A human zygote is a person, and so is everything that follows. From the moment of fertilization, there is a single developmental progression of a single organism. There isn't any other meaningful scientific or logical distinction that can be made. Before fertilization, you have the gametes from two separate organisms. Afterwards, there is a new, unique organism (a human being) who has a separate identify.

And yes, that means I think common in-vitro fertilization practices (including making multiple zygotes and throwing some out) are morally wrong and equivalent to abortion.

If anyone disagrees, I contend it's because of the moral implications, not any lack of soundness in the scientific or logical arguments.