134 RADIATION GENETICS 



The mature female mouse is permanently sterilized by a single dose of 50 r or 

 greater. 1087 - 1147 The newborn female, however, is extremely resistant and will 

 show normal fertility even after a single dose of 300 r. 1126 - At several weeks of age, 

 however, a period of extreme sensitivity occurs so that even a low dose rate such as 

 continuous exposure to about 8 milliroentgens per minute for a total dose of85rwill 

 cause complete sterility. This is in contrast to the lesser sterilizing effects of a pro- 

 tracted exposure on the mature female in which doses up to nearly 300 r of X rays 

 may be accumulated at the rate of 10 r/week before complete sterility occurs. 1085 - 1087 - 

 1114 Reproduction studies have very clearly indicated that no oogonia are present in 

 the mature ovary and that the bulk of the cells are primary oocytes. 956 These are most 

 radiosensitive in early stages of development of the follicle. The more mature stages 

 go through to ovulation but are not replaced after a sterilizing exposure. 



Females of other mammalian species are not as radiosensitive as the mouse to the 

 sterilizing effect of radiation. This phenomenon is not clearly understood and ap- 

 parently no working hypothesis has been set forth. Rats and rabbits are only tem- 

 porarily sterilized by doses above 600 r. 743 The female beagle hound that survives an 

 acute radiation syndrome induced by a near midlethal dose of 300 r whole-body X 

 irradiation shows excellent reproductive performance in terms of litter size and estrus 

 activity, with even a tendency to improved lactation. 22 - 1201 Exposure to fast neutrons 

 is more effective than X irradiation for inducing a form of sterility in dogs, but the data 

 are not adequate enough to determine if an unusually high RBE might be involved. 22 

 In this instance, the dogs bred but were unable to whelp or lactate. 



The above discussion of the sterilizing effects of ionizing radiation was not in- 

 tended to be complete and a fuller discussion will be found in a report by Oakberg. 954 

 Additional studies have been reported for dogs, rats, and mice, following single, 

 fractionated, and continuous exposure to X rays, gamma rays, and neutrons. 179 - 846 - 

 847. 937. ii94 T n summary, the degree and time of sterility varies with species, sex, 

 dose rate, age at exposure, and quality of radiation. However, the pattern of reproduc- 

 tive performance following exposure is employed to control the cellular stage of interest 

 for mutation studies and therefore must be understood by the investigator before genetic 

 analysis can be carried out. 



QUALITATIVE GENETIC EFFECTS OF RADIATION 



Hereditary partial sterility. — Some of the offspring of irradiated males were observed 

 to produce consistently small litters by Snell 1250 in one of the early radiation genetic 

 studies in mice. Litter size was reduced by about one-half, and one-half of the progeny 

 of these litters expressed the trait in the next generation. The characteristic, which 

 behaves as a dominant trait, is generally classed as an hereditary dominant partial 

 sterility and has been regularly observed in all studies to date with mice. 198, 550, 731 - 1132 

 Snell 1232 demonstrated by the use of marker genes that the characteristic definitely 

 involves a reciprocal translocation. The original interpretation was derived from the 



