Who are your mother and father?

Considering the development of embryo technology, who does Buddhist Dharma consider as your mother and father: is it the biological parents, or the legal parents? The reason I ask this question is that killing your mother and father is a heinous kamma. One of the monks said that heinous kamma is applicable only for biological mother and the father (i.e. donors of the sperm and the egg). Is this so, and why?