Definition of Prader-Willi syndrome:

Prader-Willi syndrome is a congenital (present from birth) disease that involves obesity, decreased muscle tone, decreased mental capacity, and sex glands that produce little or no hormones.

Causes, incidence, and risk factors:

Prader-Willi syndrome is caused by a gene missing on part of chromosome 15. Normally, your parents each pass down a copy of this chromosome. Most patients with Prader-Willi syndrome are missing the genetic material on part of the father's chromosome. The remaining patients frequently have two copies of the mother's chromosome 15.

The genetic changes occur randomly. Patients usually do not have a family history of the condition.