What is Chimerism?

If you recall your high school biology class, you might remember learning two key facts about DNA: first, that it serves as the "code" for all our genes, and second, that each person has a single, unique set of DNA. Otherwise, an individual could have two different blood types, which scientists once believed was impossible.

However, high school biology didn’t cover everything. Some people actually do have two sets of DNA in their bodies – a condition known as chimerism.

Chimerism, a fascinating and rare biological phenomenon, occurs when an individual possesses two or more distinct sets of DNA. This genetic mosaic can result from the fusion of two embryos, the exchange of cells between twins in the womb, or medical procedures such as organ transplants. The term "chimera" originates, a creature in Greek mythology that's part lioness, part goat, and part snake.

This unique condition can manifest in various ways, from subtle differences like varied eye colors to more complex scenarios impacting immune function and disease susceptibility. While often undetected, chimerism can have significant implications for genetic testing, forensic science, and medical treatments.

What causes Chimerism?

People may experience one of several types of chimerism. Each has a slightly different cause and may result in different symptoms.

Vanishing Twin Syndrome

The most extreme form of chimerism happens when a twin dies early in utero and the embryo disappears, a scenario known as vanishing twin syndrome. In an intriguing yet logical twist, the surviving twin absorbs some of the deceased embryo's chromosomes, resulting in two distinct sets of genes.

But you don't have to have had a vanishing twin to be a chimera: Regular fraternal twins can also have the condition. Twin embryos sometimes trade chromosomes with each other in their mom's uterus, since they have a shared blood supply. In this case, both babies would be considered chimeras and would have multiple sets of DNA.

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.

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.

Bone Marrow Transplants and Other Organ Transplants

The patient's diseased bone marrow is replaced with a healthy donor's marrow. The new bone marrow will produce new blood cells. The recipient will now have blood that is made from the marrow of the donor. This is being made by someone else's DNA. So, the recipient's blood will now contain new DNA. Even the blood type may be different now.

How is it diagnosed?

The symptoms of chimerism vary from person to person. Many people with chimerism have no clue they have a second set of cells. Most can go their whole life without being diagnosed. There is no wide screening that is mandated, and many in the medical community still don’t know about it. Usually, there will not be a diagnosis unless someone fails some type of DNA test. 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

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.

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.

Genetic chimerism is a common occurrence in adult Ceratioid anglerfish which is a crucial component of their lifespan. When a male reaches maturity, he starts looking for a female. The male, which is less than an inch long, cuts on her skin and produces an enzyme that helps to digest both his mouth as well as her body's skin, fusing the two at a time to the blood vessel point. Although the male's survival depends on this connection, this will ultimately consume him as the two anglerfish merge into a single hermaphroditic entity.

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