Genomic imprinting is an example of epigenetic inheritance that results in the maternal and paternal copies of certain genes being expressed differently.

Prader-Willi syndrome occurs when the paternal copy is partly or entirely missing.

The genes in this region are normally active on the paternal copy of the chromosome and are inactive on the maternal copy.

Due to imprinting, the maternally inherited copies of these genes are virtually silent, only the paternal copies of the genes are expressed.

PWS results from the loss of paternal copies of this region.

This locus is maternally imprinted, meaning that only the paternal copy of the locus is transcribed.

Some genes are found to be expressed from both maternal genomes while others are expressed exclusively from the lone paternal copy.

In normal mice, therefore, the paternal copy of the Mest gene presumably promotes these behaviors.

The copy from the mother's side is inactive and cannot remedy the lack of the paternal copy.

Usually, the paternal copy of H19 is methylated and silent while the maternal copy is hypomethylated or unmethylated and expressed in the offspring cell.