In a cold night of December 2002, a woman named Karen Keegan received a disturbing phone call from her doctor. Karen at that time was in urgent need of a kidney transplant and all her family members including her three sons have had their blood tested for histocompatibility. But her doctor hadn’t called to confirm a match. Instead, he informed her that two of her sons, out of three weren’t her sons at all! Yes, her children carried their father’s DNA but not her’s and according to the DNA tests – she was her children’s biological aunt and not mother.
But Karen knew that they were her children, because she had conceived them and had given birth to them. How can she, then be their aunt? Despite of what the tests said.
On the other hand, the doctors were equally adamant that Karen couldn’t be the biological mother of those boys, despite of what Karen claimed!
For the next two years a deep medical research was carried out on this case. The doctors took samples from Karen’s hair, her blood and her cheek and yet none of her DNA samples matched with that of her two sons.
It was a medical mystery!
A few years before this, Karen had undergone an operation to have a thyroid nodule removed from her throat, and it was this tissue that finally solved the mystery.
Her thyroid matched her son’s genetic code!
So, Karen Keegan was in fact, the fusion of two different indivisuals, since she carried the genetic code of two separate people. While her blood, hair and kidney had one type of DNA, her other organs such as the thyroid and possibly a good number of her gametes carried another set of DNA.
How is a chimera formed?
According to Greek mythology, Chimera is a fire breathing monster, which is part goat, part lion and part snake.
In genetics, this name Chimera has been given to an animal that has two or more different set of DNAs within it. Or in other words “Chimera” is a single organism that has the genetic coding of two or more different individuals. Chimeras are way more complex and weird than any other phenomenon ever seen in the branch of genetics. It is biologically claimed to happen when– a fetus absorbs the cells of it’s dead twin.
This can happen in case of faternal twins present in the mother’s uterus. If one of the embryo dies very early in the pregnancy, and the other live embryo absorbs some of the cells of its dead twin – then the suriving fetus will carry two diffrent population of cells with completely different set of DNAs. One will be it’s original set and another will be belonging to the deceased twin. By the time the surviving foetus’s immune system develops, it has many cells of each type of genome and therefore, the immune system recognises both type of genome as its own and thus, doesn’t attack or destroy the second type of genome.
Therefore, Karen Keegan carried a part of her own twin while her children carried the DNA of a woman who had never been born!
Chimeras formed due to organ transplant:
Chimerism can also be seen in persons who had undergone a bone marrow transplant. In several cases of Leukemia, it is often used as a treatment. Bone marrow actually consists of hematopoietic stem cells which give rise to all types of myeloid and lymphoid blood cells. So, when cancer develops in these cells leading to un-controlled growth of defective cancerous cells, they start taking up all the space within the bone marrow and start affecting the production of normal blood cells.
In order to stop this type of cancer, the defective bone marrow of the patient is destroyed and then replaced with the bone marrow of a donor with matching histocompatibility. Which means that for the rest of the patient’s life, he or she will have the blood cells carrying the DNA of the donor’s bone marrow, while the rest of her organs will have her original set of DNA.
Now, it can either be a complete chimerism, where all the blood cells will have the same DNA as that of the donor or, it could be mixed chimerism – where the patient will have both – their own cells as well as the cells of the donor.
Chimeras formed due to pregnancy:
In one of the commonest ways, a chimera could be formed when a mother is pregnant. During the fetal development cells are swapped between the mother and the fetus during the natural flow of nutriens across placenta. It was first discovered in a woman with failing liver, that suddenly started regenerating itself.
Later through extensive research, doctors found that those regenerating cells of her liver had the DNA of the stem cells from a pregnancy at least twenty years earlier. During her pregnancy, a few of the foetus’s stem cells got carried away into her own blood stream and somehow survived for the next two decades in her body. These however had logged themselves in her liver and differentiated into liver cells.
During the 1990s a study was carried out on women pregnant with male foetuses, and who had died during pregnancy or post partum and it was discovered that all of them carried some traces of Y chromosome that they had received from the male fetus in their uterus.
Later in another study even a 94 year old woman was found to have traces of cells with Y chromosomes that she had received from her son, during her pregnancy almost 50 years earlier.
Conclusion:
DNA is what makes us, the unique individuals that we are, with all our specific quirks, looks and talents. Somehow encoded into a microscopic molecule, it miraculously controls every single aspect of sorting lives, starting right from the day 1, when a zygote is formed to the multi cellular, invariably complex organism that we become. But the complexity doesn’t end there, does it? Like the chimeras- in strictly genetical terms they are not one but two indivisuals.
Sometimes, Chimerism can be detected in people who have two diffrent skin tones, hair color or each of their eye having a diffrent color. But in most cases, there are no physical signs of chimerism. Take Karen Keegan for example, who found out after 60 years of her life that she had a fratenal twin in her mother womb, a sister who was never born and whose DNA she and her sons carried.
According to a study carried out in New York, in every 10 single births, 8 started out as twin pregnancies. This only proves that although there are very few documented cases of chimerisim, there are many among us who are unknowingly carrying a part of someone else along with them and will most probably will never know about it.
References:
- Britannica encyclopedia
- Rogers, Kara. “Chimera”. Encyclopedia Britannica, 3 Dec. 2018, https://www.britannica.com/science/chimera-genetics. Accessed 3 October 2021.
- https://www.insider.com/what-is-a-human-chimera-and-how-does-it-happen-2017-11
Well done?. This story is written so well, It’s fascinating. Thank you for writing this! I love it so much!!!
This was a very interesting read. But would it be considered artificial chimerism if there was an organ transplant? Biology is weird.