670 



RADIATION BIOLOGY 



in general with this expectation. Inversions, deficiencies, and translo- 

 cations detected by genetic methods by Timofeeff-Ressovsky (1939) in- 

 creased in frequency approximately as the square of the dose (Fig. 9-14a). 

 Data furnished by Makki and Muller (Muller, 1940) and by Catsch, 

 Radu, and Kanellis (1943; see also Catsch, Peter, and Welt, 1944; Catsch, 

 1948) for translocations between the second and third chromosomes 

 induced by X rays in the range from 400-4000 r, can be fitted to a 

 curve that represents the 1.5 power of the dose. This deviation from 

 the anticipated second power is attributed to the scoring of multiple- 



(a) 



1500 3000 4500 6000 

 DOSAGE, r 



-• EXPERIMENTAL 



1000 2000 3000 4000 5000 

 (6) DOSAGE, r 



o o THEORETICAL 



Fig. 9-14. Dose-frequency relations for viable types of chromosomal rearrangements 

 induced by irradiation of spermatozoa of Drosophila milanogaster. (a) Dose-frequency 

 relations determined by genetic tests for inversions, deficiencies, and translocations. 

 Broken line represents the theoretical exponential ("two-hit") curve. The vertical 

 bars indicate the range of the standard error for each of the experimentally derived 

 values. (Data from Timofeeff-Ressovsky, 1939.) (b) Dose-frequency relations deter- 

 mined by cytological tests for chromosomal rearrangements. Exponential curve 

 represented by broken line and limits of error by vertical bars as in Fig. 9-14a. (Data 

 from Bauer, 1939c.) 



break rearrangements as two-break events, and to the disproportionately 

 greater chance for the production of inviable recombinations (dominant 

 lethals) when many breaks are available in the same nucleus. 



The frequencies of inversions, deficiencies, translocations, and more 

 complex rearrangements, as determined by salivary-gland-chromosome 

 analysis of the progeny of males irradiated at various dosage levels, in the 

 studies made by Bauer, Demerec, and Kaufmann (1938) and Bauer 

 (1939b), are indicated in Fig. 9-14b. It will be seen that the frequency of 

 cells carrying the viable types of rearrangements increases more rapidly 

 than the first power of the dose, approaching the second power at the 

 lower levels of treatment. 



The frequencies of complex rearrangements detected in these studies 

 appear to be much lower than expected at low doses, and to increase 



