X-ray-induced recessive lethal mutations in adult and foetal female mice

Tests of X-ray-induced recessive lethal mutations in adult and foetal mouse females were performed. The tests were based on family analysis which provided a possibility of making a distinction between pre-existing recessive lethal mutations and newly arisen (spontaneous + induced) ones. The way the tests were carried out provided material for the estimation of the frequency of spontaneous lethal mutations per genome. The two different estimates from this material, 0.72 and 0.91%, respectively, are similar and fall within the range previously suggested by Lüning [5]. The difference, though statistically nonsignificant, between data from irradiated mature and maturing oocytes in adult females and material from the unirradiated part, indicates a mutation rate of 13 x 10(-5) per rad per gamete according to one estimate and 8-12 x 10(-5) according to the other in comparison with 9 x 10(-5) from irradiated spermatogonia [6]. In the limited data obtained after foetal irradiation, there is no indication of oogonia and developing oocytes being more sensitive to irradiation than oocytes in adult females fertilized within 6 weeks of treatment.