Prader-Willi Syndrome is a Chromosomal Abnormality of the structural sort. In the particular case of Prader Willi Syndrome (PWS) the q 11-13 region of chromosome 15 is deleted or unexpressed. As a result, functioning of a brain structure called the hypothalamus is compromised. PWS affects one in every 12,000-15,000 people and occurs equally among both sexes and across races. The genes associated with PWS normally are expressed only from a region of chromosome 15 inherited from the father, the genes from the mother are inactivated. If the material donated from the father is compromised via deletion or inactivation then the individual experiences the effects demonstrated in the picture at left.
Individuals with PWS have incomplete sexual development and are infertile as adults. Hypothalamic dysfunction is thought to be the basis for many of the phenotypic features, such as short stature and hypogonadism. Hypogonadism presents as undescended testicles, small testes, and decreased scrotal rugae in males and small labia minora and clitoris in females. Puberty typically is delayed or incomplete and adults are infertile.