Comparison between rDNA magnification and bb lethal mutation frequencies in Drosophila melanogaster

Summary Unequal mitotic sister strand crossing over has been evoked to explain the occurrence of phenotypically bb^- males in the progeny of phenotypically bobbed males during magnification. If this is the case, complementary bb¹ loci should be obtained together with the bb¹. To test this hypothesis we compared the frequency of bb lethal mutations in the sperms of bb males with the percentage of phenotypically bb⁺ males obtained during magnification of these bb males. We then compared these values with those occurring in phenotypically bb⁺ control males. We found that, while the number of bb⁺ males obtained during magnification, though variable, is high, the bb lethal mutation occurs at a very low frequency in all the genetic conditions, whatever the phenotype of the parental male.