What's a chimera, and how common is chimerism? Most cases in humans are discovered by accident, so you could be a chimera and not even know it!
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 twin syndrome.
One type of chimerism, called tetragametic chimerism, happens when a fetus absorbs its twin in the womb.
Since this absorption happens very early on in the embryos’ development, this type of chimerism can go completely undetected. Some people live their whole lives without knowing. Others find out through confusing medical circumstances.
For example, after a failed kidney transplant, a Boston woman named Karen Keegan looked to her three sons to see if one was a match and could provide her with the needed kidney. Much to her shock, I'm sure, the doctors told Karen that while her three sons were brothers, only one of the children she gave birth to was her biological son. In case you haven’t experienced it, let me tell you that childbirth is not something you forget. Luckily her doctors believed her. They began testing her other tissues for other cell types and discovered that she had different DNA in her blood versus her body’s tissue. She was a chimera.
Getting a bone marrow transplant will make you a chimera. Bone marrow contains stem cells that develop into blood cells and platelets. So for transplant patients, their diseased bone marrow is purposefully destroyed via chemotherapy and replaced with bone marrow from a donor. That donor’s bone marrow will continue making those blood cells with the donor’s DNA once it has been transplanted in its new human home.
Getting a bone marrow transplant will make you a chimera.
If you’ve ever gotten a blood transfusion you were, albeit temporarily, a chimera. However, our bodies are programmed to destroy foreign DNA as possible invaders and these low levels of donor DNA are quickly ousted.
Microchimerism—when only a small amount of cells from another cell line are present—can happen in pregnancy. One study looked at tissue samples from 26 women who died while pregnant or just after giving birth. Identifying these different cell lines can be complicated, so researchers focused on women who were all mothers of sons so that any cells containing a Y chromosome could be easily associated with the baby’s cell line and not their own. They found those cells containing Y chromosomes in every single sample.
This may not be too surprising—there’s a whole other human being in there!—but what is shocking is how long those cells can linger on after childbirth. Another study in 2012 looked at brain tissue from 59 women ranging in age from 32 to 101 who had all had sons. Male DNA was found in 63% of the womens’ brains, including in the brain of a 94 year old.
How common are chimeras?
A google image search on plant chimeras turns up photos of beautiful bi-colored flowers and leaves. And while most human chimeras don’t know they are a chimera, there are cases where chimerism can lead to different skin pigmentations or even eye colors and thus be outwardly noticeable.
Who knows? You could be your own twin.
For example, the model and musician Taylor Muhl has openly talked about how her chimerism gives the skin on her stomach two different tones. For much of her life, she was told the darker half of her stomach was a birthmark. Then her chimerism was discovered.
Are people with heterochromia chimeras? If you happen to have two different colored eyes, it’s worth noting that there are also other ways to end up with what’s called heterochromia iridum, and chimerism is just one of those ways.
In the case of multiple births—twins, triplets, and so on—there can be some mixing of DNA at the 25-, 50- or even 75-percent level so that multiples share DNA even outside of the womb. At least one study suggests that this kind of chimerism, known as blood group chimerism, is not all that rare. Thirty-two out of 415 twin pairs tested (that’s eight percent) and 12 out of 57 triplets (or 21 percent) shared their DNA.
Most sources put the known number of cases of human chimerism at around 100, although I couldn’t find a reputable citation for that number. The known cases were mostly found by coincidence, whether through seeking out a medical procedure or a paternity test. So it's likely there are many more human chimera out there and our increased interest in genetic testing might mean we find more. Like you! Who knows? You could be your own twin.