Prader-Willi syndrome is a disease that is present from birth (congenital). It affects many parts of the body. People with this condition feel hungry all the time and become obese. They also have poor muscle tone, reduced mental ability, and underdeveloped sex organs.
Prader-Willi syndrome is caused by a missing gene on chromosome 15. Normally, parents each pass down a copy of this chromosome. The defect can occur in a couple of ways:
- The father's genes are missing on chromosome 15
- There are defects or problems with the father’s genes on chromosome 15
- There are two copies of the mother's chromosome 15 and none from the father
These genetic changes occur randomly. Persons who have this syndrome usually do not have a family history of the condition.