<P> The first imprinted genetic disorders to be described in humans were the reciprocally inherited Prader - Willi syndrome and Angelman syndrome . Both syndromes are associated with loss of the chromosomal region 15q11 - 13 (band 11 of the long arm of chromosome 15). This region contains the paternally expressed genes SNRPN and NDN and the maternally expressed gene UBE3A . </P> <Ul> <Li> Paternal inheritance of a deletion of this region is associated with Prader - Willi syndrome (characterised by hypotonia, obesity, and hypogonadism). </Li> <Li> Maternal inheritance of the same deletion is associated with Angelman syndrome (characterised by epilepsy, tremors, and a perpetually smiling facial expression). </Li> </Ul> <Li> Paternal inheritance of a deletion of this region is associated with Prader - Willi syndrome (characterised by hypotonia, obesity, and hypogonadism). </Li> <Li> Maternal inheritance of the same deletion is associated with Angelman syndrome (characterised by epilepsy, tremors, and a perpetually smiling facial expression). </Li>

The process of imprinting involves the formation of a