740 RADIATION BIOLOGY 



to X-irradiation breakage as double structures. It is concluded that the 

 type of aberration (whether chromosome or chromatid) is determined by 

 the time of X irradiation and is independent of the time at which the 

 infrared is applied. It is not yet clear why the additional reunion taking 

 place during infrared treatment does not correspond to the singleness or 

 doubleness of the chromosome as shown by X irradiation. 



The Drosophila results obtained when infrared is used as a supple- 

 mentary treatment with X irradiation differ in certain ways from those 

 obtained in Tradescantia. In an extensive series of experiments, Kauf- 

 mann and his co-workers (Kaufmann, Hollaender, and Gay, 1946; 

 Kaufmann and Hollaender, 1946; Kaufmann, 1946; Kaufmann and Gay, 

 1947; Kaufmann and Wilson, 1949) found that pretreatment results in an 

 increased frequency of translocations, but not of dominant lethals 

 (analogous to isochromatid breaks in Tradescantia) or of recessive lethals. 

 Further, posttreatment of mature sperm has no effect. If, however, 

 posttreatment is carried out at the time of fertilization (when chromo- 

 some recombination is usually assumed to occur), the frequency of trans- 

 locations is increased. The action of infrared in Drosophila thus appears 

 to be restricted to an effect on processes involved in the formation of 

 chromosomal interchanges. The failure to detect an increase in the 

 single-break type of dominant lethal indicates that the higher frequency 

 of translocations is not simply a result of an over-all increase in chromo- 

 some breakage. These observations, together with the evidence that 

 infrared alone has no detectable effect, have led to the hypothesis that 

 this radiation acts in Drosophila to facilitate recombination (as opposed 

 to restitution) among breaks produced by X radiation. It also appears 

 that breaks may fall into two qualitatively different classes, since the 

 subsequent behavior of the broken ends of some is modified by pretreat- 

 ment while that of others remains unchanged. On this basis the pro- 

 posal is made that dominant lethals in Drosophila may arise at the time 

 of irradiation as a result of sister chromatid reunion; whereas, breaks 

 resulting in translocations do not undergo recombination until the time of 

 fertilization. 



It is at present difficult to provide a comprehensive and unified inter- 

 pretation of the supplementary effects of infrared applicable to both 

 Drosophila and Tradescantia. The evidence that in Tradescantia all types 

 of breaks are increased in frequency by pretreatment appears to con- 

 stitute a real difference from Drosophila. However, the major effect of 

 pretreatment in Tradescantia seems to be on translocation frequency. 

 Again the major increase of posttreatment effects in Tradescantia, on the 

 basis of available data, appears to be on translocation frequency. 



Evidence against the idea of a weakened chromosome structure giving 

 rise to true breaks upon subsequent irradiation as postulated by Swanson 

 and Hollaender (1946) is available from the experiments of Sax (1942). 



