If you think back to your formaldehyde-scented high school biology class, you may remember learning two things about DNA: first, that it's the "code" for all our genes, and second, that each individual has one and only one set. Otherwise, a person could have two different blood types – which is impossible. Or is it?

As it turns out, high school bio didn't have all the answers. Some people's bodies do indeed contain two sets of DNA.

A person who has more than one set of DNA is a chimera, and the condition is called chimerism. The word comes from the mythical Chimera, a creature in Greek mythology that's part lioness, part goat, and part snake.

The most extreme type of chimerism occurs when a twin dies early on in utero, explains Melissa Parisi, a pediatric researcher with the U.S. National Institutes of Health. In a move that's both bizarre and logical, the surviving twin acquires some of the dead embryo's chromosomes, ending up with two distinct and separate sets of genes.

It seems the stuff of science fiction, or at the very least, high drama – and, in fact, the phenomenon has been featured in television shows like House, All My Children, Law & Order, and Grey's Anatomy.

In real life, the most well-known case is probably that of Lydia Fairchild, who nearly lost custody of her children when DNA testing "proved" she wasn't related to them. Fortunately, doctors eventually determined that she had a second set of DNA that matched.

But you don't have to have had a vanishing twin to be a chimera. Regular fraternal twins can also have the condition. "Twin embryos sometimes 'trade' chromosomes with each other, which makes sense, given their shared blood supply," says Parisi.

And yes, if the twins are boy/girl, the girl could end up with some male chromosomes and the boy with female chromosomes. Does this have visible effects? Sometimes.

"Occasionally, the cell-trading leads to a disorder of sex development, such as a girl having a small amount of testicular tissue," says Parisi. "But while we have a lot of hang-ups about this type of disorder, it's important to remember that it's simply a congenital problem, like a cleft palate or any other."

But that's rare. Most of the time, the chimerism doesn't manifest itself in any easily observed way.

Chimerism doesn't always involve twins. Even mothers and babies "trade" cells during pregnancy, usually in very tiny amounts. "A baby's DNA can end up in the mother's bloodstream, because they are linked together through the placenta," says Parisi.

The reverse is also true: A baby can acquire some of the mother's DNA, in a condition known as microchimerism.

Because chimerism usually doesn't cause problems, it's rarely diagnosed, making it hard for scientists to say how prevalent the phenomenon truly is. It's probably less rare than was once thought. Perhaps many of us are chimeras and just don't know it.