CHROMOSOME ABERRATIONS 433 



rility is passed on as a "dominant" to one-half of the viable offspring of 

 semisterile animals. 



Snell suggested that the semisterile animals carried a reciprocal trans- 

 location which was inherited by their semisterile offspring. He thus ac- 

 counted for the dying embryos as having unbalanced chromosome com- 

 plements and for the normal fertile offspring as being balanced and not 

 carrying the translocation. In one case, Snell (17, 18) was able to con- 

 firm this interpretation by linkage tests. 



With the above brief historical outline as a background, we can turn 

 to some later findings and a discussion of the results. 



The results of Lorenz et al. (12), who detected no semisterility in the 

 offspring of mice exposed to gamma rays from a radium source, have 

 been widely quoted by authors concerned with human hazards, with the 

 implication that they are at variance with the findings of Snell and Hert- 

 wig and that the explanation may lie in a difference between the effects 

 of chronic and of acute radiation. Hertwig (6), however, had shown 

 that the incidence of semisterility is high only in offspring conceived 

 within a short period (about 4 weeks) following irradiation, that is, as 

 a result of irradiating mature sex cells. If the experimental procedure 

 of Lorenz et al., as given in the earlier report of Deringer et al. (3), is 

 examined with this in mind, it is found that the exposed females were 

 not bred until a month after removal from the radiation field, and that 

 although exposed males were bred immediately after removal it is not 

 stated whether the offspring tested for fertility came from the first or 

 subsequent litters. In all, 42 offspring of exposed males were tested. 

 Even if it is assumed that all of these were conceived shortly after re- 

 moval from the radiation field, the total dose received in postspermato- 

 gonial stages was still not very high. From the rates obtained by Snell 

 and Hertwig, it could have been predicted that perhaps 1 of the 42 off- 

 spring would have been expected to show semisterility. There is, there- 

 fore, no conflict between the two sets of results. The question of de- 

 pendence of effect on intensity of radiation is not yet answered, but since 

 the effect with which we are concerned applies onh^, or mainly, to mature 

 gametes, it seems probable that, as in DrosophUa, intensity may be less 

 important than total dose. 



Additional data on the incidence of sterility and semisterility in the 

 offspring of irradiated males have been obtained by us. * They will be 

 presented in a separate paper for publication elsewhere, but they should 

 perhaps be given in summary here to show that they agree closely with 

 the results of Snell and Hertwig. The animals tested, 22 males and 15 



* Work performed under Contract No. W-7405-Eng-26 for the Atomic Energy 

 Commission. 



