Photo: Ivan Samkov

What is a chimera baby?

Chimerism occurs when a woman is pregnant with twins and one embryo dies, and the other embryo absorbs the twin's cells. (Scientifically speaking, this type of chimerism is called tetragametic because the baby was derived from four gametes – one egg and one sperm for each embryo.)

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When Taylor Muhl was born, her mother was told that her baby’s two-toned skin was a birthmark. Now at 33 years old, Muhl, a musician and model, understands her two-toned skin is actually a result of a genetic condition called chimerism, which causes an individual to have two genomes, or sets of DNA. Chimerism occurs when a woman is pregnant with twins and one embryo dies, and the other embryo absorbs the twin’s cells. (Scientifically speaking, this type of chimerism is called tetragametic because the baby was derived from four gametes – one egg and one sperm for each embryo.) Chimerism doesn’t usually have many signs or symptoms, but if people with the condition aren’t diagnosed an unexpected outcome may occur when they have children. A baby born to an individual with chimerism can be more distantly related from a genetic perspective, as if they were a niece/nephew. Most individuals with chimerism have no idea they have a second genome. Consequently, most remain undiagnosed throughout their lifetime, partly because the condition is not well known in the medical community, and partly because there is no population-wide screening practice. Most often a diagnosis doesn’t occur unless someone fails a paternity or maternity DNA test, and they seek out answers as to why. Without a diagnosis from a medical professional like a genetic counselor, chimerism can have ramifications for individuals and their families including losing parental rights or financial support. Though not common, Muhl’s noticeable two-toned skin led her to a diagnosis of chimerism. This blog discusses other signs of chimerism and what to do if you suspect you may have it.

What traits are possible signs of chimerism?

People with chimerism rarely show visible signs of their condition. Only some may have physical signs such as two different colored eyes, two different skin tones, patches of different colored or textured hair, or a disorder of sexual development.

Though rare, individuals with chimerism have also reported the following:

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Originating as a twin pregnancy, but the other twin did not survive (vanishing twin syndrome)

A failed parental (paternity or maternity) DNA test for unexplained reasons

[1] Identifying as transgender

Left-handedness or being ambidextrous [†]

Having two blood types

How common are the types of chimerism?

The majority of individuals with chimerism remain undiagnosed, so we are not sure how many people have this condition. The prevalence of diagnosed cases of chimerism depends on the specific type. The most common form is fetal-maternal microchimerism, where the fetus and mother exchange cells (through the placenta), which can remain in circulation in the mother decades later. One study found nearly 50 percent of female chimeras had Y-chromosomes (presumably from their sons) detected in heart biopsy tissue. It is worth noting that this study was not able to detect fetal cells from daughters, so it is likely a significant under-estimate. Tetragametic chimerism, where a twin pregnancy evolves into one child, is currently believed to be one of the rarer forms. However, we know that 20 to 30 percent of singleton pregnancies were originally a twin or a multiple pregnancy. Due to this statistic, it is quite possible that tetragametic chimerism is more common than current data implies.

What does a chimerism diagnosis mean?

If chimerism goes undiagnosed, it may result in false negative parental DNA paternity tests. In fact, there are several documented cases of a mother or father with chimerism nearly losing parental rights. A diagnosis of chimerism can also aid in the mental health of individuals who are experiencing a crisis of identity. In Taylor Muhl’s situation, now that she has had her condition confirmed, she understands she may fail standard DNA maternity tests. If she has children, it is quite possible that she could conceive them with eggs that contain her alternate genome. If this happens, she could test as the biological aunt to anyone she gives birth to. Muhl is now dedicated to growing awareness of chimerism, to help avoid negative outcomes for undiagnosed chimeras.

What should you do if you suspect you have chimerism?

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If you think you may have chimerism, you might consider contacting a genetic specialist such as a genetic counselor. Genetic counselors who specialize in chimerism can evaluate your medical history, interpret your standard parental DNA test results, design a chimerism-appropriate testing strategy and interpret the new results. Most often, a person’s health is not negatively impacted by chimerism. A diagnosis may, however, change your life by gaining insight into otherwise unexplained events.

Chimerism: A clinical Guide. Springer Nature (2018). https://www.springer.com/us/book/9783319898650

A Case of Chimerism-Induced Paternity Confusion: what ART practitioners can do to prevent future calamity for families. https://link.springer.com/article/10.1007/s10815-017-1064-6?wt_mc=Internal.Event.1.SEM.ArticleAuthorOnlineFirst#citeas I Am My Own Twin. Model and Musician Taylor Muhl Comes Out As A Chimera. https://www.huffingtonpost.com/entry/i-am-my-own-twin-model-and-musician-taylor-muhl-comes_us_5a47141ee4b0d86c803c76ca

Acknowledgments

A special thank you to Taylor Muhl for her contribution to this post, and for her efforts to raise awareness about chimerism. Kayla Sheets, MS, LCGC, is a board-certified genetic counselor and the founder of Vibrant Gene Consulting, LLC. She is specialized in novel uses of DNA testing to detect chimerism and to confirm relationships (such as maternity/paternity) in complex cases. [1] When chimerism also involves the sex chromosomes (X and Y), brain cells can have different sex chromosomes than other cells in the body, which is theorized to contribute to having a transgender identity. [†] Left-handedness is more prevalent in multiples, thus a left-handed singleton may have a higher likelihood of having been a twin pregnancy.