Maternal Uniparental Disomy (UPD) is seen in about 24-26% of PWS cases. [Driscoll 1994, Smith 2003] It occurs when both chromosome 15s are of maternal origin, there is no paternal contribution.

In the majority of cases this is thought to occur as a result of nondisjunction during meiosis I which produces a trisomy 15. Selective retention of only two chromosomes prevents the development of a trisomy however if by chance the two maternal chromosome are selected then Maternal UPD is the result. [Cassidy 1992]

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