are 

 adjusted for the control rale; sex chromosome 

 loss frequencies (5) are connected by broken 

 lines and are corrected for the control rate; 

 reciprocal translocation frequencies (T) be- 

 tween chromosomes II and III are connected 

 by dot-dash lines. (From I. H. Herskowitz, 

 H. J. Midler, J. S. Laugh lin. Genetics, 44:326, 

 1959.) 



