Geneticists will explain that there are two ways WHS comes about. The first is that the genetic problem is spontaneous and has no reflection from the parents genes. This is the most common reason.
The second is if a parent has a translocation of some genetic material but part of their fourth chromosome is attached to another chromasome. That is that they have all the genetic material required and have no symptoms of WHS themselves, but when hey have a child, the genes miss out the translocated gene meaning the child has 4p deletion and will have WHS.