CHROMOSOME ABERRATIONS IN ANIMALS 671 



less rapidly than would be expected on the general assumptions of the 

 "hit theory." Data summarized by Fano (1941) are presented in 

 Table 9-5. Expected frequencies (shown in parentheses) have been 

 computed on the assumption that the relative frequencies do not depend 

 on the dose. It will be seen that the relative frequencies of the different 

 types of changes — two-break, three-break, four-break, of both the 2 + 2 

 and the 4 type — are nearly constant at all doses. As a possible explana- 

 tion of deviation from expectancy, Fano (1943b) has suggested that 

 potential breaks may be influenced, by mechanical perturbations, to 

 participate in rearrangement whenever two other breaks initiate the 

 process. There is also the possibility that these mechanical disturbances 

 may in themselves cause breakage that is not attributable to the action 

 of the ionizing radiations. Because of such factors it is not possible at 

 present to formulate any simple theoretical interpretation of the relation 

 of frequency of complex chromosomal rearrangements to dose. 



In all these considerations it must be kept clearly in mind that an 

 observed rearrangement represents the climax of a series of processes 

 involving breakage and recombination. Efforts to separate these two 

 phases experimentally and thereby elucidate the process of structural 

 rearrangement have included studies of the relation between frequency 

 of aberrations and such factors as wave length, time, and intensity of the 

 ionizing radiations. It was shown by Muller and Ray Chaudhuri 

 (reported in Muller, 1940) that y rays and 50-kv X rays are equally 

 efficient per roentgen in inducing translocations. Thus 2000 r of 50-kv 

 X rays produced 2.9 + 0.3 per cent translocations between the second 

 and third chromosomes of D. melanogaster, and 2000 r of y rays produced 

 3.8 + 0.6 per cent. These values do not differ significantly, nor do those 

 obtained by Catsch, Kanellis, Radu, and Welt (1944) in a study of the 

 frequency of translocations between the second and third chromosomes 

 induced by doses of 1000, 2000, and 4000 r at wave lengths of 0.5, 0.4, and 

 0.15 A. 



It has also been shown that variation of time or intensity of treatment 

 does not alter the frequency of viable types of rearrangements induced by 

 a given dose of radiation (Muller, 1940; Kaufmann, 1941a; Dempster, 

 1941a; Makhijani, 1945). Muller and Makhijani (reported in Muller, 

 1940) found by genetic tests that doses of 2000 r given at intensities 

 ranging from 0.05 to 250 r per minute induced similar frequencies of 

 translocations between the second and third chromosomes. In these 

 experiments spermatozoa were treated after they had been transferred to 

 females in insemination; and the translocation frequencies were similar 

 for a given dose of X rays, whether it was administered in a single treat- 

 ment, with immediate or delayed opportunities for egg laying, or in a 

 series of four treatments. A like absence of effect of discontinuous treat- 

 ment when males were irradiated was established by cytological examina- 



