Prader–Willi syndrome (PWS) is a genetic disorder due to loss of function of specific genes. In newborns, symptoms include weak muscles, poor feeding, and slow development. Beginning in childhood, the person becomes constantly hungry, which often leads to obesity and type 2 diabetes.[3] Also, mild to moderate intellectual impairment and behavioral problems are typical.[3] Often, those affected have a narrow forehead, small hands and feet, short height, light skin and hair, and are unable to have children.
About 74% of cases occur when part of the father's chromosome 15 is deleted. In another 25% of cases, the person has two copies of chromosome 15 from their mother and none from their father. As parts of the chromosome from the mother are turned off, they end up with no working copies of certain genes. PWS is not generally inherited, but instead the genetic changes happen during the formation of the egg, sperm, or in early development. No risk factors are known. Those who have one child with PWS have less than a 1% chance of the next child being affected. A similar mechanism occurs in Angelman syndrome, except the defective chromosome 15 is from the mother or two copies are from the father.