It appears to have two faces; the fur color is perfectly divided in half, and they can even have eyes of different colors, making it one of the most fascinating creatures ever seen. You may have seen photos of cats with these characteristics on the internet, without knowing that this phenomenon is known as “chimerism.” But did you know that there are documented cases in humans?
The Mystery Behind Chimerism
It’s a genetic disorder in which an individual possesses cells from two or more different individuals. In other words, their bodies contain two distinct sets of DNA.
The name “chimerism” comes from the mythological chimera of Greece. This beast, the offspring of Typhon and Echidna, roamed the regions of Asia Minor, causing panic among populations and devouring domestic animals. It was recognized for its hybrid appearance, resembling a combination of a lion, a goat, and a snake or dragon.
But chimerism is not confined to mythology; it’s more real than we think, although many experts are unsure how many human cases exist in the world. The only certainty is that it’s a very unusual condition, although it may be becoming more common due to certain fertility treatments like in vitro fertilization, although nothing is proven.
To date, only about 100 cases or a little more have been documented. However, in animals, it’s more common, often presenting distinct features on one side of the body. This is because females can be fertilized by multiple males.
Characteristics and Types of Chimerism
There are different types of chimerism that can be present in a single individual. Each type has a cause and can produce different symptoms:
Twin Chimerism
The most extreme form occurs when twins are conceived, but one of the embryos dies in the womb. The surviving fetus absorbs cells from its deceased twin, giving it two sets of cells: its own and those of its twin.
Microchimerism
This is the most common type and occurs when a pregnant woman absorbs some of her fetus’s cells, or conversely, the fetus absorbs some of the mother’s cells. These cells can travel through the bloodstream of either and migrate to different organs, remaining years after childbirth.
Artificial Chimerism
Similar to microchimerism, but it occurs when a person receives a blood transfusion, a stem cell transplant, or a bone marrow transplant. The individual can absorb some cells from the donor.
Tetragametic Chimerism
In these cases, chimeras develop when two different sperm fertilize two different eggs. After the cells merge into a human embryo, crossed cell lines are formed.
Documented Unusual Cases
In 2002, a team of experts published the mysterious case of Karen Keegan in Boston, USA, in the New England Journal of Medicine. This woman was about to donate a kidney to one of her children. However, when tests were conducted to confirm compatibility, a startling discovery was made: the child was not hers, even though she had given birth to them.
This was simply impossible for Keegan to accept, and she underwent further testing, revealing that the DNA in her blood was different from that in her ovaries, where cells with different genetic material were found. These cells were the ones that developed into the eggs from which her children were born.
During the same year, an American woman named Lydia Fairchild came close to losing custody of her children after divorcing. Routine DNA testing indicated that she was not the mother, even though she had given birth to them. Subsequent examinations showed that she had the potential to be their aunt.
The Case of Taylor Muhl
Taylor Muhl is a 39-year-old American model and singer born in Hollywood. She had always noticed that her body was unusual, starting with the fact that her abdomen had two different skin tones that divided it perfectly. Similarly, she had a double tooth on the left side, many sensitivities, and allergies to various foods, medications, supplements, jewelry, or insects.
By 2010, different doctors had told her that there was nothing wrong with her, and the other skin tone on her torso, dividing it in half, was simply a “birthmark.”
However, that same year, another doctor gave her a different diagnosis. According to their explanation, in the 1980s, when she was conceived, ultrasound was not a common practice, and her mother opted for a natural birth.
She gave birth at home with the help of two midwives. During pregnancy, her mother was perfectly healthy, so there was no need for an ultrasound. For that reason, they didn’t realize that after fertilization, two zygotes had merged and formed one with two different types of cells, each with a different genetic makeup.
Taylor Muhl was born as a result of chimerism, and having two types of DNA in her body had brought her some health difficulties that she preferred to keep private. However, she did mention that her condition created a constant internal battle in her body, as she had two immune and circulatory systems.
In general, doctors explained that her body treated the second genetic composition as “foreign matter,” which negatively affected her immune system and caused the various allergies and intolerances she suffered from. Additionally, she also had Eagle syndrome, endometriosis, gastroesophageal reflux, migraines, neuropathy, chronic fatigue, and other conditions.
Muhl’s case is perhaps the most documented case of chimerism in the world, and while it’s not the only one, it has surprised both experts and ordinary people. Now, you know the reason why there are such adorable kittens with “two faces.”
References:
- Genetic chimerism a new paradigm for legal medicine – An article by Quirós Alpízar, José Luis; Alpízar Miranda, Kattia E.
https://pesquisa.bvsalud.org/portal/resource/pt/lil-637483
