Prader-Willi syndrome (PWS) is a genetic disorder that causes low muscle tone (floppiness), short stature, cognitive disabilities (mental retardation), behavioral problems, and chronic hunger that can lead to over-eating and life threatening obesity. The disorder is usually caused by an error during sperm and egg formation, not passed on from parent to child like most genetic diseases. This makes the disease nearly impossible to detect in the parents (only 5% of the time) (Cassidy). The disorder is considered "rare," but occurs in 1 out of every 14,000 people, making it the most common genetic disorders, and the most common genetic cause of obesity.
The disorder is linked with the 15th chromosome, in a small section labeled "15q11-q13". Scientists do not know exactly how many genes and which specific ones are involved, but they do know the cause of the abnormalities.
70% of all cases of PWS are caused by "parental deletion".
This makes it the most common form of the disorder. The cause is; part of chromosome 15 inherited from the father (the part containing the PWS critical genes) is missing. Typical deletions are now classified as Type 1 or Type 2, depending on the size. Deletion happens for seemingly no reason, and has a less than 1% chance of recurring).
25% of cases are cause by Maternal uniparental disomy. This less common form of PWS happens when both copies of chromosome 15 are inherited from the mother (hence, Uniparental). In these cases, the developing embryo starts out with three copies of chromosome 15, because the mothers egg had an extra one. Later, one of the three is lost (the fathers), and has the same effect as deletion. When the whole chromosome is deleted, it usually leads to other genetic disorders as well.
5% of cases are caused...