Defining a human in terms of DNA is very reasonable, up to a point, and in certain situations. When we debate the question of when human life begins, we're arguing about a continuous proess, for the most part, and so anything that looks like an identifiable milestone, like DNA combination or fertilization, is greeted with relief as the sword that promises to cut the Gordian Knot. But I don't think that the event of DNA recombination is the proper one.

Focusing on DNA in general be a red herring. There is no period of time in which an organism that will someday be unequivocably recognized as human possesses any other kind of DNA; for that matter, there's never a time when human DNA is not involved. To get really picky, I can point out that recombination isn't the creation of brand-new DNA, it's the joining of two lines of DNA that already existed; and, let us not forget, recombination also involves the "death" of half the contributed DNA.

Who mourns for the discarded DNA? It too defines a human being, but one that will now never be born. Is conception therefore also abortion?

Hmmm...the lesson here, I think, is that I think too much.