Chimerism is the genetic phenomenon in which a single organism contains the cells of two or more individuals of the same species or of different species. In any case, they have more than one set of DNA in the same body, so they contain the code to make two separate organisms.
The word “chimera” comes from the ancient Greek, meaning 'she-goat' or 'monster'. In Greek mythology, it was used to describe a fire-breathing hybrid creature composed of different animals, that was often depicted as a lion with a goat’s head on its back and a tail that ends with a snake's head.
In genetics, chimerism is a biological condition and has nothing to do with monsters. In animals, chimeras can occur after either two fertilized eggs or two early embryos fuse together into one organism. As a result, an animal chimera will carry two or more genetically distinct populations of cells, originating from different zygotes. Each population of cells keeps its own genotype and its own characters. During development, there is a chance that entire organs will possess different genotypes. For example, the animal can end up with one organ made up entirely of cells of one genotype, and another organ completely built from cells of the other genotype.
However, a condition where two or more populations of genetically distinct cells emerge in the same zygote is called a mosaic, rather than a chimera. In a mosaic, all the cells originate from the same original organism and the genetic changes were caused by genetic mutations.
Chimerism is very rare, except in cats, where it may actually be fairly common.
Humans who are chimeras usually don't know it. However, they may have different-colored eyes, patchy skin, differential hair growth, two different types of red blood cells, intersexuality, or genital ambiguity, but this is not very common. Most human chimeras do not have any observable symptoms — but may have an increased risk of autoimmune disease.
How are human chimeras created?
Human chimeras may occur naturally when two nonidentical twins merge in the womb early in development. If the early embryos fuse together, so that the cells of one are absorbed by the other in the early stages of development, they can become a single chimera. This is called fusion chimerism, or tetragametic chimerism.
Chimerism in humans can also happen as a result of a bone marrow transplant, for example, to treat leukemia or diseases like aplastic anemia, and sickle cell anemia. Because bone marrow contains stem cells that develop into red blood cells, a person with a bone marrow transplant will have blood cells that are genetically identical to those of the donor while other types of cells in their own bodies may remain the same, turning the recipient into a human chimera. This may be called “complete donor chimerism”.
There is also a kind of chimerism, called microchimerism, that particularly affects pregnant women. During pregnancy, a small number of cells from the fetus migrate into the mother's blood and travel to different organs. Because the cells are from a different individual, the mother will have two sets of DNA at least for a while. In some cases, the fetal cells remain in the mother’s organs for years. A 2015 study suggested that this happens in almost all pregnant women, at least temporarily.
In some cases, however, the fetal cells may stay in a woman's body for years. In a 2012 study, researchers analyzed the brains of 59 women ages 32 to 101, after they had died. They found that 37 (63 percent) of these women had traces of male DNA from fetal cells in their brains. The oldest woman to have fetal cells in her brain was 94 years old.
Artificial chimeras can also be created in labs. In 2017, scientists from the Salk Institute for Biological Studies in La Jolla, California, grew human cells in pig embryos to create the first human-pig chimeras.
Although the embryos were destroyed at week 4, for ethical reasons, the scientists proved that this method could possibly be used to develop new tissues and organs for transplant or other healthcare needs.
In fact, researchers from the University of Minnesota later managed to grow human muscles in pig embryos in an experiment aimed at curing volumetric muscle loss (VML), which often causes lifelong disability due to muscle function loss or limb loss.
Famous human chimeras
Famous cases of human chimeras have helped us understand a little bit more about this genetic condition. But unfortunately, chimerism has caused trouble in these people’s lives.
- Karen Keegan.
In 2002, news outlets reported the story of Karen Keegan. When she needed a kidney transplant, she and her family underwent genetic testing to see if any of her family members could donate a kidney to her. But the results showed that, genetically, she could not be the mother of two of her three children. Doctors discovered that she was a human chimera who had blood cells genetically different from other cells in her body.
- Lydia Fairchild.
In 2002, Lydia Fairchild took a DNA test along with the father of her children to apply for child support (proving the father’s paternity was one of the requirements for this). She could prove that her ex-partner was the father of the kids but the testing ruled her out as the mother of the kids.
She was accused of claiming benefits for other people's children and being part of a surrogacy scam. But when her defense attorney Alan Tindell read about Karen Keegan’s case, he asked for Lydia’s genome to be studied more deeply. This way, it was discovered that the DNA of Lydia’s hair and skin did not show her related to her children’s, but DNA taken from a cervical smear did. The conclusion was that she was a human chimera with two different sets of DNA.
- Taylor Muhl.
Taylor Muhl is an American singer and model who has two different skin colors on her torso, something that doctors always believed was a big birthmark. While trying to find a cause for her autoimmune health issues, an expert in genetics diagnosed her with chimerism.
Taylor is an only child, she had a fraternal twin that she absorbed in the womb, causing but the “birthmark” on the left side of her body — one side has a different skin pigmentation color than the other side as a result of her twin's DNA. She also has two different immune systems and two different bloodtypes, which is what’s causing her autoimmune issues. Her own immune system interprets her sister’s DNA as foreign and attacks it.
She also has sensitivities to food, jewelry, and insect bites and she says she reacts differently on the left side of her body.
- In 2015, an anonymous US-based man failed a paternity test because —as he later found out— he had two sets of DNA. The DNA extracted from his cheek swab sample didn’t match the DNA in his sperm sample. The man had absorbed his unborn twin brother’s genes in the womb. Genetically, he was his son’s uncle.
It’s important to reiterate that chimerism usually does not exhibit any symptoms and if it does, most people and even doctors do not directly relate the actual symptoms to chimerism when they observe them.
Because of this, most human chimeras do not know they’re chimeras until they have to take a DNA test for another reason. This is probably why there are only about 100 registered cases of human chimeras.
However, the high prevalence of twin loss in multiple-fetus pregnancies suggests that there might be many more human chimeras that we (or they) will ever know about. Multiple-fetus pregnancies are not so rare, either. Some studies have found that for every eight single-child births, at least one baby comes from a pregnancy that was originally multiple.