(See also Overview of Chromosome Disorders.)
About 70% of people who have Prader-Willi syndrome are missing part of chromosome 15. About 30% of people who have this syndrome have problems with the function of chromosome 15.
Many symptoms of Prader-Willi syndrome vary according to the child's age. Newborns with the defect feel limp, feed poorly, and gain weight slowly. Eventually these symptoms resolve. Then, between the ages of 1 and 6, appetite increases, often becoming insatiable. Children rapidly gain weight. The hands and feet remain small, and children remain short. Abnormalities of the face include almond-shaped eyes and a mouth with thin upper lips and down-turned corners. Children have bone disorders (such as scoliosis and kyphosis). Obsessive-compulsive behaviors are common. Hormonal problems are common, and the function of the reproductive organs is abnormally decreased, which limits growth and sexual development. Boys have undescended testes (cryptorchidism) and an underdeveloped penis and scrotum. Intellectual disability is common. Weight gain continues into adulthood and is excessive, which can lead to other health problems, such as obesity. Obesity can be severe enough to justify gastric bypass surgery.