Photo: Sarah Chai

How many humans are chimeras?

A human chimera is made up of two different sets of DNA, from two different individuals. Experts aren't quite sure how common natural chimeras are in the human population, as only 100 cases have been documented so far. However, the prevalence of natural human chimeras is hypothesized to be as high as 10%.

thetech.org - How common are chimeras? | The Tech Interactive

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In ancient Greek mythology, a chimera is a mix between a lion, goat, and serpent. Real chimeras are perhaps just as odd. A human chimera is made up of two different sets of DNA, from two different individuals. Experts aren’t quite sure how common natural chimeras are in the human population, as only 100 cases have been documented so far. However, the prevalence of natural human chimeras is hypothesized to be as high as 10%.1 If this estimate is true, most chimeras must not know that they’re chimeric. However, it is sometimes possible to find out if you are one!

Man-made chimeras (organ transplantation)

One type of chimera is actually quite common. Anyone who has received an organ or bone marrow transplantation is actually a chimera. When someone goes through either of these procedures, they are receiving cells (and DNA!) from another person. The goal is for the donor cells to permanently live in the recipient. So the recipient will have two sets of DNA: their own (most of their body), and the donor's (in the donated organ). If you've ever had a blood transfusion or organ transplantation, you are probably a man-made chimera! For example, a blood marrow recipient will have different DNA in their blood from the rest of their body. If we ever needed to look at their DNA directly, we may need to look for other tissue types instead of blood.

Microchimerism from pregnancy

It turns out that most mothers are a tiny bit chimeric. During pregnancy, some cells are exchanged between a woman and fetus. Most of these cells die off, but occasionally a few may stick around. If so, the woman essentially has a few cells that have her baby's DNA. The level of exchange is quite small, so this type of chimerism is called microchimerism. Just to give you some context, there might be 1 fetal cell for every 500,000 maternal cells.2 It may be possible to tell if a previously-pregnant woman is a microchimera. Cells from the fetus can sometimes be found in the maternal blood stream. If they have different characteristics than the mother’s blood, it could be possible to tell them apart.3 But it isn’t always possible to do this, especially if the cells have similar characteristics.

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If you can find any, and sort them away from the mom's cells, you could compare the DNA from both sets. You would then see that the genetic code is different. Remember, we all have unique DNA!

Fusion chimerism

Another type of chimerism can be more dramatic. If a woman is pregnant with twins and one embryo dies in the womb, the surviving fetus may absorb some of the cells of the deceased twin. If this happens, the surviving fetus has two sets of DNA: its own DNA, and DNA from its twin. This type of chimerism can be seen in what is known as the “vanishing twin” phenomenon. This refers to where a fetus in a multigestation pregnancy dies in utero and is partially or completely absorbed. Among twin embryos, ~30% will end up as single embryo pregnancies.4 People with chimerism rarely show any visible signs of having two distinct sets of DNA. It’s possible there may be some physical signs such as having two different colored eyes or two different skin tones. Of course, both of these can happen in non-chimeras too! (Read more about what causes two different eye colors here.) Someone may not know that they absorbed their twin, since the distribution of cells from their twin throughout the body can be random. If you’re a chimera, a DNA test might be able to reveal your chimeric status. For example, if your blood has cells from your “twin”, while your saliva has your own DNA, you should be able to detect this. If you compared the DNA in your saliva to the DNA in your blood, the samples might appear to be from “siblings” instead of the same individual. However, maybe your blood and saliva both have your DNA, while your spleen has DNA from your “twin”. In this case, comparing blood and saliva would miss the second set of DNA. You’d have to get a sample of tissue from your spleen to detect your twin’s DNA.

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So it can be really hard to detect chimerism. You might have to get testing for a lot of other tissues as well. Sometimes people do find out they’re a chimera accidentally. There have been a few cases of a parent “failing” a DNA test, when the child is truly their own! A child will inherit DNA from the cells that make sperm or eggs. And generally a parental DNA test collects the parent’s DNA from a cheek swab (or blood sample). If the sperm/egg cells have DNA from one “twin”, while the cheek cells have DNA from the “other twin”, a DNA test will say that the person is the child’s aunt or uncle. Sometimes people find out they’re a chimera accidentally, when they “fail” a parental DNA test. The most well-known example of this is Lydia Fairchild. She knew for a fact that her child was biologically hers … she gave birth, after all! And yet, a DNA test said she was not the child’s mother. Eventually, it was proven that she was a chimera with cells from her long-lost twin.

You can read more about Lydia Fairchild's fascinating story here.

Conclusion

There are many different types of chimeras, and they may be more common than we think. Most often, an individual's health is not impacted by chimerism. However, finding out that you are a chimera would definitely give you a new perspective on how DNA really is unique to each individual!

thetech.org - How common are chimeras? | The Tech Interactive

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