GENETIC EFFECTS IN MAMMALS 843 



males and females, and on the incidence in the two sexes. Is absence of 

 testes a common cause and does this occur in offspring of irradiated 

 males? Whatever the exact nature of the mutational changes, it seems 

 likely, from the high incidence, that most of the sterility will turn out to 

 be the result of chromosomal aberrations rather than gene mutations. 

 The failure to complete the later stages of spermatogenesis, which appears 

 to be the commonest cause of sterility in males, suggests chromosome 

 damage of a type that interferes with the maturation divisions. This, in 

 turn, provides a plausible explanation for the lack of sterility in the 

 progeny of poststerile-period matings of irradiated males. A mutation 

 that had such an effect would, if induced in spermatogonia, fail to pass 

 through to mature sperm. 



DOMINANT PARTIAL STERILITY 



The radiation induction of hereditary partial sterility in mammals was 

 reported first by Snell (1933a, 1934, 1935) and shortly afterward by 

 Hertwig (1935). In Snell's work, male mice were exposed to doses of 

 from 200 to 1200 r of X rays and mated to nonirradiated females. The 

 fertility of each of several offspring sired in the presterile period was 

 tested by repeated matings to nonirradiated animals. The distribution 

 of the mean sizes of the litters produced proved to be bimodal, whereas 

 that of the controls was unimodal. The upper mode in the experimental 

 group corresponded approximately to the mode in the controls, and the 

 lower mode was at about one-half the litter size of the upper mode. 

 Snell referred to the animals that consistently produced small litters, 

 that is, the animals grouped around the lower mode, as "semi-sterile." 

 The term "partially sterile," adopted by some authors, is used in prefer- 

 ence here because it avoids any implication as to the extent of reduction 

 in fertility. Snell showed that the reduction in litter size could be 

 accounted for by the death of embryos. He found that partial sterility 

 was transmitted, like a dominant, to one-half of the surviving progeny 

 of outcrosses of partially sterile animals. He pointed out that the 

 results fitted the interpretation that the partially sterile animals were 

 heterozygous for a reciprocal translocation. After laborious linkage 

 tests, Snell (1941a, 1946) finally achieved genetic proof of this in one 

 partially sterile line. In this line, the normally unlinked genes a and b 

 were found to be linked. Cytological confirmation of translocation was 

 provided by Roller and Auerbach (1941) and Roller (1944) who demon- 

 strated the association of four chromosomes at synapsis in each of three 

 partially sterile lines examined. Hertwig (1935, 1938a) confirmed Snell's 

 early findings and, in addition, showed that there was little or no induced 

 partial sterility in the offspring of poststerile-period matings of irradiated 

 males. 



