Definition of Prader-Willi syndrome:

Prader-Willi syndrome is disease that is present from birth (congenital). People with this condition are obese, have reduced muscle tone and mental ability, and have 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 rest of patients with this condition often 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.