A rare medical case in Brazil involving a woman whose blood carried one genetic code while the rest of her body carried another has prompted at least one Israeli physician to reconsider how genetic testing is used and what may go undetected.
Professor Idit Maya, a senior physician and head of the Nephrogenetic Clinic and the Genomic School for Physicians at Rabin Medical Center, said understanding this kind of genetic abnormality could have significant medical implications. She told The Media Line that it may help advance approaches that reduce transplant rejection without relying heavily on immunosuppressive drugs.
Israel performs hundreds of organ transplants each year, including live kidney transplants. According to the Health Ministry’s National Transplant Center, 656 organ transplants were carried out in 2024. Official figures for the total number of transplants performed in Israel in 2025 have not yet been published.
The Brazilian case involved a 35-year-old woman whose blood tests revealed male chromosomes, even though she appeared entirely female. Doctors ordered a karyotype test, an analysis used to examine chromosomes for abnormalities, after she experienced a single early pregnancy loss. The test showed two distinct genetic lineages in her body, pointing to chimerism, a rare condition in which one person carries two different sets of DNA. In simple terms, different cells in her body were not genetically identical.
'This is like two gas stations, one next to the other, that are delivering all the good things that the mother has to bring to the fetuses. But we know that many times they are connected, they are very close together, and then they can be mixed'
Maya noted that it is unusual to order a karyotype after only one miscarriage. Most people never undergo such testing, she said, meaning similar cases often go undetected, even in Israel, where genetic testing is relatively accessible for other reasons. The woman had typical female anatomy and hormone levels and reported normal puberty, with regular menstruation beginning at age 13. Her blood results showed a 46, XY karyotype, which is typical for males. By contrast, skin, buccal and other cells carried a 46, XX karyotype, which is typical for females. Because all tissues originate from the same zygote, the initial cell that forms an embryo, doctors generally expect sex chromosomes to be consistent across blood, skin and saliva samples.
Chimerism is most often seen after a bone marrow transplant, when donor stem cells begin producing blood cells that carry the donor’s DNA. In this case, however, the woman had never received a transplant. She did have a twin brother, and further analysis showed that the genetic profile in her blood matched his. This indicated that her bloodstream was effectively functioning using her brother’s genetic material.
Maya said that when an unusual genetic pattern appears in a blood test, doctors typically suspect mosaicism rather than chimerism. Mosaicism is another rare condition in which not all cells are genetically identical, even though they originate from the same zygote. In mosaicism, only part of the genome differs, such as when some cells carry an extra chromosome while others do not. This pattern can be identified through karyotyping or more advanced genetic tests and is seen in conditions such as mosaic Down syndrome. The fact that the Brazilian woman’s entire blood genome differed from her other tissues made the case especially unusual. Maya explained that when male and female twins are conceived, they originate from two separate eggs and sperm. “They are like two siblings sharing the same womb,” she said. “They have the same mother and father, so their genomes are 50 percent similar.”
In fraternal twin pregnancies, there are two placentas. Because space in the womb is limited, those placentas can sometimes connect, allowing blood to pass between the fetuses. Maya compared it to two neighboring gas stations supplying nutrients from the mother. When connections form, she said, blood and stem cells can mix.
In the Brazilian case, stem cells from the male fetus entered the female fetus’ blood system and took over. Those cells implanted in her bone marrow and began producing blood cells. As a result, all of the woman’s tissues came from one zygote with one DNA profile, while her blood cells originated from another. In effect, her body developed as female, but her blood was genetically linked to her male twin. The woman lived this way for 35 years without any medical complications. If she had not miscarried, Maya said, she might never have discovered the condition. “She had nothing outside or inside to say that her blood cells are from her brother,” Maya said.
According to Maya, this unusual biology could be an advantage if the woman ever needed an organ transplant. Under normal circumstances, transplant recipients must take lifelong immunosuppressive medication to prevent rejection. Another option, now used in Israel, involves transplanting a kidney along with bone marrow from the same donor to induce immune tolerance and potentially eliminate the need for anti-rejection drugs. In this case, Maya said, the situation might be simpler. Because the woman’s blood cells already match her brother’s genetic profile, the risk of rejecting an organ from him could be significantly lower, possibly without requiring an additional bone marrow procedure.
'Many times, we have to make a decision, do we test both if we think they are identical twins? Sometimes we test only one. Why? Because we say it’s the same DNA. Why should we test it?'
Genetic testing is more common in Israel than in many countries, largely because certain inherited diseases, such as Tay-Sachs, are well known in specific communities and because couples often seek screening before pregnancy. Israel also conducts genetic testing in specialized laboratories to establish family relationships for legal or administrative purposes. One of the most common prenatal tests is amniocentesis, which analyzes fetal DNA. It is also frequently considered in twin pregnancies, which are relatively common in Israel due in part to high rates of in vitro fertilization. “Many times, we have to decide whether to test both twins if we think they are identical,” Maya said. “Sometimes we test only one, because we assume it is the same DNA. Why should we test it?”
The Brazilian case, she said, suggests that such assumptions may need to be reconsidered, as biology does not always follow expectations. Maya added that there are intrauterine procedures designed to limit blood sharing between twins, because extensive sharing can be dangerous. In some cases, one twin receives too much blood while the other receives too little, making medical intervention necessary.
After reading about the Brazilian case, Maya began to wonder how often similar situations occur without being detected. “Maybe it happens more frequently, and we don’t know,” she said. She added that as diagnostic technologies continue to advance, physicians should think more carefully about what to test and how many gaps in medical knowledge may still exist. Shortly after her miscarriage, the Brazilian woman became pregnant again and gave birth to a healthy child.