Prader-Willi syndrome occurs when several genes on chromosome 15 are deleted or not expressed. This happens by chance and is not passed down in families. Why it happens is unknown. This genetic change affects the part of your brain (the hypothalamus) that regulates hormones and appetite, as well as the development of muscle tone and expression of emotions.
Video: Prader Willi, Understanding and Managing Behaviour part 2
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(Prader-Willi Syndrome Association NZ, 2015)