Abstract
Male flies of Drosophila auraria have about 14 bristles on the 6th sternite, while those of D. biauraria have generally none. Backcross experiments were made by using mutant markers of D. auraria chromosome. It was found that both the X and A-chromosomes were obviously responsible for the bristle manifestation. A chromosome assay was made by using a mutant strain of D. auraria being homozygous for y, cn and cu. The A-chromosome had a major effect which manifested 8.6 bristles on an average and the X had a moderate one being conformable to 4.1. The effect of the B-chromosome was a little, corresponding to 1.1 at the most. Interactions between the effects of chromosomes were negligibly small in all combinations.
