Figure 1.

Variegated expression of a gene on packaging into a heterochromatic structure. (A) Cells expressing the wild-type ADE2 gene from its endogenous, euchromatic locus produce colonies that are white, (B) whereas those lacking the ADE2 gene appear red. (C) Juxtaposition of ADE2 to heterochromatin results in its silencing without changing the underlying coding sequence. Although inherited, the packaging state of ADE2 (euchromatic versus heterochromatic) can switch at a low frequency. This results in a variegating phenotype in a clonal population of cells. An example of a telomeric position effect (TPE) (Gottschling et al, 1990) in S. cerevisiae is shown here.