Abstract
In a previous study (Minamori and Ito 1971) the extrachromosomal element [delta b] was shown to induce recessive lethal, semilethal and visible mutations on 2nd chromosomes. In this study the mutagenic action of this delta to X chromosomes as examined. The frequency of recessive lethals induced was 0.08%, being four times the spontaneous mutation rate (0.02%). The difference, however, was statistically insignificant. Accordingly, the conclusion may be that [delta b] has no effective mutagenicity for X chromosomes. The differential behaviors of X and 2nd chromosome to mutagenicity of [delta b] were discussed from the aspect of natural selection.
