Diagnostic dilemma: Woman had her twin brother's XY chromosomes — but only in her blood

image of an X chromosome next to a Y chromosome on a blue background — A woman had XY sex chromosomes, but only in her blood cells.

(Image credit: BSIP / Contributor via Getty Images)

The patient: A 35-year-old woman in Brazil

The symptoms: The woman was referred to a hospital following a miscarriage at seven weeks of pregnancy. Leading up to the hospital visit, the woman's gynecologist had decided to check the patient's chromosomes to see if there was an underlying genetic reason for the pregnancy loss.

The doctors then conducted a physical examination of the patient. Her visible traits were female, and they found no abnormalities in her genitals or reproductive system. Her hormone production was also within the normal range. As an adolescent, her sexual development had been normal, she told the doctors, with regular menstruation that began when she was 13 years old. There were no unusual health incidents in her medical history, and she was the first person in her family to be tested for a potential genetic disorder, the physicians wrote in a report describing her case.

This can sometimes happen after a blood transfusion or organ transplant, in which one person receives tissues (and thus DNA) from another individual. But chimerism can also happen naturally, when several fertilized eggs fuse in the uterus to produce a single fetus, or when cells and their DNA are "traded" in the womb. This swap can occur between a mother and an embryo, and sometimes between twins that share the same placenta or whose individual placentas interact in some way.

The patient in this case had a twin brother. When the doctors analyzed samples from the woman's brother and parents, they found genetic variants in the twin brother's XY blood cells that matched those in the patient's blood. They concluded that during fetal development, the woman had incorporated her twin's DNA into her blood cells while still retaining a different copy of DNA with XX chromosomes in the rest of her body's tissues.

What makes the case unique: In most cases of people with chimerism involving sex chromosomes, their condition carries substantial physical or reproductive issues. This is the first known example of complete blood chimerism in a woman who otherwise had typical female traits, had no health issues and was able to get pregnant. The case suggests that chimerism in humans is not necessarily a barrier to reproduction.

And notably, even the karyotyping test for identifying the woman's condition is not commonly performed. It is not part of a standard medical visit. Therefore, it is unknown how common it is for individuals to carry hidden chromosomal anomalies, and incidences of blood chimerism are likely underreported as a result, the authors wrote. Indeed, studies have suggested that other types of chromosomal variations, such as having extra sex chromosomes, are also underreported.

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Mindy Weisberger is a science journalist and author of "Rise of the Zombie Bugs: The Surprising Science of Parasitic Mind-Control" (Hopkins Press). She formerly edited for Scholastic and was a channel editor and senior writer for Live Science. She has reported on general science, covering climate change, paleontology, biology and space. Mindy studied film at Columbia University; prior to LS, she produced, wrote and directed media for the American Museum of Natural History in NYC. Her videos about dinosaurs, astrophysics, biodiversity and evolution appear in museums and science centers worldwide, earning awards such as the CINE Golden Eagle and the Communicator Award of Excellence. Her writing has also appeared in Scientific American, The Washington Post, How It Works Magazine and CNN.