Position effect variegation

Position-effect variegation is a variegation caused by the inactivation of a gene in some cells through its abnormal juxtaposition with heterochromatin.

The classical example is the Drosophila w[m4](speak white-mottled-4) translocation. In this mutation, an inversion on the X chromosome places the white gene next to pericentric heterochromatin. Normally, the white gene is expressed in every cell of the adult Drosophila eye resulting in a red eye phenotype. In the w[m4] mutant the eye color is variegated (red-white mosaic colored) where the white gene is expressed in some cells in the eyes and not in others. The mutation was described first by Hermann Muller in 1930.