My understanding is that the Virgin Birth, assuming it is true, is like many of Jesus' miracles: a sign. The Logos could certainly take to himself a human born in the usual way. But having God's direct participation in his birth is a sign that he is the Word made flesh.
This raises an interesting question - why? Why have a virgin birth for Jesus? As mentioned above, it wasn't needed for "sin" reasons.
Was it done as a "sign", like the many miracles in John done to prove that Jesus is God (the opposite of the other three gospels, where Jesus explicitly refuses to do them for that reason)? But a "sign" doesn't really work either, since there is nothing to see. After all, who could have known back then exactly what Mary was doing every minute of every day 4 months ago?
Perhaps a good reason was because many famous people back then were expected to have a virgin birth from the gods. That was true of Roman/Greek heroes like Romulus, perseus, Hercules, Dionysus, kings like Theseus, many Egyptian pharaohs, and even Alexander the Great. Maybe, if Christians were to gain followers, God knew that Jesus too would have to have been said to be virgin/divinely born, to be taken seriously?
After that, the Isaiah verse could be used, even though it really isn't a prophecy of a virgin birth.
I agree that when you start looking at genetics all kinds of difficult questions begin to arise. (If you want to do that kind of speculation, my favorite is that God used Joseph's genetic material.) But that's probably not helpful for understanding the original meaning.
Yes. As Jack pointed out, people back then thought that the man's DNA was everything and didn't know about the woman's egg. It sounds like Jesus looked like Mary and Joseph (mk 6 and Mt 13), suggesting he had DNA like them or similar to them. Now, if God made all his DNA from scratch, then he could have looked like anything. You mentioned doing the virgin birth as a sign - but if a sign was desired, and one is making the DNA from scratch anyway, why not have, say, gold skin (and eyes like gemstones, reflecting the ark of the covenant, which he is replacing), or glowing eyes, or 4 arms (like Hindu gods)?
If one is making the DNA from scratch, then even if it is to look like a human, why make Jesus look like a child of Joseph and Mary? Since he represents all humans, why not make him a racial mix of caucasian, African, Australian, Native American, Chinese, Indian? Or maybe just, say, Australian aborigine, to emphasize that all races are included in God's plan?
After all, if he's not the normal biological child of Joseph and Mary, why make him look exactly as if he was the normal biological child of Joseph and Mary? Is that dishonest?
On the other hand, if he had half Mary's DNA and half that God made, then all the questions above still apply (you can still have gold skin from the God half), and other problems come up too.
All human DNA has all kinds of imperfect stuff, like thousands of broken, defective gene carcasses (pseudogenes), transposons (the DNA equivalent of chain letters), ERV (virus carcasses from past infections), and so more. So Mary's DNA would have had all that, plus any genetic defects that we all have a few of. So would God have "fixed" those in Jesus? If so, then a good chunk - more than 20% - of Mary's DNA would have been deleted. Would Jesus have been "fully human" if he didn't have all that? But if he did, it doesn't seem that Jesus was "perfect".
Jesus would have to have had Mary's mitochondrial (mt) DNA, and thus her specific haplotype. Does that mean that the haplotype Jesus had was "special"? Are people with that halplotype more "blessed"? Or did God make all new mt DNA for him? In which case all the earlier questions apply again - including the fact that human mt function is not "perfect".
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In Christ-
Papias