A chimera is a person who has two different sets of DNA inside their body. How is it possible? While the condition can be rare and experts are not exactly certain how many human chimeras exist in the world, the condition can occur naturally. In fact, some people — even you — may not know they possess twin sets of DNA.

Here are 3 ways a person can become a chimera.

1. Bone marrow transplant

A medical operation, namely a bone marrow transplant, can cause you to become a chimera. The bone marrow is responsible for producing white blood cells, red blood cells, and platelets.

When a person undergoes a bone marrow transplant and a healthy marrow is placed inside his body, the donated bone marrow will keep producing blood cells that contain the donor’s DNA. Through this process, the recipient of the marrow becomes a chimera.

In some cases, 100% of the recipient’s blood cells contain the donor’s DNA, causing “complete chimerism.” In other cases, the recipient’s blood cells can contain a mix of DNA both from themselves and their donor. This is called “mixed chimerism.”

2. Normal pregnancy

A lot of wonderful (and sometimes weird) things happen to the body during pregnancy, but did you know that pregnancy can also result in chimerism? Scientists found that a pregnant woman may retain some of her baby’s DNA if these cells migrate outside the uterus. This phenomenon is called “microchimerism.”

Mothers who have just given birth to boys have been tested and all possessed cells with Y chromosomes, which are only present in males. To test the theory on whether microchimerism can persist longer, researchers studied mothers who have sons but did not necessarily just give birth to one, the oldest of which was 94 years old. They found traces of male DNA in 63% of the women, proving the aforementioned theory.

Experts believe that the phenomenon is very common, if not universal, among pregnant women.

3. Fusion chimerism

Finally, there’s also the condition called fusion chimerism. This is probably the most interesting type of chimerism there is as it involves an embryo absorbing its twin in utero. When a mother carries fraternal twins, one of the twins can die very early during the pregnancy. What happens is that the surviving embryo can absorb some of the cells of their deceased twin.

During DNA testing, a human chimera can be surprised to find out that they have different DNA profiles from their DNA samples. In some cases, a person’s DNA from their saliva might be different from the DNA from their semen. This is because when they absorbed the deceased embryo’s cells in utero, they also absorbed some of their DNA.

Twin loss is actually common and can occur in as many as 30% of multiple pregnancies. Hence, it is possible that many people out there are actually human chimeras who will never find out unless they undergo genetic testing for some reason.

Chimerism is an interesting biological phenomenon, and they can be more common than previously thought. More often than not, chimerism does not have any effect on an individual’s health and well-being.


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