How many loci on the X-chromosome of Drosophila melanogaster can mutate to recessive lethals?

The sensitivity of the sex-linked recessive lethal test is due to the fact that a very large number of loci are included i the mutation study. From extensive studies on the spontaneous sex-linked recessive lethal frequency and spontaneous specific locus mutation rates, it is possible to derive an estimate of the number of loci included in the recessive lethal test. The average number derived from three estimates on male and female germ cells is 563 loci. A second independent approach derives from published data which analyzed short regions of he genome and the proportion of loci within these regions which mutate to lethality. This analysis suggests that 830 loci are potentially lethal mutables. We describe the reasons for concluding that 600 to 800 loci of the approximately 1,000 loci on the X-chromosome are involved in the X-linked recessive lethal test.