n 2018, a woman went public discussing her unusual birthmark on her abdomen. After years of questions, she had finally gained an explanation for her birthmark – chimerism. She reported that doctors were never able to explain why half of her abdomen was a different color from the other half. Most doctors just said it was an unusual birthmark. Finally, in 2009 it was explained that her birthmark is actually a result of two different genetic cell lines within the same person. When this woman was in the womb, she was a twin. That twin died very early on in pregnancy and was absorbed into this woman, which produced two different set of genetics in the same woman.
Chimerism is the fusion of two different cell lines, or two genetically distinct types of cells. In Greek mythology, the chimera was a creature with the head of a lion, the body of a goat, and the tail of a serpent, representing a mix of different species. In humans, chimerism was mainly discovered through blood typing. Some people were found to have more than one blood type, which was thought to have occurred from non-identical twins who shared a blood supply in the uterus. Approximately 8% of non-identical twins are chimeras.
A common example of chimerism in our daily life is the chimera cat, or “two-faced cat”. These cats have one color scheme on half of its face/body and a different color scheme on the other half. These halves are typically split right down the middle of the cat. Similar to chimera humans, the chimera cat has two sets of genetic cell lines due to two embryos fusing together in the womb. Calicos and tortoiseshell cats are often mistaken for chimera cats, but actually look the way they do due to a genetic variant which is carried on the X chromosome.
Tetragametic chimerism is when the presence of two genetically different cell lines are discovered in various organs in the body including the gonads, or sex organs. Often chimerism is discovered due to a baby being born with both female and male sex organs or ambiguous genitalia. There is also another form of chimerism called microchimerism, which occurs due to the blood connection between a mother and a fetus. It is thought that the genetic instructions of mother and fetus can sometimes fuse together.
Referring back to the woman reported in the news, she reported that because she has two sets of genetically different cell lines, her body views her twin’s genetic cell line as foreign. As a result, she has been diagnosed with an autoimmune condition with allergies to foods, medications, etc.
Back in 2002 there was another woman reported to have chimerism after needing a kidney transplant. After testing her family members for possible donors, it was discovered that she could not be the mother of two of her three sons based on the genetic results. Doctors then discovered she was a chimera, and her genetics in her blood was different from other organs.
If you are interested in learning more about chimerism or are interested in genetic testing, please contact AT-GC to schedule an appointment with a genetic counselor.