840 RADIATION BIOLOGY 



the 1- to 8-day and 8- to 1-i-day mating groups. Unpublished data on 

 the sex ratio at birth from our experiments, tabulated according to the 

 division into time intervals made by Parkes, show, for male mice exposed 

 to 600 r, 54.1 per cent males in 218 offspring of 0- to 4-day matings and 

 54.6 per cent males in 370 offspring of 5- to 14-day matings. Thus 

 Parkes' results are not confirmed by later work at higher doses. Further- 

 more, the low proportion of males in the 5- to 18-day mating group in 

 Parkes' data is hard to explain. An increased death rate of male embryos 

 can scarcely be the cause, because the litter size is only slightly depressed. 

 The question, therefore, arises as to whether the disturbance in sex ratio 

 may have been the result of causes other than irradiation. It is also 

 possible that the fluctuations were random ones. The probability of 

 this is not necessarily as remote as the tests of significance indicate, 

 because the time sequences were apparently chosen on the basis of the 

 differences shown by them and not on a priori biological grounds. 



The sex ratio at birth in the offspring of poststerile-period matings of 

 mice shows, in the data of Hertwig (1938a), a larger proportion of males 

 than in the controls in all except the lowest of seven dose groups ranging 

 from 400 to 1600 r. The difference from the controls is significant for 

 doses of 1200 r and above. The total number of animals in the irradiated 

 group was 2240, in the controls 2595. On the other hand, the sex ratio, 

 recorded at three weeks of age, in unpublished data from our extensive 

 experiment with exposure of male mice to 600 r shows a slight decrease in 

 proportion of males in progeny of poststerile-period matings (50.35 per 

 cent males in 72,472 offspring as against 51.00 per cent in 55,828 controls). 



DOMINANT STERILITY 



The sterility described in this section is, like the lethality already dis- 

 cussed, called "dominant" solely because it appears in the Fi of an 

 irradiated parent. Its dominance precludes transmission to later genera- 

 tions and consequently makes analysis of its nature difficult. For 

 dominant lethals, there is, as has already been mentioned, cytological 

 evidence that chromosomal aberration is at least the major cause. The 

 cytology of the chromosomes of dominant steriles has not yet been 

 described. The available information on dominant sterility is limited to 

 the incidence and to the morphology and histology of the defect. 



Data from various experiments with mice on the incidence of dominant 

 sterility in the offspring of irradiated male or female parents is given in 

 Table 12-8. Although far more attention has been paid by all investigators 

 to the partial sterility described in the next section, it is apparent that 

 the rate of induction of sterility in the offspring of presterile-period 

 matings of irradiated males is far from negligible. In the male offspring 

 it may be taken as about 10 per cent for a 700-r X-ray exposure of the 



