The symptoms of Prader-Willi syndrome are most likely due to the dysfunction of a portion of the brain called the hypothalamus.
PWS is caused by a lack of active genetic material in a particular region of chromosome 15 (15q11-q13). Normally, individuals get one chromosome 15 from the mother and one from the father. The genes in the 15q11-q13 region only function when they come from the father. In PWS, a genetic defect causes the inactivity of chromosome 15 from the father (paternal chromosome 15).
These genetic problems almost always (98%) occur sporadically: this means that they are not present in one of the parents. The chance that this will happen again in the next child is therefore very low, except in specific cases. It is therefore wise to discuss this risk of recurrence with a clinical geneticist.