GENETIC EFFECTS IN MAMMALS 831 



rate were sterile at first, but recovered fertility after two months. Expo- 

 sure to 1100 r at 4.4 r per day did not affect fertility. Hybrid females 

 exposed to total doses of from 770 to 880 r, regardless of rate, and then 

 mated, were either sterile or became so after the production of one, or 

 rarely two, litters of reduced size. When inbred strains were used 

 instead of hybrids, the same effects in males and females were produced 

 at lower doses. Histological observations on the testis showed reduction 

 in spermatogenic elements to stable levels which were dependent on dose 

 rate. The relative proportions of the cell types were, however, normal 

 except in males exposed to 8.8 r per day. In these animals, after ten 

 months, the sperm were relatively few in number and multinucleated 

 spermatids were common. Eschenbrenner et al. suggest that these effects 

 resulted from the degeneration of Sertoli cells which was observed only in 

 this material. 



Acute X-ray exposure of female mice with a dose of 150 r, or perhaps 

 lower, results in permanent sterility. One, and occasionally two, litters 

 can be obtained, even at much higher doses, before the sterility sets in. 

 The figures, 800 to 1500 r, given by Glucksmann (1947) as the permanent 

 sterilizing dose for the female mouse and rat are too high by a factor of 

 about 10, and are perhaps either a misprint or refer to the dose at which 

 even the temporary fertility following irradiation is suppressed. The 

 litter size in this temporary fertile period is reduced, and Snell and Ames 

 (1939) report that it falls off more rapidly with increasing dose than does 

 the litter size from irradiated males. 



DOMINANT LETHALS, SEMILETHALS, AND SUBVITALS 



For purposes of discussion here, lethals are defined as mutations that 

 cause death usually before birth, semilethals as those which cause death 

 usually between birth and reproductive age, and subvitals as mutations 

 that sometimes cause death. 



Offspring of Presterile-period Matings of Irradiated Males. Radiation- 

 induced dominant lethal effects in mammals were reported as early as 

 1908 by Regaud and Dubreuil who observed a high proportion of abnor- 

 mal embryos in rabbits sired by males that had been exposed to X rays. 

 It was not until many years later, however, that it became probable, and 

 was finally demonstrated, that induced chromosomal aberrations are a 

 cause of this class of abnormalities. Strandskov (1932) found a mark- 

 edly reduced litter size in the immediate progeny of X-rayed male guinea 

 pigs. Since electrically induced ejaculations showed numerous motile 

 sperm during the times the litters were produced, Strandskov assumed 

 that the normal number of eggs was fertilized, but that some were later 

 resorbed. He cited the results of Regaud and Dubreuil, and similar 

 findings in amphibia, in support of the view that some embryos died. 



