The Immaculate Conception: How to have a baby without getting busy

Special thanks to Denis "the Menace" Lanno for suggesting this festive topic.

Of all the Christmas stories calling out for a scientific explanation, the virgin birth stands in a league of its own. While science and religion so often choose to avoid each other completely the immaculate conception all but demands a closer look. Whether or not you believe that a couple thousand years ago a middle eastern woman got pregnant while still a virgin, the question of whether or not such a thing is possible is undeniably interesting. There may even be an off chance that this sort of thing is fairly common. In a recent self-report study, 1% of US women reported having children without ever having had sex. We wouldn’t want to dismiss their claims without giving the subject a fair, open-minded glance. So come along with us on this magical Christmas Eve as we dig into the science of unilateral reproduction!

Virgin birth is a biological problem that involves things we actually know a good amount about. In normal reproduction an egg is produced by a women which contains copies of half of her 46 chromosomes. When that egg and its 23 chromosomes meet up with a sperm, containing 23 chromosomes from the man who produced it, the result is a “zygote” which eventually divides into a cluster of cells called and embryo and on into a baby somewhere down the line. If we are looking to explain a virgin birth we somehow need to find a way to get that second set of chromosomes into an egg and give it the ability to divide using only the resources available in a normal woman’s body. It’s like that part of Apollo 13 where they have to fix the air filter with duct tape and coffee filters.

When we take a look at the tools we have available there is one fairly important thing that is missing: A Y-chromosome. As it turns out, the biggest problem with the story the Bible provides is that Mary (a woman… presumably) gives birth to Jesus (a male). The thing is, chromosomes come in pairs. The pair of chromosomes that determine gender in humans can be either X or Y. Women have two X chromosomes (XX) and men have one of each (XY). As you can see, Mary doesn’t have a spare Y chromosome to provide her baby with.

If Mary were a reptile, we might have something to work with. In fact, asexual reproduction has actually in observed in boa constrictors, komodo dragons, a few birds, and a couple other kinds of vertebrates. The process basically amounts to cloning since the genetic material involved is all identical to what is found in the mother. The cool thing about reptiles is that females are defined by their ZW sex chromosomes while males have a ZZ combination. That means that it is actually theoretically possible for a female snake to give birth to male offspring without mating, by giving an egg 2 Z chromosomes. What makes the whole situation mind-blowing is that, in practice, when scientists first observed this process in nature, all the offspring tested had 2 W chromosomes, meaning that they were an entirely new gender! This process is called parthenogenesis and it is useful when an animal is cut off from potential mates.

That doesn’t really help us explain Jesus, though. Fortunately, we are left with one pretty cool, albeit slightly complicated, possibility. To understand it we need to appreciate why parthenogenesis doesn’t happen in mammals. To put things very simply, the genes that a growing mammalian egg cell is provided with do not allow it to develop past a certain point. This is called “genetic imprinting” and you can think of it as a sort of cellular traffic signal. Under normal conditions, eggs cells grown in female mammals contain a “stop” code that prevents them from growing into embryos on their own and it is only when a male sperm cell containing the green-light “go” code arrives that things can get underway.

Research from Japan and South Korea has shown that a good part of the stop and go process is controlled by two genes called Igf2 and H19. In experiments with mouse embryos, researchers determined that the H19 (stop) gene normally blocks the growth-stimulating Igf2 (go) gene in egg cells. When scientists, who were very keen to play god in the most literal sense, mutated the H19 gene, deleting 13,000 of its bases, they managed to produce 26 viable, unfertilized eggs (out of around 600 eggs they started with) that led to 10 live mouse babies. Only one of those pups survived to adulthood, but to back up the Christmas origin story we only really need one anyway.

So what are we left with? Is the Immaculate Conception a scientific possibility? The short answer is maybe.. but not exactly as written in the Bible. First, we need to assume that human reproduction is similar enough to mouse reproduction that we have any evidence at all to go on. Next it would require a genetic mutation so precise and so unlikely that the scientific method has never in modern history found evidence for it ever happening naturally. Then, and only then, might we concede that after having produced an egg with a crippling mutation on its H19 gene, and having suffered no other mutations that would damage it beyond repair, that a woman named Mary might have given birth to baby without having sex… but we still don’t know where she would have gotten that Y chromosome.