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. Also, mild to moderate intellectual impairment and behavioral problems are typical. Often, the forehead is narrow, hands and feet are small, height is short, skin is light in color, and most of the affected 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.Prader–Willi syndrome has no cure. Treatment, however, may improve outcomes, especially if carried out early. In newborns, feeding difficulties may be supported with feeding tubes. Strict food supervision is typically required starting around the age of three in combination with an exercise program. Growth hormone therapy also improves outcomes. Counseling and medications may help with some behavioral problems. Group homes are often necessary in adulthood.PWS affects between one in 10,000 and one in 30,000 people. Males and females are affected equally. The condition is named after Andrea Prader, Heinrich Willi, and Alexis Labhart, who described it in detail in 1956. An earlier description was made in 1887 by John Langdon Down.