Maternal Disomy or UPD - 25%
In approximately 25 percent of subjects with Prader-Willi syndrome, both
chromosome 15s are inherited from the mother (maternal disomy or UPD)
and no chromosome 15 is present from the father. The missing
chromosome 15 from the father which contains the active or paternally expressed
genes required for normal development causes Prader-Willi syndrome in individuals
in which both chromosome 15s are inherited from the mother.
Genetic testing is required to identify maternal disomy 15 or UPD using
DNA obtained from the parents and the person with Prader-Willi syndrome.
Polymorphic DNA markers from chromosome 15 are studied
to identify whether both chromosome 15s are from the mother.