86 MUTATION AND PLANT BREEDING 



her of nucleoprotein strands in a Tradescantia chromosome, accord- 

 ing to recent work with the electron microscope, may be as high as 

 32 before synthesis (177). One would expect that the number of 

 strands or fibrils in a a chromatid, as well as their diameter, should 

 have a bearing on the probability of breakage of that chromatid by a 

 given ionization track or cluster. 



3. Dosage response curve for one-hit events 



Chromosome or chromatid breakage is considered to result from 

 a cluster of 15 to 20 ionizations either within or very close to the 

 chromatin strand in question (93). A sufficient density of ionization 

 may occur at any position along the more dense tracks, such as those 

 of alpha particles, or only near the end, or "tails", of less dense tracks. 

 While each ionization is an independent event, in radiobiological 

 parlance it is also possible to regard any large cluster or tail as a single 

 event. When such a cluster or track passes through a chromosome 

 (or chromatid), the event is called a "hit". Such clusters or tracks have 

 a certain probability of producing a break (in contrast, it is often 

 considered that gene mutation can be caused by as little as one ion 

 pair). It has been shown that various one-hit chromosomal events 

 (one-hit interstitial deletions, terminal deletions) are independent of 

 the dose rate, of the time of exposure, and of dose fractionation. They, 

 therefore, increase linearly with dose (Figure 8). In general, the 

 kinetics of such events are similar to those of gene mutations, but the 

 relative biological efficiency of different radiations in producing these 

 end results would be expected to differ. (See also Section VIII A.) 



4. Dosage response curves for two-hit events 



Certain types of chromosome or chromatid aberrations 

 (exchanges, inversions, rings, etc.) are two-hit aberrations as far as 

 X-rays and gamma rays are concerned and behave in a different 

 fashion with respect to several of the variables mentioned above from 

 one-hit chromosome aberrations. Whereas with one-hit aberrations 

 the frequency is strictly proportional to dose, with two-hit aberrations 

 the frequency increases disproportionately with increasing dose and 

 with sufficiently high intensities becomes almost proportional to the 

 square of the dose (Figure 8). The dose-squared relationship is best 

 observed at the higher intensities when the exposure time is kept 

 constant but the intensity varied (Figure 9). From these and similar 



