654 RADIATION BIOLOGY 



evidence conform with this interpretation. It has been reported that the 

 exceptional flies which result from interchange between X and Y chromo- 

 somes in attached-X females of D. melanogaster (Kaufmann, 1933) fre- 

 quently occur in clusters, suggesting that they originate in a gonial cell 

 (Neuhaus, 1936; Cooper, 1946). Whittinghill (1947) has observed that 

 the presence of a Curly inversion in the second chromosome of males of 

 D. melanogaster does not influence the induced frequency of crossing over 

 in the third chromosome, although it is known (e.g., Schultz and Redfield, 

 1932) that the presence of an inversion may affect the frequency of 

 recombination when crossing over occurs in the course of meiosis in the 

 female. 



On the other hand, Parker (1948) suggests that induced crossing over 

 in the male may result from breakage in the spermatocyte of both 

 homologous chromosomes by the action of a single ionizing particle — a 

 phenomenon that will be discussed subsequently. This hypothesis 

 assumes that synapsis of homologues occurs within a restricted region of 

 the chromosomes being tested, since it was found that crossing over is 

 limited to a short portion of the chromosome at any time. The produc- 

 tion of clusters is attributed to similarity of all cells in a cyst with respect 

 to their crossing-over potentialities. Dose-frequency relations as deter- 

 mined by Parker for the 500- to 2000-r range were in accord with the 

 assumption that a single ionizing particle is concerned; there was an 

 increase in crossing over with increase in dosage, but no increase in the 

 proportion of double crossovers. Shapiro (1945), however, observed a 

 much higher frequency of crossing over at 3000 than at 1500 r (0.69 as 

 compared with 0.23 per cent in one experiment), but only a slight addi- 

 tional increase (to 0.79 per cent) when the dose was increased to 4500 r. 

 He concluded that crossing over may be the result of breakage of chromo- 

 somes by ionization, especially in late spermatogonia or in spermatocytes, 

 but that selective elimination of germ cells may account for the declining 

 proportion of detectable recombinants with increasing dosage. A further 

 line of evidence that the frequency of crossing over is a function of the 

 distribution of ionizations is afforded by the observation of Lefevre (1948) 

 that neutrons are relatively more effective per ionization than are y 

 rays in inducing somatic crossing over in Drosophila. It thus seems 

 probable that irradiation-stimulated crossing over involves the produc- 

 tion of chromosomal breaks as a consequence of ionization; but the 

 period of gametogenesis during which this occurs is not clearly defined. 



3. THE PROCESS OF STRUCTURAL REARRANGEMENT 



From the foregoing discussion it is evident that the breakage and 

 recombination phases must both be considered in an analysis of the 

 process of induced structural rearrangement. Whether breakage pre- 



