668 RADIATION BIOLOGY 



1945b, P. W. Whiting, 1938a, b), it was found that the proportion of 

 adult wasps failing to emerge among the offspring of irradiated haploid 

 males gave a dose-frequency curve closely paralleling that for dominant 

 lethals in Drosophila. The results are illustrated in Fig. 9-13b. 



Dose-frequency relations determined in studies of induced crossing 

 over have been discussed previously. Related studies concern the fre- 

 quency of separation of attached-X chromosomes in irradiated cells. 



Separation in nonirradiated XXY females involves crossing over with 

 the Y chromosome, so that the detached X carries either the short or the 

 long arm of the Y (Kaufmann, 1933). Mainx (1940) found that the 

 frequency of exceptional flies resulting from separation of attached-X 

 chromosomes after irradiation of unfertilized females increased approxi- 

 mately linearly with increased dose in the 1000- to 3000-r range. There 

 was a marked increase over values expected on the basis of linear propor- 

 tionality, however, when the dose was increased to 4000 and 4500 r. It 

 is assumed that at the higher dose a considerable proportion of the 

 exceptional progeny was derived from eggs in which the X chromosomes 

 were broken by processes other than the process involved in crossing 

 over with the Y. 



The frequency of production of small deficiencies, detected by genetic 

 methods, has been reported in some cases as proportional to dose (e.g., 

 Muller, 1940, 1941) and in others as approaching a "two-hit" curve 

 (e.g., Panschin, Panschina, and Peyrou, 1946). Rick (1940) also 

 reported dose-frequency relations for small spherical fragments induced 

 by irradiation of Tradescantia microspores which suggest that many of 

 the rearrangements are produced by a "two-hit " process. In Drosophila 

 the small deficiencies detected by genetic methods constitute a graded 

 series, some of which are discernible in salivary-gland chromosomes and 

 some of which are not (see Slizynski, 1938). From an analysis of the 

 frequency of deficiencies in the Notch region of the X chromosome of D. 

 melanogaster, Demerec and Fano (1941) concluded that the longer defi- 

 ciencies involve two breaks and depend for their production on the pass- 

 age of two separate ionizing particles. Deficiencies of sections shorter 

 than 15 bands on the salivary-gland chromosome (or ca. 60 m/x on the 

 sperm chromosome) presumably involve single events, and therefore 

 increase in number in proportion to dose. Breakage in these cases may 

 be visualized as a process whereby the effect of the radiation spreads 

 along the chromosome for a short distance and produces two uncouplings 

 rather than one. 



Another type of two-break rearrangement that is apparently produced 

 with a frequency proportional to the dose involves breaks, often in 

 separate chromosomes, induced by irradiation with neutrons (e.g., the 

 studies of Giles, 1940, and Thoday, 1942, on Tradescantia). The results 

 suggest that both strands involved in the exchange are disrupted simul- 



