Question: Two disorders, Angelman syndrome and Prader-Willi syndrome are caused by the inheritance of the same genetic information, but they have widely differing symptoms why is this?

Tristan Smith answered on 9 Mar 2015:

Ok so these two diseases are actually caused by deletions of different genes, but from the same region of the same chromosome. Everyone contains two copies of every chromosome, one from each parent. However, we don’t need two copies of every gene all the time, so some cells, particularly during development, turn different parts of some chromosomes off so that they only use one copy, either the maternal or paternal one.
This is called imprinting, AS and PWS are one of the few cases of human imprinting diseases. So these diseases involve one region of chromosome 15 that is heavily imprinted. One of the genes there is called UBE3A, and it is imprinted so that only the mother’s copy is used. There are other genes in the same place, including one called SNORD116, and only the father’s copy is used.
However, this region of the chromosome can be deleted. If both the mother and father’s chromosome are damaged, then it would never get to the embryo stage. However, if the mother’s chromosome is damaged, then UBE3A is lost, the other copy is silent and so the baby has Angelman syndrome, and vice versa for SNORD116 and Prader-Willi syndrome.
So even though both diseases involve the same region of the chromosome, different diseases occur because of which genes are ON or OFF, depending on whether the chromosome came form the mother or father.