MUTATION AND PLANT BREEDING 



500 



1 r 



300 

 DOSE OF X RAYS, r 



Figure 9. — Effect of dosages of X-rays at different intensities upon the 

 yield of chromosome exchanges. Data of Sax, after Giles (49). 



Certain types of chromosomal aberrations behave in a fashion 

 intermediate between one-hit and two-hit events. For instance, the 

 yield of interstitial deletions increases as the 1.5 power of the dose 

 as shown by Rick (127), whereas isochromatid deletions have a value 

 between 1.0 and 1.5. The exact value depends on the particular 

 radiation used (84, 188). 



It has been known for some time that some types of two-hit 

 aberrations fail to show the nonlinear response when cells are exposed 

 to certain densely ionizing particles, such as neutrons, alpha rays, 

 protons, etc. (Figure 10). The reason for this situation is that these 

 particles have sufficient density of ionization and sufficient length of 

 track to produce more than one break for each passage through a 

 nucleus. Thus, what would for X- or gamma rays be a two-particle 

 event (two separate ionization "tails") is, in actual fact, a one-particle 

 event (one ionization track) for the densely ionizing particles men- 

 tioned above. (See Swanson (183) for references.) 



