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DNA -

Ever since DNA was first discovered towards the end of the 19th century, it has proven itself useful in more ways than we could ever possibly have imagined.

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Applications -

From convicting someone of a crime to identifying blood relatives, there are many applications of DNA sampling that are extremely important to modern life.

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Why is it so useful? -

DNA is so useful because, the overwhelming majority of the time, each individual is composed of cells that are made from DNA unique to them.

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Complicating matters -

Its value lies in the fact that DNA testing will point us to a single individual. This picture is complicated, however, by a genetic phenomenon that scientists are just beginning to understand.

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Introducing chimerism -

A genetic chimerism, or chimera, is a single organism that contains the cells of two different and distinct individuals. Its body contains two sets of DNA.

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Rare phenomenon -

Although it is extremely rare (only around 100 cases have been recorded in modern medical literature), chimerism can affect humans as well as plants and animals.

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Different types of chimerism -

There are four types of chimerism: microchimerism, artificial chimerism, twin chimerism, and tetragametic chimerism. Each has a different cause and potentially different symptoms.

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Microchimerism -

The most common way for microchimerism to occur is for a pregnant mother to absorb some of her fetus’ cells, or vice versa.

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Microchimerism -

If this happens, the new cells may be absorbed into the bloodstream of their new host and travel to the organs. They can remain in the body for up to a decade.

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Artificial chimerism -

Similarly, a person who receives a blood transfusion, a stem cell transplant, or a bone marrow transplant may incorporate some of the donor’s cells into their body.

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Artificial chimerism -

This is called artificial chimerism and, although it happens, it used to be more common in the past. Nowadays donated organs are treated with radiation to stop this from happening.

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Twin chimerism -

There is also twin chimerism, which tends to be more extreme. This occurs when one fetus in a set of twins dies in the womb and the surviving twin absorbs some of its cells.

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Twin chimerism -

Finally there is tetragametic chimerism. This occurs when two sperm cells fertilize two egg cells, and then these cells all fuse together to form a single embryo.

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Symptoms -

There are a number of possible symptoms of chimerism, although they vary from person to person and many chimeras notice no symptoms at all.

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Skin symptoms -

More common symptoms include hyperpigmentation or hypopigmentation. These present as either patches of darker or lighter skin across areas up to half of the body.

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Other visible symptoms -

Chimeras may have different colored eyes, for example one blue and the other brown, or both male and female sex organs. This can sometimes cause infertility.

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Non-visible symptoms -

Other symptoms include red blood cells that contain two or more sets of DNA, and possible autoimmune diseases, particularly those related to the skin and nervous system.

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Testing is rare -

Although it is possible to test for chimerism, many cases go undiscovered because the condition is so rare that people are not normally tested for it.

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Left to chance -

More often than not, chimerism is discovered by accident when a person undergoes genetic testing for an unrelated reason, such as an organ transplant.

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High-profile cases -

Chimera cases are so rare that they make the headlines, and often the reported cases are ones that were discovered by accident.

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Failed paternity test -

In 2015, the Independent reported on a 34-year-old male chimera who failed a paternity test because the DNA in his saliva was different from the DNA in his sperm.

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Failed paternity test -

It turned out that the man was a twin chimera, i.e. he absorbed his twin in the womb, and the genetic father of the child in question was that twin.

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Failed maternity test -

In another high-profile case, this time from 2006, a mother failed a maternity test because she was a twin chimera. The DNA in her egg cells was different from that in other cells in her body.

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Failed maternity test -

She passed on those different cells to her children who, when tested, showed as being genetically unrelated to the woman who had given birth to them. You can imagine the confusion.

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Taylor Muhl -

More recently, in 2018, Californian singer Taylor Muhl revealed that she was a twin chimera. Her main symptom is having two different shades of skin on her torso.

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Interesting facts -

Now that you’ve heard some cases of chimerism in action, time for some fun facts. First up: male tortoiseshell cats are very often chimeras. Their coloration is the result of two fused embryos.

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Bone marrow transplants -

Following a bone marrow transplant, the person who received the bone marrow will have a mix of DNA: some of their own and some from their donor.

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Bone marrow transplants -

After a time, however, the DNA in their bone marrow may match the donor only. This is because bone marrow continues to regenerate.

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Pregnancy -

Although it is extremely rare, researchers believe that it is possible for microchimerism in the direction fetus to mother to occur in almost every pregnant mother.

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Pregnancy -

Finally, despite what you might assume, fertility treatments such as IVF do not increase the likelihood of a pregnant mother giving birth to a chimera.

Sources: (Healthline) (The Independent)

See also: A brief history of anaesthetic

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