The loss of genes in a specific region of chromosome 15.
PWS is caused by a lack of active genetic material in a particular region of chromosome 15 (15q11-q13). Normally, individuals inherit one copy of chromosome 15 from their mother and one from their father. The genes in the PWS region are normally only active on the chromosome that came from the father.  Knowledge center. Prader-Willi syndrome, also known as PWS is an uncommon genetic disorder (present at birth) in which seven genes (or subsets) on chromosome 15 are deleted or unexpressed. Patients with PWS may have physical, mental and behavioral problems-the main one being an unrelenting feeling of hunger. There is no cure for Prader-Willi syndrome, however, several treatments are available which help to lessen the signs and symptoms of the condition, including infant/child nutrition, growth hormone therapy, sex hormone therapy, physical therapy, speech therapy, occupational therapy, and developmental therapy. Causes of Prader-Willi syndrome. Prader-Willi syndrome (PWS) is caused by a genetic defect on chromosome number 15. Genes contain the instructions for making a human being. They're made up of DNA and packaged onto strands called chromosomes. A person has two copies of all their genes, which means chromosomes come in pairs.  Prader-Willi syndrome is caused by the loss of function of genes in a particular region of chromosome 15. People normally inherit one copy of this chromosome from each parent. Most cases of Prader-Willi syndrome are not inherited, particularly those caused by a deletion in the paternal chromosome 15 or by maternal uniparental disomy. Prader-Willi syndrome is caused by the loss of genes in a specific region of chromosome 15. People normally inherit one copy of this chromosome from each parent. Some genes are turned on (active) only on the copy that is inherited from a person's father (the paternal copy). This phenomenon is called maternal uniparental disomy. Rarely, Prader-Willi syndrome can also be caused by a chromosomal rearrangement called a translocation, or by a mutation or other defect that abnormally turns off (inactivates) genes on the paternal What is Prader-Willi Syndrome. Prader-Willi Syndrome (PWS) is a genetic disorder that occurs in approximately one out of every 15,000 births. PWS affects males and females with equal frequency and affects all races and ethnicities. PWS is recognized as a common genetic cause of childhood obesity.  Causes. 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. 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. Prader-Willi syndrome is a complex genetic condition that affects many parts of the body. In infancy, this condition is characterized by weak muscle tone (hypotonia), feeding difficulties, poor growth, and delayed development. Most cases of Prader-Willi syndrome are not inherited, particularly those caused by a deletion in the paternal chromosome 15 or by maternal uniparental disomy. Back to Top Causes. 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 persons with Prader-Willi syndrome are missing the genetic material on part of the father's chromosome. 
https://www.fpwr.org/about-prader-willi-syndrome http://www.medicalnewstoday.com/articles/182287.php http://www.nhs.uk/Conditions/prader-willi-syndrome/Pages/Causes.aspx http://ghr.nlm.nih.gov/condition/prader-willi-syndrome http://www.medicinenet.com/prader-willi_syndrome/page2.htm http://www.fpwr.ca/about-prader-willi-syndrome/ http://www.mayoclinic.org/diseases-conditions/prader-willi-syndrome/basics/causes/CON-20028982 http://ghr.nlm.nih.gov/condition/prader-willi-syndrome http://www.nytimes.com/health/guides/disease/prader-willi-syndrome/overview.html
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