Descent from Jesus

At this point, I doubt that I'm spoiling The da Vinci code for anyone by revealing that the author posits, for plot purposes, that not only that Jesus married and fathered children, but also that his line reaches the present day: Exactly one contemporary person is a descendant of Jesus.

This posit is demographically extremely improbable, of course. Almost everyone who was alive in 30 A.D. has either no present-day descendants, or literally millions of them. Having exactly one eightieth-generation descendant is so rare that the odds are against any of Jesus's contemporaries being in that situation, let alone Jesus himself.

Leaving that aside, however, I find it even more peculiar that the creators of the movie assume that Jesus's eightieth-times-great-grandchild is somehow different from the rest of us, sharing Jesus's magical powers or charisma. At one point, for instance, the present-day descendant makes a casual try at walking on water, and there is a moment of disappointment when the attempt fails.

This strikes me as ridiculous, for several reasons; but the one that I'd like to focus on concerns genetic inheritance. Jesus's sons and daughters, if there were any, received about half of his genes. His grandchildren, if there were any, received about a quarter. Assuming no inbreeding, descendants in the nth generation derive about 1/2ⁿ of their genes from Jesus. So the fraction of Jesus's genetic constitution that his descendants in the eightieth generation would receive is 1/1208925819614629174706176 -- which means that it is extremely unlikely that such a descendant inherits even a single gene from him.

The probability would be slightly higher if some of the ancestral lines crossed -- but the premise of the movie is that at no time has the set of Jesus's descendants been large; it is likely that they would all have known one another, making inbreeding improbable.

The probability would be much higher if everyone in the ancestral line leading to the present-day descendant were male, for then most or all of the genes on the Y chromosome would survive. But the movie makes it clear that this is not the case either. Perhaps that's why the water-walking attempt fails: it's a sex-linked trait, and was lost when the line descent reached the first female.

Of course, this last point presupposes that Jesus had a Y chromosome, which seems plausible given his human form but implausible given his posited parentage. (Old academic joke. Q: “What does Jesus H. Christ's middle initial stand for?” A: “Haploid.”)

I suppose this is one mark of a creatively Bad movie: Every viewer has a different reason for being unable to suspend disbelief.