This has been well described in Drosophila with respect to eye color and is known as position effect variegation (PEV).
[1] The phenotype is well characterised by unstable expression of a gene that results in the red eye coloration.
These phenotypes are often due to a chromosomal translocation such that the color gene is now close to a region of heterochromatin.
Regions of heterochromatin can spread and influence transcription, which may result in the cessation of gene expression and subsequently, white eye sectors.
Position effect is also used to describe the variation of expression exhibited by identical transgenes that insert into different regions of a genome.