What would Jesus' DNA do?

What would it mean if Jesus did have sex and let his DNA loose into the general population? How special would that make his would-be descendants, such as a character in the fictional "The Da Vinci Code"?

And what kind of DNA would Jesus have, considering that his mother was reportedly a virgin?

I wrote about genetics and the virgin birth last fall and found two surprising conclusions. First, most Catholic and Protestant theologians do not get insulted by this question. Second, they don't agree.

They concur that Jesus was not supernatural. The doctrine of incarnation says he's fully human, wrote Georgetown University professor of theology John Haught. "To imply that Jesus is somehow exempt from ordinary natural laws and biological patterns [including having DNA and male chromosomes] would, in my view, be a failure to take the incarnation seriously."

This is hard to square with the virgin birth in light of modern biology. It's true that asexual reproduction, called parthenogenesis, happens in some fish, insects, and even a lizard species, and artificially in a few mammals, through cloning.

But if cloning or parthenogenesis were involved, Jesus would look a lot more like Mary. He'd be a woman, for one thing, since females always beget females.

In humans, females package some of their DNA in two matched X chromosomes, males in a single X and Y. So if you're a male, there's only one way you could have gotten your Y chromosome, and that's from your biological father.

Where would Jesus have gotten his Y? Some, such as Protestant theologian Wesley Wildman of Boston University, say Jesus got his Y chromosome and half his DNA from a human father, most likely Joseph. What he got from God was something more spiritual.

Others, however, say that God must have fashioned at least part of Jesus' DNA himself, through a miracle. "It's not God's sperm ... but God creates something like a sperm and caused it to fertilize Mary's egg," said Ron Cole-Turner, a professor and ordained minister at the Presbyterian Pittsburgh Theological Seminary. That was also the majority view expressed by those who wrote in after my initial column.

If that's the case, maybe there's reason to feel uneasy about "The Da Vinci Code's" premise that this DNA was disseminated. Evolution tells us the 3 billion-character genetic code we carry in our cells was shaped by several billion years of evolution. The Jesus DNA would, by contrast, be specially created just 2,000 years ago.

That said, even miraculous DNA wouldn't render those in Jesus' bloodline all that special. Historically, bloodlines were restricted by rules about firstborn, legitimate sons, but true biological inheritance tends to fan out.

A few years ago, Joseph Chang, a mathematician at Yale, created a computer model designed to mimic the dynamics of the human population over the last 40,000 years. He concluded that all members of today's population share common ancestors if you go back just a few thousand years. Geographic and social boundaries keep us from all sharing the same ancestors 1,000 years back.

Working from the other direction, he said, those people alive 2,000 years ago almost certainly left either no descendants or millions. "It's called a branching process," he said. If your line of descendants doesn't peter out quickly and gets up to 20 or so, it will approximately double each subsequent generation and become quite numerous.

If the "Da Vinci Code" character were descended from Jesus, it's almost certain that millions of other people would be too, Chang said.

Not only that, the character might not have gotten a trace of Jesus' DNA. After so many generations, his genetic code would have been diluted by a factor of more than a trillion. There are only three billion chemical "letters" in the human genetic code, and it gets passed down in chunks.

Whether any genes get passed down, Chang said, is up to chance. So in the unlikely event that Jesus' DNA got out, it would have scattered much too far for anyone to find it.

Faye Flam's Carnal Knowledge column appears Wednesdays

in The Seattle Times