Human Chimeras: One Body, Two Individuals


Normally, each person is born with one blood type and one set of DNA; but, is this always the case? Years ago, we found out that a person might actually be living with two or more sets of DNA and two blood types; in other words, there might be two or more individuals inhabiting your own body without you even knowing! This phenomenon is known as “chimera”; as strange as it may seem, it is actually true, and there are three cases in which human chimeras may exist.

You Are Your Own Twin

One way human chimeras can exist is when a fetus absorbs its twin. During pregnancy, if one embryo of fraternal twins dies very early, the other twin might absorb some of its cells, which means that the remaining fetus will have its set of cells, in addition to the ones from its twin.

However, these chimeras are not usually aware of their condition until they undergo a medical examination for any reason. In 2002, the story of a woman named Karen Keegan was all over the news when she needed a kidney transplant and was genetically tested to identify a matching donor among her family members, only to discover that none of her children could genetically be hers. Doctors were finally able to solve this medical mystery by concluding that Karen was a chimera and that she had one set of DNA in her blood cells and another completely different set in other tissues in her body.

Bone Marrow Transplant

The second case a chimera may exist is when a person undergoes a bone marrow transplant; such as in the treatment of leukemia. In such cases, the bone marrow of a person will be destroyed and replaced with the donor’s bone marrow. As a result, the person who receives the bone marrow transplant will have genetically identical blood cells to those of the donor, because bone marrow contains stem cells that develop into red blood cells; yet, other body cells will not be genetically the same.


When a woman is pregnant, she often hears comments such as: you are now carrying a part of you, your baby needs all your care because the baby is sharing your body, your food, etc. She never thinks that she is the one who is carrying part of her baby, does she? It never occurs to her that she is actually sharing her baby’s cells, and not the other way round! During pregnancy, it is very common that a small number of the fetus’ cells migrate into the pregnant woman’s blood, then travel to various organs. This is known as “microchimerism”; it basically means having a small fraction of your cells from someone else.

One way to make sure a woman is carrying fetal cells is by examining tissue samples from various parts of her body. In 2015, researchers conducted a study suggesting that almost all pregnant women experience microchimerism, by examining tissue samples from the livers, kidneys, lungs, spleens, brains, and hearts of 26 women. Some of the women had died while pregnant, while others died within one month of giving birth. What confirmed to these researchers that these cells were the fetus’ not the mother’s is that the presence “Y” chromosomes, which can only be found in males, in these cells and all the women were pregnant with boys.

Can a woman live with her son’s fetal cells all her life? In a 2012 study, researchers examined the brains of 59 dead women to trace “Y” chromosomes from fetal cells in their brains; traces were found in 63% of these women aged 39–101. What led the researchers to conclude that the fetal cells could stay in a woman’s brain for a lifetime is that the oldest woman of those examined aged 94 and had fetal cells in her brain.

Possible Signs of Chimerism

Human chimeras do not usually show physical signs of their condition. However, there are some possible physical signs that could be linked to chimerism, such as having two different skin tones, disorders of genital development, having two differently colored eyes, or patches of different color or texture of hair.

The world is full of wonders, and this condition is definitely one of those! The most exciting part of it all is that any of us might be a chimera; any of us might actually be two in one!


The article was first published in print in SCIplanet, Summer 2020.

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