What Is a Chimera? Could You Be One?

In Greek mythology, a chimera is a female fire-breathing hybrid creature pieced together from the parts of a lion, a goat, and a snake. As mythological beasts go, the chimera is a pretty fearsome creature.

More recently, animal chimeras have appeared in pop culture everywhere from Power Rangers to My Little Pony and, of course, in lots and lots of science fiction. Storylines of human chimeras evading DNA tests pop up in crime dramas like CSI and NCIS, and mothers are told their children are not really theirs on daytime soaps like All My Children.

But how much truth is there to the existence of chimerism? 

What is a chimera?

Biologically speaking, a chimera is an organism that has the cellular make up of two separate organisms—think two separate sets of DNA or the blueprints to make two different individuals. As offspring, we are typically a mix of the genetic makeup of both of our parents—I have my father’s nose and my mother’s eyes. But a chimera has two complete sets of genetic information.  

Plant chimeras are usually caused by mutations that happen during ordinary cell division. So a zygote—that’s the first stage after fertilization has happened and two sets of DNA have combined—forms as usual but then can later mutate. By studying plant chimeras, biologists have been able to determine which plant traits (like coloration and size) are cell-autonomous, or derived from a single cell versus those that are more widespread. 

A chimera has two complete sets of genetic information rather than the usual mixed set.

Animal chimeras are formed from two or more different zygotes, which means yes, your favorite crime drama wasn’t making it up—a chimera can have two blood types. If the two zygotes were different sexes, it’s also possible to have both male and female sex organs as a chimera. 

Are there human chimeras?

Not only do human chimeras exist, but many human chimeras don’t know their chimera status.

One type of chimerism, called tetragametic chimerism, happens when a fetus absorbs its twin in the womb. In the case of fraternal twins, two eggs are fertilized by two sperm cells and form two zygotes. But if one embryo dies early on, those cells can get absorbed by its womb-mate and merge to form one fetus with two different cell lines. The whole process is sometimes referred to as vanishing...

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