It is known that in Angelman syndrome, the chromosome 15 undergoes deletion in the maternal chromosome while in Prader-Willi it's in the paternal.
I understand it can be detected by FISH or other chromosomal staining methods (which would have been used those times), the chromosome number which underwent the deletion.
How could they figure out which chromosome (maternal or paternal) had undergone the change. My question is, which technique allows the chromosome to be marked maternal or paternal? Which technique was originally used?