At birth, babies with Prader-Willi syndrome are usually small and floppy, with low muscle tone, and have problems feeding. They may have small hands and feet, and boys may have undescended testicles. The babies are slow to start walking and poor motor skills may persist into adult life.
After about six months to a year, especially as the child becomes mobile, they develop an interest in food that may become an insatiable obsession. Weight gain can be rapid, leading to severe obesity that results in diabetes, strain on the heart, lungs and skeleton, and even early death.
Other symptoms can include:
Learning difficulties
Short stature
Underdeveloped genital organs
Behavioural problems
People with Prader-Willi syndrome often have obsessive-compulsive behaviour - they may hoard possessions and show repetitive habits such as picking at skin or hair. They are unsettled by changes in routine and can show unpredictable rages and aggression. Some individuals are more severely affected than others.
Some people have also noticed that children with Prader-Willi syndrome may be unusually blonde and blue-eyed.