Prader-Willi syndrome is a genetic disorder, a condition caused by an error in one or more genes. Although the exact genes responsible for Prader-Willi syndrome haven't been identified, the problem lies in a particular region of chromosome 15.

With the exception of genes related to sex characteristics, all genes come in pairs — one copy inherited from your father (paternal gene) and one copy inherited from your mother (maternal gene). For most types of genes, if one copy is "active," or expressed, then the other copy also is expressed.

However, some types of genes act alone. For example, with certain genes it's normal for the paternal gene to be expressed and the maternal gene to be "silent," or repressed.

What happens in Prader-Willi syndrome

Prader-Willi syndrome occurs because certain paternal genes that should be expressed are not for one of these reasons:

  • Paternal genes on chromosome 15 are missing.
  • The child inherited two copies of chromosome 15 from the mother and no chromosome 15 from the father.
  • There's some error or defect in paternal genes on chromosome 15.

Prader-Willi syndrome disrupts the normal functions of a portion of the brain called the hypothalamus. The hypothalamus controls hunger and thirst and releases hormones that prompt the release of other substances responsible for growth and sexual development. A hypothalamus that isn't functioning properly — caused by the defect on chromosome 15 — interferes with each of these processes, resulting in uncontrollable hunger, stunted growth, sexual underdevelopment and other characteristics of Prader-Willi syndrome.

April 17, 2014