A chimera is a legendary creature from Greek mythology that has the physical traits of a dragon, goat, and lion, and it breathes fire.
Derived from this fantastical creature is the human version of a chimera, which is a person whose body contains two separate sets of DNA. And yes human chimeras do happen to exist.
While they aren’t really mutant beings or superhero experiments gone wrong, there are ways people can unwittingly end with a second DNA in a normal naturally occurring way.
Here are a few ways human chimera can exist.
Bone marrow transplant
A life-saving procedure, bone marrow transplants are used to replace a person’s infected and diseased bone marrow with a healthier one from a donor. This is usually done after a patient’s bone marrow has been destroyed by chemotherapy or radiation.
The donor’s bone marrow fights to create healthy blood cells in the patient’s body, and in turn, creates blood cells with the donor’s DNA. The patient in this procedure becomes a chimera with two sets of DNA in their body.
When there are two DNAs found in the body of recipient it is called ‘mixed chimerism’. However, there have also been cases where the recipient’s blood cells end up containing 100% of the donor’s DNA. This is called ‘complete chimerism’.
In a handful of cases, when a woman is carrying fraternal twins, on embryo dies prematurely early on during pregnancy. When this happens, the embryo sibling tends to absorb the diseased embryo’s DNA resulting in a baby born with two sets of DNA.
There have been many such cases of human chimeras. For instance, in 2015, a man from Washington took a paternity test with a cheek swab, and the test result stated that the man was the kid’s uncle and not his father. On further examination, it was revealed that the man was a chimera who had absorbed some of his DNA from his diseased fraternal twin’s embryo. When tested further they discovered that the man had different DNA in his saliva and his sperm.
Twin loss has become a common occurrence and usually takes place in 21 to 30% of multiple-fetus pregnancies. So, human chimeras born through fraternal twins may be more common than you think.
In some cases, pregnant women tend to absorb some of the DNA from her baby. Foetal cells sometimes migrate outside the uterus and the mother’s body ends up retaining these cells. This is known as ‘microchimerism’, or as New York Times calls it — ‘pregnancy souvenir’.
This usually gets detected better in mothers of baby boys, as it is simpler to detect Y chromosomes (which is only present in males) in the mother.
In a study reported by the Scientific American, 63% of the women were found to have had traces of male DNA. Scientists claim that although it may not be universal it is quite common among pregnant women.