Section 5 — Mutagenesis 



4 non-irradiated females. The females (328) were 

 dissected on the 13th to 16th day of pregnancy 

 for counts of corpora Iutea, implantations, and 

 post-implantation losses. The results obtained 

 show that the treatment both of the spermatozoon 

 and of spermatids led to the exponential depend- 

 ence of the dominant lethals frequency upon the 

 dose. The average probability of one dominant 

 lethal appearance per dose unit (mutation rate) 

 was 6.0 x 10 -3 for the spermatozoon irradiation 

 and ll.OxlO -3 for the spermatid irradiation. 

 The dose which leads to the death of 50 per cent 

 of embryos, calculated by probit-method, was 

 106 ± 10 rad for the spermatozoon irradiation 

 and 64 ± 9 rad for the spermatid irradiation. 

 Thus the spermatids appeared to be ~ 1.7 times 

 more genetically radiosensitive than the sperma- 

 tozoon. 



The probability of dominant lethal appearance 

 which leads to the death of embryos before im- 

 plantation was ~ 0.3 both for the spermatozoon 

 and spermatid irradiation. 



The comparison of the results obtained with 

 the data in the literature leads us to the conclusion 

 that in the case of the mature spermatozoon irradi- 

 ation the genetical effect of fast neutrons is about 4 

 times higher than that of X-rays. In the case of 

 the spermatid irradiation the RBE of fast neu- 

 trons is somewhat lower. 



5.67. A Dose-rate Effect in Mice with Chromosomal 

 Mutations. A. G. Searle and R. J. S. Phillips 

 (Harwell, Great Britain). 



Lyon, Phillips and Searle have shown that 

 1200 r acute X-irradiation of mouse spermatogo- 

 nia delivered in two equal fractions 8 weeks apart, 

 leads to a decrease of 1 5 per cent in litter-size of 

 offspring at birth. This is due to the induction 

 of dominant lethals acting around embryonic 

 implantation. Translocations, leading to semi- 

 sterility (or complete sterility in the male if sex- 

 linked), were induced in about 4 per cent of 

 survivors. The dominant effects of 1 200 r chromic 

 gamma-irradiation of spermatogonia, spread over 

 12 weeks at 100r/week, have now been investi- 

 gated, using the same mouse stocks. Results so 

 far show that the dominant lethal effect is much 

 reduced, with a decrease in litter-size of only 

 about 2 per cent in the irradiated series. In ad- 

 dition, the rate of induction of translocations is 

 decreased to about 1 per cent. 



Two different phenomena are probably re- 

 sponsible for the lower yield of chromosomal 

 mutations: (i) a reduction in the magnitude of 

 the non-linear component of the dose response 

 when the dose-rate is low, and (ii) a greater capa- 



city for "repair" with low than with high dose- 

 rate irradiation, as found with specific locus 

 mutations. 



5.68. The Influence of Radiation on the Genetic Con- 

 trol of Neonatal Mortality in Swine. D. F. Cox 



(Ames, U.S.A.). 



Duroc and Hampshire swine are used in a 

 continuing experiment designed to measure the 

 genetic effects of radiation. The first generation 

 offspring produced by normal males and females 

 are compared with offspring from males given a 

 single exposure of 300 r X-irradiation at least six 

 months prior to matings with untreated females. 

 Present data include 951 litters representing 

 9640 pigs. 



Survival from birth to 42 days was 72 per cent 

 in the control group compared with 68 per cent 

 in the group sired by irradiated males. No signi- 

 ficant changes in the number born, the sex ratio 

 or the number of late fetal deaths have been 

 found. The rate of mortality was determined in 

 three intervals of postnatal life; day one including 

 those born dead, the period between day one and 

 day six, and the period from day six through 

 day 42. The rations of the mortality rates in the 

 irradiated group to those in the control group for 

 the three intervals studied were 1.090, 1.529 

 and 1.020, respectively. The mortality rates in the 

 offspring from irradiated males exceeded the con- 

 trols in each interval but the period between the 

 first and sixth day after birth appears particularly 

 sensitive to the genetic effects of radiation. Inter- 

 pretation of this pattern of mortality resulting 

 from paternal irradiation will be presented on 

 the basis of information concerning the breed, 

 sex, and individual genetic factors contributing 

 to variation in the probability of survival. 



This work is supported by Contract AT(1 1-1) 

 707 from the U.S. Atomic Energy Commission. 



5.69. Heterosis in the Response of Mouse Embryos to 

 Irradiation. Donald J. Nash and John W. 

 Gowen (New Brunswick and Ames, U.S.A.). 



Mice of three inbred strains, BALB/Gw, K, 

 and S, and the hybrids derived from them were 

 exposed in utero on day 6 J, 10|, 14| or 17^ of 

 gestation to single whole-body doses of X-rays 

 of from 20 to 320 r. Data were obtained on the 

 incidence of malformations at birth, postnatal 

 growth from birth to maturity, life span, and 



78 



