Fact: 75% are due to a deletion in the long arm of the chromosome 15.
Input: This question tricked me up because when I saw a deletion in the chr15 as an option I though "I got it" but actually I did not because it is not in the short arm is in the long arm.
Prader Willi = Paternal deletion (partial or full). Noted for imprinting. Cousin to Angelmann (opposite deletion).
25% of cases due to maternal uniparental disomy (two maternally imprinted genes are received; no paternal gene received).