Fig. 3. Dose-dependent effect of Z4 on wm4 position-effect variegation. (A) Representative eye phenotypes of males derived from a cross between homozygous wm4 females and males heterozygous for the Z4 mutants balanced over TM3, Sb. Compared with the males with the TM3 chromosomes, the eyes of sibling males mutant for Z4 have increased numbers of red ommatidia. (B) Quantitative measurement of eye pigments of the phenotypes shown in (A). Eye pigments were extracted from the heads of 20 representative individual males for each cross and their absorbance at 480 nm was determined. The columns show the ratios of absorbances between the Z4 mutants and their TM3 siblings derived from the identical cross. The control shows the ratios obtained from w^m4/Y;e,st,spo/+ and w^m4/Y;TM3,Sb/+ siblings. (C) Eye phenotypes of males from a cross between w^m4/w^m4 females and w¹¹¹⁸/Y;P[Z4myc]/CyO males. Males with three copies of the Z4 gene have significantly reduced numbers of red ommatidia compared with their sibling males with two doses of Z4.