An Analysis of the Effects of the Different Rays of Radium in Producing Lethal Mutations in Drosophila

Abstract

Exposure of D. melanogaster to radium rays produces both viable and lethal gene mutations. Under heavy exposure, the lethals are so numerous as to lend themselves to exact quantitative work. Flies exposed to 150 mgm. of radium for 9 hrs. show 13.6% of lethal mutations. Interposing a series of Pb filters between the Ra and the flies gives a decreasing mutation rate dependent on the increasing thickness of the filters and a mutation curve can be constructed showing this relationship. The thickest Pb screen was 8 mm. thick and the thickness was halved each time until the thinnest screen was 1/8 mm. thick. When Ra rays are passed through the same series of filters into an ionization chamber, the air is ionized and the connecting electroscope is discharged. The ionization value for each filter was computed and these data used to plot an ionization curve, which if made arbitrarily to fit the mutation curve at one point also fits it with a remarkable exactitude at the other 7 points. Since all the a particles were held back by the silver in the Ra needles and the [gamma] rays do not ionize a gas, except indirectly through excited [beta] radiation, [alpha] and [gamma] rays seem to be excluded as agents of gene mutations. Ionization is due to the [beta] radiation and since the ionization curve can be superimposed upon the mutation curve the evidence is all but complete that the [beta] particle is the effective agent in gene change. These results apply to x-rays as well as radium.