The relation between recessive lethals, dominant lethals, and chromosome aberrations inDrosophila

Abstract

The suggestion is put forward that radiation-induced recessive lethals, or a large proportion of them, are due to chromosome breaks. About one-third of all the chromosome breaks primarily induced by the radiation are lethals. If the break restitutes, a lethal unaccompanied by chromosomal aberration (type A lethal) results. If the break takes part in chromosome interchange a type C lethal, which is associated with chromosomal structural change, results. Arguments are given against the alternative position-effect explanation of type C lethals. A quantitative theory of dominant lethals is developed on the basis that the dominant lethals are a mixture of single breaks which fail either to restitute or to interchange but instead undergo sister-union, and of non-viable chromosomal structural changes involving two or more breaks. The experimental curve of variation with dose of the yield of dominant lethals is successfully fitted, and also the curve of the yield of viable structural changes. It is shown that the recessive lethal and the dominant lethal theories are consistent in that they require the same postulated number of primarily produced chromosome breaks per unit dose (namely, 0·75 breaks per sperm per 1000 r.). Experiments on the distortion of the sex ratio in the progeny of irradiated males with ring-shaped or rod-shapedX-chromosomes are also shown to be consistent with the theory.