Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes Prader-Willi syndrome ?
    ### Response:
    What causes Prader-Willi syndrome? Prader-Willi syndrome (PWS) is caused by the loss of active genes in a specific region of chromosome 15. People normally inherit one copy of chromosome 15 from each parent. Some genes on chromosome 15 are only active (or "expressed") on the copy that is inherited from a person's father (the paternal copy).
    