iGenetics 3rd Edition by Peter Russell

Exercise 9

The brick red eye color of normal Drosophila results from pigment deposition controlled by the white gene, which lies on the X chromosome at map position 1.5, far from centromeric heterochromatin (which starts at about map position 66). Hermann Muller screened for new white mutants by irradiating wild-type Drosophila males ( w + /Y) and mating them to white-eyed ( w / w ) females. He isolated several mutant females bearing mottled red eyes-red eyes with varying amounts of white spotting. One mutant, w M5 , was associated with a reciprocal translocation with breakpoints near the white locus and centromeric heterochromatin of chromosome 4. A different mutant, w M4 , was associated with an X-chromosome inversion with breakpoints near the white locus and centromeric heterochromatin. Kenneth Tartof screened for revertants of the mottled eye phenotype of w M4 by crossing irradiated w M4 /Y males with w/w females. He recovered three different normal-eyed female revertants, each associated with a new X-chromosome inversion. In addition to having the original w M4 breakpoints near the white locus and in centromeric heterochromatin, each had a third euchromatic breakpoint.
a. Based on these results, is the mottled eye phenotype in Muller's mutants due to a mutation within the white gene? If not, what is its most likely cause?
b. How can the w M4 mutation be reverted by an additional inversion with a euchromatic breakpoint?