CHROMOSOME ABERRATIONS IN ANIMALS 675 



by McClintock (1939). If a broken chromosome is present in the endo- 

 sperm or gametophyte tissue of maize, union of sister chromatids will 

 occur at the breakage point, but no such sister-strand reunion occurs in 

 the tissues of the embryo. 



Another aspect of the problem of qualitative differences among breaks 

 concerns the different fates of the breakage ends produced in cells of a 

 single type, such as the spermatozoa of Drosophila. Of the breaks pri- 

 marily induced in a cell by irradiation, some may be restituted, some may 

 participate in structural rearrangement, and others may remain "open" 

 or "unhealed." When breakage ends are capable of rejoining, the prob- 

 ability of restitution or participation in structural change "appears not 

 to depend on a difference in the breakage process, but mainly on whether 

 other breaks are available with which interchange can occur" (Lea, 1946). 

 Existing data suggest that restitution and recombination occur with 

 approximately equal frequencies (Lea and Catcheside, 1945; Baker, 

 1949). 



If neither restitution nor recombination takes place, it is difficult to 

 determine whether the breakage ends are different in some way from 

 other breakage ends, or whether they are capable of joining but have been 

 prevented from doing so by chance circumstances (Lea, 1947a). Analysis 

 of the proportions of breakage ends that failed to recombine in experi- 

 ments treating Tradescantia chromosomes with different types of radia- 

 tion favored the former of these alternatives (Catcheside, Lea, and 

 Thoday, 1946a; Lea, 1946). It thus seems probable that some breaks 

 may be qualitatively different from others with respect to their capacities 

 for subsequent recombination. 



Data obtained in studies of Drosophila suggest that such qualitative 

 differences in breakage ends may exist from the time of their origin 

 (Kaufmann, Hollaender, and Gay, 1946; Kaufmann and Wilson, 1949). 

 These studies involved treatment of spermatozoa with near-infrared 

 radiation before exposure to X rays. The near-infrared portion of the 

 spectrum, centering around wave length 10,000 A, was not in itself effec- 

 tive in inducing chromosome breaks or gene mutations; but when used 

 before X rays it significantly increased the frequency of viable types of 

 rearrangements as compared with the frequency in controls receiving 

 only the X rays. On the other hand, treatment with near-infrared radia- 

 tion after exposure to X rays did not increase the frequency of rearrange- 

 ments. The frequency of induced dominant or recessive lethals was not 

 modified by near-infrared pretreatment or posttreatment. The experi- 

 mental data are summarized in Table 9-7. Additional posttreatment 

 data were obtained by Kaufmann and Wilson (1949) from exposure of 

 females inseminated by X-ray-treated spermatozoa to near-infrared 

 radiation. As in the experiments summarized above there was no sig- 

 nificant increase in frequency of rearrangements or percentage of breaks 



