If you’re familiar with the word “chimera,” it’s probably from Greek mythology, where it refers to a fire-breathing, three-in-one monster — lion-like overall, with a goat’s head protruding from its back and a snake’s head at the tip of its tail.

The modern, scientific worldview leaves little room for these grotesque creatures, but it has replaced them with something just as strange and wonderful: human chimeras.

Most of us are the product of a single genetic code. When our parents conceive us, their chromosomes combine to form the unique sequence of DNA that we carry with us for the rest of our lives. Under the right circumstances, however, it’s possible for one body to contain two distinct genomes.

Borrowing from the Greeks, scientists call this condition chimerism. It isn’t typically a health concern, and it won’t cause horns or extra heads to sprout from inconvenient locations. You’d never know someone was a chimera simply by looking at them. That said, the condition can put unwitting chimeras in bizarre situations, and it’s more common than you’d expect. In fact, there’s a decent chance you could be one yourself.

The Twins Within: Vanishing Twin Syndrome

One recipe for chimerism begins during pregnancy with twins (or triplets, octuplets, etc.). In cases of vanishing twin syndrome, where one of the embryos dies in utero, the other may absorb its cells and become a being divided — it will still have its original genes, but they’ll be mixed with those of its less fortunate womb-mate. (Of course, this only works with fraternal twins, since identical twins already share the exact same DNA.)

That’s what happened to Lydia Fairchild, a woman who almost lost custody of her children in 2002 after DNA testing showed she wasn’t a genetic match with them. Luckily a second test, this time on cells from a different part of her body, set the record straight: Fairchild was a hybrid, partly herself and partly a twin sister she’d carried with her all her life. So, she was indeed her children’s mother — but also, genetically speaking, she was their aunt.

Read More: Identical Twins: Just How Genetically Alike Are They?

The First Confirmed Chimera

Even if both twins survive to term, it’s possible for their cells to mingle. In 1953 the first confirmed chimera, a woman known in scientific literature as Mrs. McK, learned after donating blood for the first time that she had two different blood groups, A and O. This would’ve made sense if she’d recently received a blood transfusion, but she hadn’t.

A single explanation remained: One group rightly belonged to her; the other was a relic of her twin brother, who died at just three months old. Their blood cells had somehow blended in the womb, and after all those years his were still living within her. It was, as the doctors who described her case in the British Medical Journal called it, “Nature’s experiment on Mrs. McK.” 

Read More: Why Scientists Have Been Creating Chimeras in the Lab for Decades

Fetal Cell Migration

These twin scenarios may sound extraordinary, but there are even more routine ways to gain a second set of DNA. When a woman is pregnant with just one child, either (or both) of them may become chimeric if their respective cells cross the placental barrier that separates mother and fetus. These stowaways can enter the other’s bloodstream, travel to distant organs, and establish themselves alongside native cells.

In 2012, for example, a team of medical researchers from Washington state performed autopsies on the brains of 59 women. Astonishingly, 37 of them — nearly two thirds — harbored cells with Y chromosomes, which are only present in males. Clearly, they weren’t homegrown.

The women could’ve gotten these anomalous genes from twins, perhaps even vanished twins they never knew existed. But the much more likely source was their own offspring: Sons they’d given birth to, in some cases many decades before. Talk about a maternal bond — if it feels like your mom always knows what’s going on inside your head, maybe it’s because there’s a bit of you in hers.

Studies like this have led experts to conclude that chimerism isn’t the rarity they once thought it was. The rise of accessible genetic testing in recent years has shown that it is, as the Washington researchers write, “frequent and widely distributed.”

Read More: The Next Human Genome Challenge

Post-Op Chimeras

Everything thus far falls under the category of “natural” chimeras, people who come by their dual DNA in the course of normal human development. But, later in life, a person can also become an “artificial” chimera. All it takes is a quick medical operation.

Returning to Mrs. McK’s story, doctors at first assumed her second blood group came from a blood transfusion. That’s because it’s well known that a donor’s blood can persist in the recipient’s body for a while, making them a temporary chimera.

On the other hand, some medical treatments result in permanent chimerism. When a person receives a donated organ (say, a kidney), the tissue retains its old genes. The same is true for bone marrow — when someone’s diseased marrow is swapped out for a healthy substitute, the new blood cells it produces have the donor’s DNA. The recipient may even wind up with a new blood group.

Read More: How Do Bone Marrow Donations Work?

Living Double Lives

What at first glance seemed like science fiction turns out to be an everyday phenomenon. But since chimerism doesn’t generally come with adverse side effects, most people don’t get a diagnosis and never realize they’re living a kind of double life.

That doesn’t mean their hidden doppelgangers can’t affect them in unexpected ways, though. Their main challenge lies in faulty DNA tests, like Fairchild’s. These can make it hard to prove maternity or paternity, and may pose other complications — if the results aren’t always reliable, what does that mean for criminal proceedings that use genetic evidence? And how can you tailor a drug treatment to someone’s specific genetic makeup if your picture of it is incomplete?

Kamlesh Madan, a geneticist at the Leiden University in the Netherlands, summarized the problem in a 2020 paper: “If human chimeras are more common than hitherto thought, there could be many medical, social, forensic, and legal implications.”

Read More: The Next Human Genome Challenge