In some fruit flies, an inversion involving the white-eyed locus (w) moves the w+gene from one end of the X chromosome to a location nearer to the centromere. Flies with a w+ or w+/w genotype are normally expected to have red eyes

Question

However, in flies with the inversion, the eyes are mottled red and white. Explain why.

Answer 1

The mottled phenotype is caused by a position effect. During the inversion, the w+ gene is moved from a euchromatic region on the X to a region near the heterochromatin at the centromere. Sometimes genes in this region are not transcribed because the heterochromatin remains condensed throughout the cell cycle. The eye cells in which the w+ gene has become inactivated produce white spots in the eyes. The cells in which the gene is not inactivated produce red spots.