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2) Fertilized eggs don't hold to a rule 1 egg = 1 person, a rule that seems implicit with saying that life begins at conception. But rather, eggs can end up resulting in multiple people as noted with identical twins, or two eggs can end up becoming one person, such as with chimeras, which do occur in rare circumstances with humans.
http://en.wikipedia.org/wiki/Chimera_(genetics) Either of these variations ultimately bring question to the basic rule, as we have one human being split, which human beings cannot do, or we have two human beings becoming one human being, which is also similarly impossible for humans. Both circumstances bring into heavy question the basic rule used in the idea that life begins at conception.
That seems a pretty good point, is a new being created just right after conception? if not, then life doesn't start at conception. The issue about eggs spliting to make twins, triplets, etc, seems to indicate that those multiple beings didn't exist right after conception but a time later.
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3) Finally, personhood generally isn't a trait we assign simply based upon genetic facts, or anything else, but rather when we talk about something being human a human being, we mean something significantly more substantive. Something which includes psychological traits, and perhaps even other things. Because of this, it is very difficult to see how a person really begins at conception, when the entity involved lacks the traits that we tend to consider important, even implicitly, in determining whether something is a person or not.
I'm not sure how pro-lifers make their case about this, I assume somehow that they probably don't care about where personhood begins, rather life.
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