GENETIC EFFECTS IN MAMMALS 847 



in dominant lethals does occur, the absence of translocations is probably 

 not attributable to lack of breaks. Thus, it appears that it is the process 

 of segmental interchange which fails to occur. 



Using 7 rays from a radium source, Lorenz et al. (1947) found, by 

 fertility tests, no evidence of translocations in the offspring of male mice 

 exposed to 8.8 r given in 8 hours per day to a total dose of 1100 r, or in 

 the offspring of female mice exposed to 8.8 r given 24 hours per day to a 

 total dose of 770 r. The details of this experiment were presented in an 

 earlier report by Deringer et al. (1946). In all, forty-two offspring of 

 irradiated males and thirty-one offspring of irradiated females were tested. 

 The offspring of the irradiated males came partly from matings made 

 immediately after removal of the males from the radium field and partly 

 from later matings, but the proportions and time intervals are not given. 

 The absence of partial sterility in the offspring of the irradiated males in 

 this experiment is just what would have been expected from the earlier 

 work of Snell and Hertwig. For, firstly, even if all the offspring had been 

 obtained from matings made immediately after removal from the field, 

 the total dose received in spermatozoal stages, and perhaps in all post- 

 spermatogonial stages, would hardly have been high enough, even if it 

 had been given as an acute dose, to produce any translocations in the 

 number of animals tested. Secondly, although the average dose received 

 in spermatogonial stages may have been close to the total dose of 1100 r 

 given, Hertwig had already found no translocations in a slightly larger 

 sample of offspring of poststerile-period matings of males exposed to 

 acute doses of from 1200 to 1600 r (Table 12-9). Since the dose of radia- 

 tion to which the sperm were exposed in the experiment of Lorenz et al. 

 was not sufficient, even if it had been given as acute radiation, to have 

 produced any translocations in the sample tested, the data do not answer 

 the question of whether chronic irradiation of sperm is less effective in 

 translocation production than acute irradiation, or equally effective, as 

 would be expected if the chromosome breaks in mouse sperm are like 

 those in Drosophila sperm and remain open until after fertilization. 

 Taking SnelPs data on the incidence of partial sterility in the offspring 

 of females exposed to acute irradiation at face value and comparing them 

 with the results of Lorenz et al. for irradiated females, it appears that, if 

 translocations are induced in oocytes, chronic irradiation is less effective. 



The only information on the incidence of partial sterility in the descend- 

 ants of mammals exposed to repeated small doses of high intensity radia- 

 tion is that provided by Charles (1950). He exposed male mice to 0.1, 

 0.5, 1.0, or 10.0 r of X rays daily, for 6 days a week, mated them to 

 unexposed females throughout the weeks of irradiation and tested the 

 fertility of the offspring. The average accumulated exposures at time of 

 mating for the different dose levels were 13, 69, 134, and 238 r, respectively, 

 and 60 r for all dose levels combined, but there was a wide range of 



