A human chimera

March 4, 2018 • 10:45 am

Reader Tom Alves called my attention to the singer Taylor Muhl, a human chimera. Chimerism is the situation in which an individual results from the fusion of two early-stage fertilized eggs (zygotes), and thus has the genetic constitution of two separate individuals. This is a very rare condition: geneticists estimate that there are fewer than 100 people on Earth known to have it. That’s probably a big underestimate, though, as it’s often hard to diagnose. As Live Science explains:

Muhl has a type of chimerism called tetragametic chimerism. This can happen in cases of fraternal twins, where there are two separate eggs fertilized by two separate sperm, and the two zygotes “merge and form one human being with two different cell lines,” said Dr. Brocha Tarshish, a clinical geneticist at Nicklaus Children’s Hospital, in Miami, who is not involved in Muhl’s case. This happens very early in embryonic development, Tarshish said.

Most of the time, people with chimerism probably go undiagnosed, Tarshish said. Indeed, without specific biomedical tests (such as genetic testing), it’s impossible for doctors to tell that a patient is a chimera, according to a 2009 paper about the condition. But there may be subtle clues to this condition: Some people with chimerism have “patchy” skin coloration (like Muhl does) or different-colored eyes, the paper said. In some cases, chimerism is diagnosed when a person is found to have two different blood types.

Muhl discovered her condition when she noticed a difference in pigmentation on her torso that runs right down the middle, as shown in this picture from her Instagram site:

She also noticed, and you can see this on some pictures on the internet, that features on her left side are larger than those on the right. This suggests that the twins differed in genes affecting body size. (She also has a double tooth on the left side of her mouth.) I see that in her eyes and nose:

Finally, she’s been ill most of her life. That’s explained by the fact that she’s a fusion of two different people with genetically different tissues (since she’s fused with her sister’s embryo, the two sides share only half of their genes), so her immune system from one twin is constantly trying to reject the foreign tissue of the other—and there are also two immune systems.  Apparently Muhl has an allergic reaction to some metals on one side of her body but not the other.

Further, Live Science, the Independent, and The Daily Mail (actually, the Mail‘s story is decent), report that Muhl has two different lymphatic systems and “two bloodstreams”. The circulatory system thing I don’t understand, for Muhl has only one heart, and that has to pump all the blood through her body. That means that the circulatory systems, if they are genetically different, still have to be fused into one functional system. She’s also reported to have two blood types.

Sometimes human chimeras are detected through paternity tests: a woman’s somatic cells, which are sampled to get her genotype, might be from one zygote, while her ovaries, which produce the eggs, could be from another. That means that her children would share only 1/4 of her genes instead of 1/2, something that could be detected genetically. The American woman Lydia Fairchild is one of these cases. She separated from her husband, and having had three children with him, had to take a paternity test when she and her ex-husband were dealing with child support. The tests showed that while her ex-husband was the father of all the kids, she didn’t seem to be. Although her kids were almost removed from her, they eventually found out that Ms. Fairchild was a chimera: her skin and hair cells didn’t match the genotype of her kids to the required degree, but her cervical cells did. Fairchild was a woman with two different genotypes.

One wonders if a male twin could fuse with a female, and produce a double-sex person. I know of no such cases, though male cells from a woman’s own fetus can be incorporated in parts of a woman’s body—a phenomenon called “microchimerism.” As Scientific American reported:

A 2015 study suggested that this happens in almost all pregnant women, at least temporarily. The researchers tested tissue samples from the kidneys, livers, spleens, lungs, hearts, and brains of 26 women who tragically died while pregnant or within one month of giving birth. The study found that the women had fetal cells in all of these tissues. The researchers knew that the cells were from the fetus, and not from the mother, because the cells contained a Y chromosome (found only in males) and the women had all been carrying sons.

Here’s a video of Muhl from a television show that tells a little bit more of her story.


92 thoughts on “A human chimera

  1. That is absolutely 100% bonkers bananas. I had no idea this condition exists.

    I hope David Cronenberg just read about this. I assume he didn’t know because, if he did, there would already be a movie about it. Dead Ringers 2?

    So, what happens if a human chimera needs a blood transfusion? Is it impossible? Can they accept one of the two blood types? Combining the two blood types?

      1. Should be, but there are other blood cell surface antigens that can have an effect on transfusion acceptance apart from the well-known ABO typing system.

        1. Yes. But I don’t think those are even checked in most blood transfusions because they are less of a problem. Not sure why that is, but perhaps those are other antigen markers that are less abundant so they do not cause much a a reaction.

          1. For pretransfusion testing, a “type (that’s the ABO/Rh) and screen (testing for atypical antibodies)” is performed. If antibodies to red cell antigens are detected, then the donor unit is tested for the corresponding antigens. Most people don’t have atypical antibodies. These antibodies can be formed in response to blood exposure, via transfusion or pregnancy.
            In fraternal twins, about 8 percent have two different red cell lines, due to engraftment of stem cells in the embryonic stage.
            Typically, this extremely early exposure to different cells leads to immune tolerance so it is no problem.

            1. I was going to comment the same – that her immune system must be fully tolerant to antigens of both cell lines, like in the fraternal calf twins in the pioneer work of Owen.

  2. Very interesting and she does state in the video that she has two bloodstreams.
    How indeed can this work with one heart?
    I would be fascinated to know or is there a simple explanation.
    Beyond my expertise that is for certain.

        1. Perhaps it means the circulatory system is slightly different on each side of her body, which could be described as “two” circulatory systems, combined to be one whole.

    1. I think it more likely means that she has one circulatory system, and two genetically different populations of blood cells in circulation, from different regions of bone marrow that produce blood cells.

      1. My thought too. The lymph nodes (very important components of the immune system) might also be differentiated between one side and the other.

  3. Way back in 1977, an Oxford biologist studying and cloning frog embryos (as I recall) told me about the lady who became sunburnt in patches, `two-tone’, over her body: a chimera. He suggested that there were probably two fathers in this case… long before DNA testing.

    1. This might also be a consequence of Lyonization, where different X chromosomes were inactivated at the early embryo stage. The calico cat phenomenon!

      1. Scratching my head, thinking back to 1977, the differential sunburn was the result of two skin pigmentations suggesting fathers of very different skin colouration..

      2. I was thinking along those lines. Also, since only one X chromosome is active in any female cell aren’t all female mammals in some sense chimeric? Isn’t mosaicism chimerism from a single zygote?


  4. this is a CSI nightmare, and has already happened in a few legal cases. I have been covering chimerism in my Forensic Biology course for many years at Ohio University. Sometimes students think that I am making it up, LOL.

    1. Do you have a more accurate estimate for the prevalence?
      “100 people known to have it” would imply at first glance one person in 70 million. But if it’s 100 people in “western world”, it’s more like one person in 10 million, and if it’s 100 people in the US, it’s one person in about 4 million. Quite a difference.

      1. Chimerism is not rare, but rarely discovered, because it seldom generates any observable anomalies. I quote a part of a 2007 EMBO REPORT but it is worth reading the whole thing excluding the silly first two paragraphs:

        A chimeric human, or human chimera, has two or more populations of genetically distinct cells that originated from different zygotes. Cells with the ‘extra’ genotype might be found in any part of the body, but have not yet been found mixed through all tissues of a body. The extra cell line(s) can be acquired through transplantation, transfusion or transfer of fetal cells into the circulation of the mother, but most frequently chimerism occurs spontaneously during embryogenesis. In vitro fertilization apparently increases the likelihood of chimerism as a side effect of the increased frequency of twinning in all pregnancies involving artificially induced ovulation. Except in the case of feto-maternal transfers, spontaneous human chimeras are di-zygotic twins whose bodies have incorporated cells of both genotypes during their embryogenesis from a single mass of cells.

        A significant portion of the population might be affected by chimerism, with repercussions for transfusion medicine, transplantation and forensic science. “About one-eighth of all conceptions and about one-eighth of live births are twins—the majority of whom are born alone without a live twin. About one in eight of everybody walking around is a twin who was born single,” said Charles Boklage, a developmental biologist at the Brody School of Medicine at East Carolina University (Greenville, NC, USA) who has studied chimerism for more than 25 years. For the public, the idea of carrying around parts of an unborn twin is not easy to accept. “Look up the definition of ‘chimera’ in an ordinary dictionary. A major fraction of the definition has to do with something fanciful, imaginary, mythical. It has a really big ‘yuck factor’ for most people, to imagine a substantial fraction of us are made of cells from what might have been two people,” Boklage explained.

        For the public, the idea of carrying around parts of an unborn twin is not easy to accept

        The frequency of spontaneous human chimerism might have been greatly underestimated, because such people are usually discovered only by chance. “When both cell lines in a chimeric individual are normal and of the same sex, there is nothing to make us investigate them. They are totally indistinguishable by ordinary observation from people with one genotype,” Boklage said. “The first few [cases] reported were found in blood banks, when genotype testing showed three or four versions of each of several genes instead of the expected one or two.” Even in those cases, the condition often goes unnoticed unless the second cell line is present at a proportion of 25% or more. “It has shown up when a surgical patient had a dramatic reaction to a unit of blood with a small fraction of a chimeric second line—the wrong kind of cells,” Boklage said, referring to a case in which a transfusion of mixed cells caused an acute intravascular haemolysis (Pruss et al, 2003).

        In 2001, transfusion medicine specialist Willy Flegel and colleagues at the University Hospital in Ulm, Germany, reported on a chimeric blood donor whose mixed blood had transformed the RhD antibody profiles in some transfusion recipients (Wagner et al, 2001). Their discovery led to changes in the policy for routine blood screening at German blood centres in 2002, and later in Austria, Switzerland and the Netherlands, and sparked discussions of possible changes in routine blood testing elsewhere in Europe and North America (Flegel, 2005)

        1. Very interesting, thanks.
          Prevalence versus detection is always a point to remember. Absence of evidence is not evidence of absence – but checking a dozen or a thousand samples does permit one to put limits in the likely prevalence.

        2. “About one-eighth of all conceptions and about one-eighth of live births are twins…”

          This surprises me! Double ovulations or identical twins, one wonders?

          Also, I suppose there are identical twin chimeras, but how would you tell? 😉

          1. I’m in that slightly fuzzy state that occurs halfway through the 2nd bottle of a beautiful British wheat beer & you hit me with the concept of “identical twin chimeras.” You are a cruel woman! I tried to ignore you because I’m halfway through 3rd season, Episode 1 of Homeland on Netflix UK [Homeland is new to me believe it or not]…

            QUORA SEZ…

            1. Well, thanks for the research!

              If I’m reading the Quora responses correctly, it sounds as if they’re referring to sets of identical twins being born with both babies being chimeras. I was thinking of a single baby, the result of one identical twin zygote fusing with its womb-mate. I’m easily confused, however.

              If they do mean twin babies who are both chimeras–would that mean that the, er, litter started out as identical quadruplets?


  5. Several of you brought up her comment as to having separate bloodstreams; of course, that is impossible, she is not a biologist, what she was making reference too was two blood types

    1. The part of blood that defines blood type:

      What part is it?
      Where – what organ[s] I mean, make that part? I can imagine the marrow being different in the two halves of the body for example.

      Cheers Zeus

      1. ABO blood type refers to particular cell surface antigens, which are molecules that the immune system can recognize and respond to using antibodies. The marrow makes blood cells and might well produce different blood types, as you surmise, if the marrows are genetically distinct, as can be the case in bilateral chimerism.

  6. If a male egg fused with a female egg then I would expect the result to be male, since the sex is determined by hormones circulating in the blood. Since there is a Y chromosome the resulting hormones would produce a male.

    1. Ummm, I bet it’s not that simple. It’s biology, things rarely are that simple.
      Ah – does the occurrence of at least one problem in a recent Olympics from a “female” athlete being found to have an XXY genome illuminate the question? I’m not going to claim knowledge to interpret the genetics/ biology of such cases, and I may have mis-heard the sport news as I scrabbled for the tuner to move to an interesting channel.

      1. There are a few women who have a Y chromosome, but this is very unusual. I think it is something to do with not having a certain receptor (for testosterone?) on the cell membrane. Usually a Y chromosomes does result in a male.

  7. It sounded cool at first, like she could be a super hero, “The Chimera” but then I read this part, “she’s been ill most of her life.”

  8. I don’t quite get why she has health problems from one immune system trying to reject the ‘foreign’ tissue.
    It has been a very long time since I had immunology, but from I had been told the immune system is “educated” to accept ones’ cells as part of you during development. Based on experiments in animals, doing grafts between two different embryos results in the graft being accepted since the immune system, when it develops, learns to accept both of them.
    But if her claim is true, then obviously the view I describe is a bit wrong.

    1. That was what I understood as well. And I have not heard of any other chimeras having similar problems. (That does not mean none of them do, of course)

      1. Gotta say, for all its cheesiness the original ST had music bits that were damn good.
        Would make a good addition to a workout mix tape.

      2. The Star Trek episode that comes to my mind is about the alien society where one race is black on the left side and white on the right side and the other race is coloured the opposite way round.

    2. Many complications. My daughter is a chimera twin. She has x & y chromosomes..2 sets of dna…has the red ‘birthmark’…immunology issues, mast cell activation sydrome…etc etc. Learned of chimerism voa Rare disease dept at Vanderbilt…yesterday! This answers YEARS of questions for us

  9. Disgraced American cyclist Tyler Hamilton once tried to escape blood doping charges by claiming he was a chimera.

  10. I am surprised so few people seem to have heard of chimerism since there was a House episode that dealt with it. (Very inaccurately, IIRC)

    I have read about a XX/XY chimera that had both male and female organs. A baby that was conceived by IVF. It was a very long time ago though and I cannot give a link, I’m afraid.

    1. There was also an episode of CSI (the original series) where chimerism explained the inability to DNA match the suspect.

      1. I’ve never seen an episode of CSI (but the genus of programme is common enough…), but I’m going to guess that the alleged perpetrator committed the offence with his penis/ right hand/ head of one genotype, while the DNA samples and/ or blood were take from the left hand/ arm hair or a chimerically different part of the body. Or something similar.
        Blood samples shouldn’t be different – they’d be a mixture of the two gnomes. If the chimerism involved any of the active blood-forming bones. Which change through life, so there’s a potential plot twist.

        1. I can’t abide any of the CSI TV thingies, but a quick Google reveals that in one episode a rapist almost walks free until crime scene investigators realize he is a chimera: the semen found at the crime scene came from his unborn twin. [see link in my other comment]

        2. “they’d be a mixture of the two gnomes.”

          What, are we bringing dwarfism into now? 😉


          1. I really need to replace the keyboard on this laptop. But since the casing of the screen is also cracked … I’ll probably just use it until it dies totally.

    1. Good comment! Identical twins by evangelical doctrine should have one soul between them, endowed at conception and then segregated during embryo division. The second twin is just faking it. In a chimera, one soul probably eats the other. I find it curious that Taylor Muhe’s linked video essentially claims that her isolated created soul destroyed that her sister. An alternative view is that the “soul”, unitary self-awareness, is a epiphenomenon of neurological integration and comes about at the end of embryological development.

  11. When I read the subject line, I had no idea what to expect as I’ve only heard of the mythical chimera. Thanks for the education of this strange biological phenomenon.

    Creationists: please explain.

    1. Creationists: please explain.

      Ummm, is the phrase [crickets] appropriate?
      More crickets.
      I don’t intend to hold my breath waiting for an answer.

    2. She says in the video you have to work with “what God gave you.” So she believes this is His doing. Hrmm…

  12. actually, the Mail‘s story is decent

    Perhaps surprisingly, the Daily Mail’s science coverage is pretty good. I know from what appears in my own research field that they will often carry pure science stories that the “quality” papers (Times, Guardian, Indie, Telegraph) just don’t.

    1. I was thinking – “I know what President Trump would think, if he understood this.”
      Glad to see I’m not the only one who can scrape that barrel.

  13. Doesn’t everyone have one hand, one foot, one leg, one arm, one eye a little bit larger or longer than the other, usually the right side? Place your palms together and see if one of your hands is a wee bit longer than the other. 🙂

  14. What if a woman were inseminated by two different men producing two zygotes, one from each man, that then fused?

    And what if these two men were Donald Trump and Jason Alexander?

  15. Is her brain also split with the same symmetry? If so does that mean the left side of her body is controlled by the right side of her brain and the two are not from the same person?

  16. “The tests showed that while her ex-husband was the father of all the kids, she didn’t seem to be.”

    Say what???

    (You probably meant to write “parent” rather than “father.”)

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