Abstract
We have been studying the genetic effects of X-ray irradiation on the silkworm, Bombyx mori, since 1927. In the present paper is described one of the obtained results.
The larval marking genes S(=PS, striped), P (normal), and p (plain) are multiple allelomorphs situating on the same locus of the second chromosome, and are linked with the blood colour gene Y (yellow) and y (white). The standard crossing over value between S and Y in the heterozygous male is known to be 25.6%.
Several male moths from a Chinese SY strain were treated with X-rays, and then mated by Py females from a Japanese race. In F1 we got 15 mosaics among 2907 individuals, and one of the mosaic females was mated to a Py male of the same strain as used in the P-generation crossing. The offspring of this mating continued to show very peculiar modes of segregation: 1) Complete repulsion of P and Y in both sexes instead of partial linkage in the male side in the normal occasion; 2) when Y and y segregate in the present strain, Y always shows deficit to the expectation.
The peculiarities mentioned above may be accounted for as follows. In the F1 S-P mosaic, S mutated to p in a number of somatic as well as germinal cells which gave rise to pYPy offspring, hence repulsion between P and Y! The mutated pY chromosome is represented as pY. No crossing over occurs, in either sex, between pY and Py chromosomes, perhaps in virtue of inversion which supposedly took place in pY chromosome. The pY chromosome behaves as a partial lethal which chiefly acts in the postembryonal life, thus the shortage of Y individuals is resulted, the counts being made only in full-grown larvae. The lethal action of pY is of course strongest in the homozygote, but it does not lose the lethal action even in the heterozygous state, i.e. it behaves as a partial dominant lethal.
