Overview Ancient Greek mythology includes stories of a fire-breathing creature called a chimera. This fearsome beast was a mix between a lion, goat, and serpent. But chimeras are not just a part of mythology. In real life, chimeras are animals or humans that contain the cells of two or more individuals. Their bodies contain two different sets of DNA.

How common is it? Experts aren’t sure how many human chimeras exist in the world. But the condition is believed to be quite rare. It could be becoming more common with certain fertility treatments like in vitro fertilization, but this isn’t proven. Only about 100 or so cases of chimerism have been recorded in modern medical literature. Chimerism can also affect nonhuman animals. Often, it causes two distinct types of colorings on different halves of the same animal, such as two different-colored eyes.

What causes chimerism? People may experience one of several types of chimerism. Each has a slightly different cause and may result in different symptoms. Microchimerism In humans, chimerism most commonly occurs when a pregnant woman absorbs a few cells from her fetus. The opposite may also happen, where a fetus absorbs a few cells from its mother. These cells may travel into the mother’s or fetus’s bloodstream and migrate to different organs. They may remain in a mother’s body or a child’s body for a decade or more following childbirth. This condition is called microchimerism. Artificial chimerism A similar kind of chimerism can occur when a person receives a blood transfusion, stem cell transplant, or bone marrow transplant from another person and absorbs some of that person’s cells. This is called artificial chimerism. Artificial chimerism was more common in the past. Today, transfused blood is usually treated with radiation. This helps the transfusion or transplant recipient better absorb the new cells without permanently incorporating them into their body. Twin chimerism A more extreme form of chimerism can occur when a pair of twins is conceived and one embryo dies in the womb. The surviving fetus may absorb some of the cells of its deceased twin. This gives the surviving fetus two sets of cells: its own, and some of its twin’s. Tetragametic chimerism In other cases, human chimeras develop when two different sperm cells fertilize two different egg cells. Then, these cells all fuse together into one human embryo with crossed cell lines. This is called tetragametic chimerism.

What are the symptoms of chimerism? The symptoms of chimerism vary from person to person. Many with this condition show no signs, or they may not recognize these signs as chimerism. Some symptoms include: hyperpigmentation (increased skin darkness) or hypopigmentation (increased skin lightness) in small patches or across areas as large as half of the body

two different-colored eyes

genitals that have both male and female parts (intersex), or that look sexually unclear (this sometimes results in infertility)

two or more sets of DNA present in the body’s red blood cells

possible autoimmune issues, such as those related to the skin and nervous system

How is chimerism diagnosed? People most often discover they are chimeras by accident. There are cases of chimerism that have been discovered during genetic testing for medical reasons other than chimerism, such as for organ transplants. Genetic tests can help uncover whether or not a person’s blood cells contain DNA that’s not present in the rest of their bodies. Multiple sets of DNA in the bloodstream are a classic sign of chimerism. But people may go their entire life without knowing they are chimeras because the condition is rare and people aren’t usually tested for it.

Interesting facts Human and animal chimeras can have two different blood types at the same time. It may be similar amounts of each blood type. For example, in one case, a female chimera had blood that was 61 percent type O and 39 percent type A.

Male tortoiseshell cats are often chimeras. Their split coloration is the result of two different embryos fusing together. While it’s possible for these cats to be fertile, most often they’re not. This is because the extra DNA they receive links the trait for their coloration to infertility.

Human fertility treatments like IVF and multiple embryo transfer, which sometimes can cause double pregnancies and twins, haven’t been proven to increase a person’s chance of giving birth to a chimera.

For many chimeras, the mixing of DNA happens in the blood. But it’s possible for it to happen elsewhere in the body. This includes in the sexual reproductive organs. This means it’s possible for a parent with chimerism to pass on two or more sets of DNA to their child. A child may get two sets of DNA from their mom and one from their father, for example.

After a bone marrow transplant, a person will have a mix of DNA from their original blood cells and those from their donor. In other cases, their bone marrow may match the DNA of their donor only. This is because bone marrow continues to regenerate.

Microchimerism going from a fetus to a mother may happen in almost every pregnant woman, according to researchers. In one small study, all women who died while pregnant or within one month of giving birth had fetal cells in some of their body tissues. Experts are not sure exactly what effects this chimerism has on the mother and child.

High-profile cases A small number of chimera stories have appeared in popular news headlines over the past few decades. Recently, a singer from California named Taylor Muhl was profiled as a chimera. She reports that she has twin chimerism, meaning she absorbed some of her twin’s cells while she was in her mother’s womb. This has left her with a half-white, half-reddish pigmentation on the skin covering her abdomen, according to Live Science. In another recent story, a male chimera failed a paternity test because the DNA his child inherited came from the twin he absorbed in the womb. Similarly, a mother didn’t pass a maternity test for the baby she gave birth to for the same reason: The DNA she presented in testing wasn’t the same as the DNA she passed down to her children. This happens because chimeras can carry different DNA in different parts of their bodies, including in their reproductive cells.