I used to be a cytogenetic technologist and I can tell you that it is not necessarily true that this will be seen by karyotype. This type of disorder can be inherited if the baby inherited two copies of the mother's chromosome 15 or two copies of the father's chromosome 15. Depending on which copy is missing it will determine which syndrome she has. I am not sure if that made sense. You can look up uniparental disomy - that may explain it better.
I am sure the genetic counselor will tell you it is important to find out because although the deletion is on the same chromosome in the same area, they are two very different anamolies. If you have any questions feel free to FM.
Best of luck to you and your family. You are in my prayers...
