In Greek mythology, a chimera was a fire-breathing creature with physical traits of a lion, goat, and dragon.
In human beings, a chimera is a person who has two totally different sets of DNA inside their body. It’s a bit less dramatic than a fire-breathing monster, sure, but it’s still pretty wild.
Even wilder: Human chimeras aren’t the result of futuristic genetic tinkering. They can occur naturally, and some people don’t even know that they’ve doubled up on DNA.
Here’s a quick guide to the ways a person can become a human chimera.
It can happen after a bone marrow transplant
Bone marrow is the tissue inside our bones that’s responsible for making white blood cells, red blood cells, and platelets. In bone marrow transplants, doctor uses chemotherapy or radiation to destroy all the recipient’s diseased bone marrow, then a donor’s healthy marrow is put in its place.
The donor’s bone marrow will keep on making blood cells that have the donor’s DNA, according to a Scientific American report. That’s how the recipient becomes a chimera.
In “complete chimerism“, 100 percent of the recipient’s blood cells have the donor’s DNA, a paper in the journal Nature explained. But the blood can also contain a mix of DNA from both the donor and the recipient – that’s called “mixed chimerism”.
This type of chimera has inspired one some interesting storylines in pop culture, Motherboard reports. The 2015 film Bad Blood is all about a cancer-patient-turned-serial-killer using the DNA in his blood to implicate his bone marrow donor.
It can happen when fraternal twins are in utero
Scientific American explains that, when a mother is carrying fraternal twins, one of the embryos might die very early in the pregnancy. Then, the other embryo can absorb some cells from the deceased one. The resulting baby ends up with two sets of DNA.
Sometimes these chimeras make the news.
In 2015, a man from Washington took a cheek swab paternity test that said he was technically his son’s uncle, not his father. Further testing revealed that the man had different DNA in his saliva and his sperm.
Genetic experts believed he was a human chimera, and he had absorbed some of his DNA from a fraternal twin’s embryo, BuzzFeed reported.
A woman named Karen Keegan wound up in a similar situation.
Tests said she wasn’t the biological mother of her children, but it turned out that the DNA in her blood was different than the DNA in her ovaries. Doctors said her extra DNA most likely came from a…