826 RADIATION BIOLOGY 



available cytological and genetic tests for structural chromosomal changes 

 are far less critical. The category "point mutations," or mutations in 

 which structural change has not been detected, is not, therefore, a par- 

 ticularly useful one in mammals at the present time. In this chapter, 

 mutations are classified according to their dominance relations and 

 phenotypic effects. Wherever there is proof, or evidence, that a chromo- 

 somal change is involved, this is, of course, discussed. 



Effect of Radiation on Reproduction. In investigating the genetic 

 results of radiation in mammals the experimental procedures are in some 

 respects limited, and in others aided, by the effects of the radiation on the 

 reproduction of the exposed animals. It is desirable, therefore, to give 

 an outline of these effects before considering the results of genetic studies. 

 Furthermore, the two subjects are not unrelated: some of the effects on 

 reproduction are themselves the result of damage to the genetic material 

 of the germ cells. 



The procedure used in most genetic studies has been the exposure of 

 males to a single, high intensity dose of X rays. Considering, first, the 

 effects on reproduction of this treatment, it is now well established by the 

 results of many investigators that within the range of approximately 

 400 1000 r, the limits depending on the species and on other factors, a 

 period of fertility immediately following irradiation is succeeded by an 

 interval of sterility after which fertility returns. A sample of data on the 

 lengths of the initial fertile period and the temporary sterile period is 

 given in Table 12-1. The length of the initial fertile period is probably 

 underestimated in some of the experiments because of its dependence on 

 number of copulations (Snell, 1933b; Brenneke, 1937; Hertwig, 1938a), or 

 because of insufficient testing of males. The data are erratic and there is 

 no clear-cut dependence on dose. The length of the sterile period, how- 

 ever, is apparently affected by dose. This conclusion is confirmed by 

 the findings of Strandskov (1932) who showed that the length of time 

 during which no motile sperm were present in electrically induced ejacula- 

 tions of guinea pigs was related to the dose received by the males. Litter 

 size in the first fertile period is reduced and is related to dose. After the 

 sterile period the litter size is probably slightly below normal, but so close 

 to it that a difference is hard to establish. Details on litter size are 

 presented later. 



For doses below about 400 r there is no clear-cut period of complete 

 sterility. As the dose increases above 1000 r the litter size in the period 

 immediately following irradiation reduces toward zero, most matings 

 producing no young at all. At very high doses it appears that some 

 permanent sterility is induced (Table 12-2), but whether this results from 

 direct damage to the testis or from the systemic effects, which are notice- 

 able even with the partial-body irradiation, is not known. In either case, 

 there appear to be differences in response attributable to the strains of 



