genomic imprinting
For most genes you inherit two working copies, one from each parent, and either can do the job. For a small set of genes, though, the cell remembers which parent a copy came from and uses only the maternal or only the paternal version. Genomic imprinting is this parent-of-origin labeling of genes.
The label is not a change in DNA sequence but an epigenetic mark, chiefly DNA methylation, placed during the formation of eggs and sperm. Because the mark is reset and re-applied each generation according to the parent's sex, the same gene can be silenced when inherited from the mother in one person and from the father in another.
Imprinting means that, for these genes, the two parental copies are not interchangeable. Losing or duplicating the active copy can cause disease even when the other copy is structurally normal, which is why some disorders depend on whether a deletion was inherited from the mother or the father.
Only a few hundred human genes are thought to be imprinted, and many cluster together under shared control regions. Why imprinting evolved at all is still debated, with the leading idea involving a tug-of-war between parental interests over resource allocation to offspring.
Prader-Willi and Angelman syndromes can arise from the same region of chromosome 15, with the outcome depending on whether the affected copy came from the father or the mother.
One region, two disorders, set by parent of origin.
Imprinting can break the simple expectations of Mendelian inheritance, because a trait may depend not just on which allele you carry but on which parent it came from.