Position-effect variegation in Drosophila . The stochastic expansion of heterochromatin proteins in each cell can cause a variegated eye color phenotype in Drosophila. At the top of each figure is a representation of DNA within a cell, the middle is a representation of the location of heterochromatic proteins within a locus, and at the bottom is the observed eye color phenotype. (A) A fly whose cells contain the white gene located exclusively in heterochromatin, and thus inaccessible for transcription, will have white eyes devoid of red pigment. (B) A combination of cells with the white gene located in heterochromatin and cells with the white gene located in euchromatin, and thus available for transcription, will have a mottled phenotype with some cells producing red pigment and some cells producing no pigment. (C) A fly whose cells contain the white gene located exclusively in euchromatin will have fully pigmented red eyes.