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?
I suppose you can separate mother and father into those of nature and those of nurture. Whoever provides the genetic code becomes the natural parent, whoever raises the kid becomes the nurturing parent. I wouldn't place much value on the code though - the seriousness of the act of killing a parent is probably more tied to the fact that they have devoted a large fraction of their lives to the rasing of a child.