GENETIC EFFECTS IN MAMMALS 855 



dominant defects, such as complete sterility and lowered viability in 

 postnatal or late prenatal life, which are important at the individual 

 level, is far from negligible. Fortunately, there is ample evidence to show 

 that the high incidence in the offspring is found only after irradiation of 

 later germ-cell stages. It is clear that if the gonads of a man are exposed 

 to a considerable dose of radiation at one time, or within the few weeks 

 required for a spermatogenic cycle, the chance of transmission of the 

 mutational changes responsible for these defects can be greatly reduced 

 by abstention from fertile matings for a period of a few weeks following 

 exposure. 



Turning to a consideration of point mutations and small deficiencies or 

 other minor chromosomal changes, it must be remembered that the ratio 

 of induced mutation rates in spermatogonia and spermatozoa for these 

 types of mutation has not been measured in mammals. However, from 

 results on Drosophila, it seems likely that the practice recommended above 

 for avoiding the transmission of major chromosomal changes would 

 result in much less of a reduction in the probability of transmitting point 

 mutations and minor chromosomal changes. If this is true, then, for 

 these mutations, it is more important to know the induced mutation rate 

 in spermatogonia than that in spermatozoa, because most of the total 

 dose received in the ontogeny of a spermatozoon in man will usually have 

 been accumulated in the spermatogonial stage. 



The induced mutation rate in spermatogonia for mutations with clear- 

 cut dominant effects has not been adequately determined in mammals. 

 The results of Charles (1950) indicate that the rate to dominant visibles 

 may be appreciable, but, in his data, the effects of irradiation on early 

 and late germ-cell stages cannot be separated. In the investigation by 

 Russell (1951), irradiation of spermatogonia gave a low over-all mutation 

 rate to dominants affecting coat color, tail, and ears, but a high rate at 

 one locus to dominants causing reduction in body size and possibly 

 lowered viability. 



The reduction in litter size in litters from poststerile-period matings 

 of irradiated males (Table 12-7) may prove to be useful in estimating the 

 total effect from the induction in spermatogonia of dominants that cause 

 mortality in early development. This effect is apparently not large 

 enough for easy measurement by a small sample, because of the presence 

 of variation in the many biological factors that influence litter size, but 

 the extensive data now being collected may prove informative when 

 fully analyzed. 



The specific-loci mutations induced in spermatogonia in the mouse 

 (Russell, 1951) have not yet been thoroughly investigated for deleterious 

 effects. However, the majority of those tested have proved to be lethal 

 or semilethal in homozygous condition (Table 12-11). Work on Dro- 

 sophila has shown that most recessive lethals have a deleterious effect on 



