Indirect effects of X-rays on chromosomes.

Abstract

Not long after Muller's discovery of the mutagenic effect of X rays the so-called “target theory” of the mode of action of the rays was put forward with the co-operation of several investigators. According to this theory, which was generally accepted, mutations induced by irradiations are the direct effect of ionizations in the gene molecules. Different criteria have repeatedly been shown to be fulfilled and amongst them the most commonly established one is the linear dependence of mutation frequency upon dosage, measured in röntgen units. Some years ago I studied this question together with my co-worker Dr. Lüning (Bonnier and Lüning, 1949). We irradiated wild-type Drosophila males using four different doses. The males were then mated to females homozygous for a few sexlinked recessives, and the mutation rate was determined by the occurrence of recessive daughters in the progeny. These experiments were made on a large scale, the total number of daughters inspected being over one million. We found an excellent fit to the straight line, as expected on the target theory. However, when extrapolating the straight line to the zero dose, the frequency was higher than that observed to occur spontaneously and the statistical analysis showed that it was very unlikely that the difference was due to chance alone. Consequently, it must be assumed that the relation between mutation rate and dose is curvilinear for small doses. To test this adequately, closer studies ought to be made over a range of small doses.