Need a simple explaination for this. what is prader-willi syndrome?

PRADER WILLI SYNDROM: A genetic disease with a deletion at the long arm of chromosome#15. Signs: lack of muscle tone at birth (hypotonia), obesity, short stature, small hands and feet, hypogonadism, small testicles, mental retardation.

70% of cases of P-W: Occur when the paternal chromosome 15 genes q11-q13 are missing. In 25%, the entire paternal chromosome 15 is missing ; both come from mother. Rarely, in <5%, father's genes are present but their activity has been turned off by faulty "imprinting", which can be either accidental ; not hereditary or from the genes he inherited from his mother, with a 50% chance of p-w for each child he has.