GENETIC EFFECTS IN MAMMALS 



835 



embryos examined on the eighth or ninth day of pregnancy of females 

 mated to males exposed to 1000 r. There is some evidence, from a com- 

 parison of implantation data for 1200 r with the two sets of data for 

 800 r (Table 12-6), that the effect of raising the dose is to increase the 

 ratio of preimplantation to postimplantation death; for, although no 

 controls are given for the 1200 r data, the low mean number of implanta- 

 tions indicates a high incidence of preimplantation death. This effect of 

 an increase in dose, as Lea (1947) pointed out, could be attributed to 

 more of the sperm carrying more than one lethal chromosome change and 

 to this resulting in a relatively greater death in early stages. The data 

 of Amoroso and Parkes (1947) on irradiation of rabbit spermatozoa in 



Table 12-6. Time of Death of Embryos Carrying X-ray-induced Dominant 



Lethals 



" Excludes an insignificant proportion of unclassified embryos. 



vitro show that, when very high doses are given to the sperm, death of 

 embryos takes place still earlier in preimplantation development. 



The exact nature of the chromosomal aberrations that cause dominant 

 lethality in mammals is not known. Snell (1933b) pointed out that his 

 data, Table 12-5, fit the interpretation that litter size falls off logarith- 

 mically with dose. This would be expected if the dominant lethals were 

 the result of single-hit effects. However, the data are not extensive 

 enough to warrant definite conclusions on the relation of lethality to dose. 

 It is difficult to obtain accurate information on this relation because with 

 increasing dose there is an increasing proportion of fertile matings that 

 produce no young at term and possibly an increasing proportion of still- 

 births, some of which may be missed. The cytological observations of 

 Brenneke (1937) and Amoroso and Parkes (1947), as far as they go, 

 strengthen the view that dominant lethals in the mouse and rat are 

 analogous to those of Drosophila. The much higher rate of induction in 



