7-7 a -™ population at an essentially linear rate of 0.4 ~ 0.5% per generation, a rate consistent with known mutation rates. In the populations treated with a large dose of x-rays (7000r for males, 1000r for females), the lethal chromosomes dropped from an origingl fequency of 18% to 10%.4n four generations and subsequently have increased in frequensy at a rate comparable to that of the control, The initial decrease is that expected from the elimination of translocations which were coincident with recessive lethals. It seems, therefore, that induced recessive lethals which are not associated with translosations have remained in the pepulation and are included in the concealed variability of the population, whereas those induced lethals which were associated with translocations were eliminated rapidly as a result of the concomitant reduction in fertility associated with thetranslocations, To determine the frequency of allelisa of recessivelethals, a primary source of elimination of these lethals from any population, series of cross tests were made between lethals (induced by gawea s-rays) af independent origin, The mmber of lec! calculated from our results is 400 with possible values ranging fran 23% te 718 (99% confidence interval). This number does not differ significantiy from that determined by Ives from atudying lethals of spontaneous origin. The magnitude of this number makes it highly improbable that any appreciable elimination of recessive isthals has ceccurred through homozygosity. Analyses of the viabilities of those files hososygous for non-lethal, non-eemi-lethal chromosomes (i.e., the bulk of the tested chromosomes which seex to be "normal" at first glance) reveal that their average viability is relatively constant but varianee of their distribution, analyses indicate an increase in the This implies that a coumteraction of mutation of deleterious genes (sub~vitals) and the selection for advantageous genes (gene combinations) have, to date maintained the constant average viability. Since, however, the detailed analyses of this problem have just begun, it is best that this conclusion be taken with some reserve. It seems highly probable that this mechanism is operative in the contrel population but it ia too early to discuss the experimental populations. Gamma Radiation. A conference was held at Oak Ridge June 6 to consider a project originating with the Toxicity Laboratory of the University of Chicago on the effects of gamma radiation on mamzals. The project sponsored jointly by the Division ef Biology and Medicine and the Division of Military Application was originally planned to bs carried out at Dugway. Since burros are adaptable to the enviroment at Dugway and are approxinately the size of man, they will be used as the experimental animal, The animals will beirrediated with tantalum. Jtudles will be made on the effect of radiation on sperm physlotogy, work capacity and blood sonstituents. CoORnte \ ey ero 16--56741-1 OFFICE > | &.” MEP => | DOR ARCHIVES —