RADIATION— INDUCED ABERRATIONS 269 



tracks, with the result that the frequency of interchange aberrations (for 

 example, dicentrics and rings) increases as the square of the dose (a 

 two-hit curve) when the time of irradiation is kept constant. Because 

 such aberrations are two-hit phenomena, there is also an intensity ef- 

 fect — the yield of interchanges decreasing when a constant dosage of 

 radiation is administered over periods of increasing duration. The 

 average time during which a break may remain "open" before restitu- 

 tion or reunion occurs (that is, its half life) is at least 4 min. 



For other radiations (for example, recoil protons from fast neutrons), 

 ionization distribution along tracks is such that the probability of 

 breakage of a chromosome by a traversing particle is close to 1, and as a 

 consequence the efficiency of such radiations in producing chromosomal 

 changes is very high compared to those of gamma or x-rays. In addition, 

 both the breaks in the two separate chromosomes (or chromosome arms) 

 taking part in an interchange are usually produced by a single particle 

 track, resulting in a linear relationship between interchange frequency 

 and dose (a one-hit curve) and the absence of an intensity effect. 



In addition to aberrations resulting from breaks in two separate chro- 

 mosomes, certain types are produced oy breaks in a single chromosome 

 arm (either divided or single). The majority of these aberrations are 

 one-hit types resulting from the passage of a single ionizing particle, and 

 exhibit no intensity effect with any type of radiation. 



The interpretation of chromosome-aberration production as just out- 

 lined has been quite generally successful in accounting for most of the 

 quantitative results of radiation experiments w^ith Tradescantia. How- 

 ever, experimental data of two sorts have been obtained which indicate 

 that this hypothesis in its simplest form is not entirely adequate. The 

 first evidence was that obtained by Kotval and Gray (17) in their studies 

 with alpha particles. On the basis of comparative ionization distribu- 

 tion and particle numbers, the hypothesis predicts that a given amount 

 of ionization produced by alpha particles should be considerably less 

 efficient in producing chromosome breaks than an equal ionization dose 

 produced by fast neutrons, whereas the experimental results indicate 

 that for equal ionization doses alpha particles are somewhat more 

 efficient. It was concluded that a proportion of the breaks produced by 

 alpha particles arises from ionization produced in the immediate vicinity 

 of, but not within, a chromosome, thus suggesting the involvement of 

 an indirect as well as a direct mechanism. The second, and even more 

 striking, evidence was that obtained by Thoday and Read (28), who 

 noted a pronounced effect of oxygen on the frequency of x-ray-induced 

 aberrations in the root-tip mitoses of the broad bean, Vicia faha. Their 

 experiments indicated that the absence of oxygen during irradiation re- 



