844 RADIATION BIOLOGY 



The above historical account summarizes the major findings. Detailed 

 aspects, and some additional results, follow. It should be kept in mind 

 that most of the partially sterile animals obtained in the experiments to 

 be discussed were not actually proved cases of translocation and that 

 some of them were not even tested for transmission of partial sterility to 

 descendants. Furthermore, if any translocations in heterozygous condi- 

 tion show considerably more than 50 per cent fertility, these would usually 

 not have been detected. It is also possible that some of the partially 

 sterile animals carried more than one translocation. In spite of these 

 qualifications, however, it seems safe to assume that the frequency of 

 induction of translocations is represented, in at least a rough way, by 

 tabulation of partial sterility. 



The frequency of occurrence of partial sterility in the offspring of pre- 

 sterile-period matings of male mice exposed to various doses of X rays is 

 shown in Table 12-9. Since Snell found no clear-cut cases of transloca- 

 tion in his controls, and Charles (1950) only two in 2755 animals, it is 

 likely that in Hertwig's controls, which were not tested for transmission of 

 partial sterility to their descendants, most, or all, of the observed partial 

 sterility was not due to translocations, but was simply an expression of 

 the variation, apparently large in Hertwig's material, of other factors 

 affecting fertility. Although these factors presumably did not contribute 

 greatly to the results in Hertwig's irradiated material, some of which, in 

 contrast to the controls, was tested for transmission, it is possible that 

 they accounted for some of the recorded partial sterility and perhaps for 

 more in females than in males. Making allowance for this, the data in 

 Table 12-9, for all investigations combined, indicate that induced partial 

 sterility occurs with approximately equal frequencies in the male and 

 female offspring of presterile-period matings of irradiated mice. The 

 data are still not sufficient for an accurate analysis of the relation between 

 frequency and dose. They do, however, show that the rate is much 

 higher than that for translocations in Drosophila. The yield from a dose 

 of about 700 r in the mouse is comparable to that from 4000 r in Dro- 

 sophila melanogaster. A difference of about this magnitude would be 

 expected from the larger number of chromosomes in the mouse, but 

 whether or not other factors are involved is not known. 



Since, as will be discussed later, the yield of partially sterile offspring 

 from irradiated oocytes may be lower than that from irradiated sperm, it 

 might be expected that the yield from irradiated spermatocytes would 

 also be low. If this assumption is correct and if, as Hertwig has main- 

 tained, later matings within the presterile period of irradiated males 

 utilize some sperm that was irradiated in spermatocyte stage, then these 

 later matings should give a lower percentage of partial sterility than the 

 earlier matings. The difference between spermatocytes and sperm would 

 thus be in the opposite direction to that obtained for dominant lethals. 



