Science: Three Types of Human Chimeras That Already Exist


The Chimera from Greek mythology, not the genetic anomaly that shares its name.


By Fariha Fawziah

Researchers have recently declared that they wish to create human-animal “chimeras,” a genetic intervention which has sparked major medical and ethical debates.

The reason behind this may be the experiments of creating hybrid animals by human geneticists such as, Tigon (Male Tiger + Female Lion), and Grolar Bear (Polar Bear + Brown Bear). Additionally, another reason this topic is controversial may be the mythic and science fiction stories that suggest that monstrous human-animal hybrids might be possible. However, chimeras aren’t always man-made, and there are various examples of human chimeras that already exist.
As a term borrowed by scientists from Greek mythology, a genetic chimera is a single organism or person composed of two genetically distinct types of cells. For example, about 8% of non-identical twins are “blood chimeras” who carry more than one blood type. This happens because they shared a blood supply in the placenta where stem cells can pass from one twin to settle in the bone marrow of the other.


In theory, the blood cells of genetic chimeras contain two sets of DNA, each having sufficient code to make two distinctive organisms. Another way for chimeras to happen naturally in humans is if a fetus absorbs its own twin. This can occur with fraternal twins, if one of the fetuses dies very early in a pregnancy, and some of its cells are absorbed by the remaining fetus. The surviving fetus will now have two sets of DNA, its own original set and the one from its twin.  These individuals usually don’t know they are a chimera. For instance, in 2002, there was a story which reported a woman named Karen Keegan, who needed a kidney transplant. She went through a genetic testing along with her family to check if a family member could donate one kidney to her. The tests suggested that Karen could not be the mother of her sons since she had different sets of DNA in her blood cells compared to other tissues in her body. Additionally, another way for a person to be a chimera is to have a bone marrow transplant. These transplants treat leukemia, which is a type of cancer. The person with a bone marrow transplant will have blood cells from the donor for the rest of their life, and those cells will not be genetically the same as the other cells in their own body.



In some cases, all of the blood cells in a person who received a bone marrow transplant will match the DNA of their donor. On the other hand, there are other cases in which the recipient may have a mix of both their own cells and the donor’s, according to a 2004 review paper in the journal Bone Marrow Transplantation.
These transplanted blood cells are not temporary in the body, but are permanent, according to the Tech Museum of Innovation in California. But sometimes chimeras may display something called “microchimerism”, which is when a small fraction of their cells are from someone else. This can happen when women are pregnant, and a few cells from the fetus migrate into her blood and travel to different organs. A 2015 study suggested that this happens in the majority of pregnant women, at least temporarily. Researchers tested out tissue samples from the kidneys, livers, spleens, lungs, hearts and brains of 26 women who died while pregnant or within one month of giving birth. This study discovered that those cells were from the fetus since the cells contained a Y chromosome, which is found only in males and the pregnant women had all been carrying sons.


In numerous cases, those fetal cells may remain in a woman’s body for years. In a 2012 study, researchers analyzed the brains of 59 women aged from 32 to 101, after they had died. They found 63 percent of these women had traces of male DNA from the cells of a fetus. The oldest woman to have those cells in her brain was a 94-year-old, showing that these fetal cells can remain with the mother for their lifetime as well.

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Future Questions:
Does the migration of the cells from the fetus hurt the fetus in any way?
How does having two sets of DNA affect the chimeras’ children in the future?

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