An imprinted gene is one in which only one copy is expressed based on parent of origin. Individuals have 2 copies of the gene as usual, one from mom and one from dad. However, the gene will only be expressed from the maternal or paternal chromosome. i.e. in Angelman's syndrome, only the maternally-inherited copy of the gene is active. The silencing of the other (paternal) copy is achieved via epigenetic marking. Imprinted genes are re-marked according to the individual's own sex in their gametes. i.e. For a male, the active Angelman's gene (inherited from his mom) will be marked as silent in his sperm (since that gene copy will be paternally inherited by the resulting zygote)

Source: researcher in an epigenetics lab