The Mind-Blowing World of Human Chimeras

Maggie Koerth-Baker is a guest blogger on Boing Boing. A freelance science and health journalist, Maggie lives in Minneapolis, brain dumps on Twitter, and writes quite often for mental_floss magazine.

One person outside: But two people "inside": That's the gist of the chimera, a human being who carries the DNA (and sometimes the body parts) for two. It sounds crazy, but it happens. In fact, doctors think it probably happens more often than we realize. Unless there were some reason to test the DNA from cells in different parts of your body, you could easily be a chimera and never know it. Happy Freaky Friday, everybody.
So how's it happen? In this excerpt from my book, Be Amazing, I explained how chimeras happen, and how confusing it can be to be one.

First: Get That Meddling Sibling Out of Your Way
Imagine you're a fertilized egg, just a few days old. There you are, floating around the womb and minding your own business, when, BAM! You run smack into another just like you. Well, not just like you. But certainly close enough to be a threat. Now, you have a choice. You can roll over and let yourself be born as just another fraternal twin, or you can stand up for your individuality and absorb the interloper. Naturally, you do the smart thing, and nine months later your parents take home one healthy baby.

Then: Discover That They Aren't As Dead As You Thought
Like a horror-movie villain locked into a three-picture contract, your twin never really died. Instead, she'll end up hiding in plain sight--within your very cells--rendering you a chimera, a single human who carries the genetic makeup of two different people. Most of the time, there aren't any outward signs that your body is harboring a stowaway. But when you do notice, things get a little crazy. Take Karen Keegan, who discovered her chimera-ness at age 52. When Keegan needed a kidney transplant, she and her two adult children underwent DNA testing to figure out which kid's kidney would be the best match for mom. Surprisingly, the tests showed neither. In fact, according to DNA, Keegan's children weren't her children at all. The case confounded doctors for more than two years until, in 2000, the docs finally realized that Keegan's blood cells carried different genes from the cells in her ovaries---the long-absorbed twin was found.

Perhaps you're wondering whether chimeras can incorporate twins of two different sexes. The answer is yes, and the results are often much stranger. In 1998, Scottish doctors reported treating a teenage boy for an undescended testicle. But when they put the kid under the knife, no second testicle could be found to pull down. Instead, where the ball should have been, doctors discovered an ovary and fallopian tube. Chimera strikes again.

For some fun further reading, check out the story of Lydia Fairchild. Like Karen Keegan, Fairchild's chimeric nature was discovered after DNA tests said she wasn't the mother of the children she was pretty sure she remembered giving birth to. Unlike Keegan, however, Fairchild's kids were still young and the initial DNA test almost cost her custody.

Much like Professor Xavier of the X-Men, illustrator Michael Rogalski is locked in deadly, psychic battle with his evil, chimeric twin.


  1. I’ve read about women who were found to have some random cells with male DNA in them because they had given birth to boys and some of their son’s cells had stayed behind and survived.

  2. the most common form of a chimera is part goat, part snake, part goat and part fox… news.

  3. The Lydia Fairchild story is chilling and a good example of how courts and the legal system can be fallible. Saying that DNA is “100% foolproof” is stupid. Nothing is 100% foolproof.

  4. I have a chimera friend.

    The top half of his eyes are blue, and the bottom half brown.

    He has to take immune-system suppressants to stop his body rejecting various organs, so he always has a cold.

    1. This blows my mind. I have identical twins, 11 months old. One of them has a half-and-half eye. Does that mean we were supposed to have triplets and the one absorbed the third?

  5. My son is a liver transplant recipient. He was had his new liver for 24 yrs. he has been enrolled in a study to see if he can stop using the anti rejection medication and see if his body adapts to the new cells in the liver….chimerism at it best

  6. I’ve always been partial to the legendary Esquilax, the horse with the head of a rabbit, and the body of a rabbit.

  7. A friend of mine underwent a complete bone marrow transplant to treat a dangerous blood disorder.

    Her new marrow came from stem cells harvested from her brother and infused into her own blood stream.

    She now has her brother’s blood flowing through her body.

    I guess that makes her a chimera.

  8. A fairly common example of this is a calico cat. The orange spots have a diferent DNA makeup than the white parts.

    Another example is probably Corky, the guy with Downs syndrome, from the TV show Life Goes On. His trisomy 21 probably didn’t occur until later on and thus only a part of him has Downs. Thus he appears, and is, much more high functioning (can act, memorize scripts) than the vast majority of people with Downs syndrome.

  9. @8

    Dr. House knew what it was all along and just didn’t bother to tell anyone until the end of the episode, because of how he is such a smug, yet (supposedly) lovable, jerk.

  10. This explains The Great Roe: “The great roe is a mythological beast with the head of a lion and the body of a lion, though not the same lion.” (Woody Allen, from Without Feathers)

  11. One of the US cable channels aired a program about chimerae about 2 years ago, but I can’t find a link to it. Anybody else remember?

    Was probably Discovery or TLC, but I don’t remember.

    Fascinating stuff…and makes your head spin thinking of the possiblities and problems that it raises.

    1. Discovery fit & health is where I just learned about chimeria. They did a special about Lydia Fairchild with Karen Keegan’s story in it as well. Maybe what you watched before?

  12. #6 – From your description of the esquillax, either you’ve got a normal rabbit or a rabbit with the neck of a horse.

    I need pictures, please.

  13. Discovery Health Channel just reaired a documentary about chimerism the other day. It’s called “I Am My Own Twin.” This is why I minored in biology – the human body is so fascinating!

  14. #12 posted by skabob, May 1, 2009 8:50 AM
    #6 – From your description of the esquillax, either you’ve got a normal rabbit or a rabbit with the neck of a horse.
    I need pictures, please.
    Isn’t that used in the Comcast commercial? A genetically modified jackrabbit, ridden by an overly caffinated fighter pilot, blah-blah-blah…

  15. How awkward it must be to learn that you not only have a phantom twin, but that she’s been sleeping with your husband.

  16. When originally nabbed for doping in 2005, former pro-cyclist Tyler Hamilton claimed the strange blood in his system came from his chimeric twin:

    His case was rejected and he was suspended from cycling for two years.

    Surprising no one, he recently left cycling for good after testing positive again and finally admitting to doping the whole time.

  17. I have to agree w/ dculberson – the description of the Fairchild’s ordeal – from the callous/abusive social workers on through the legal system and the way she was treated by state officials (irrespective of hospital records, character witnesses, and even direct observation)… is quite sobering.

  18. Chimeras are crazy and mind blowing and all, but I’m still just chuckling at the people trying to dissect a Simpson’s quote.

  19. I WANT to be a chimera!. My liver is failing and I need a transplant. I often wonder what it will be like to have 2 pounds-ish of someone else’s DNA.
    If you’re not an organ donor. Please consider it. There’s a lot of people waiting for organs… but not enough to go around.

  20. Normal females (including tortoiseshell and calico cats) are mosaics, not chimeras. Very similar, in the variable expression of X-linked traits, but from a single DNA profile.

  21. I think I’m a chimera… My hair on my head, eyelashes, etc. is blond on one half and brown on the other. And it isn’t a pigmentation thing because my skin is fine, my eyes are the same color. It’s just my hair.

  22. #9 and #30 – I believe Corky is also probably a mosaic, not a chimera. Trisomy mosaics are not uncommon, and in fact mosaics are able to survive with trisomies (e.g. trisomy of chromosome 16) that are otherwise completely lethal.

  23. I heard about this on WNYC’s RadioLab a while back. That show does for me the same thing Boing Boing does, makes my mind do double takes.

  24. Fun example in the Venture Bros where Dr. Venture’s Chimera twin leaves the body to exact revenge and take his place as the true son of Dr Jonas Venture

  25. Chimeric humans have been made as a way to eliminate the use of anti-rejection drugs. You take bone marrow from the organ donor and implant it into the transplantee, then subject them to radiation. The chimeric immune system doesn’t reject either the original body nor the replacement organ, so no drugs are required.

  26. I recently started writing a sci-fi story about a girl who had two sets of DNA and had to time travel backwards to find out why. I had never heard about chimeras as double DNA, only as ‘animal’ creatures. Endless possibilities here …

  27. @ 9, 23, 30: X chromosome inactivation in “normal” females is not the same thing as mosaicism. X-inactivation results in the formation of Barr bodies (condensed chromosomes) and accounts for tortoiseshell coat color in cats (along with the expression of many other X-linked traits in humans and other animals), but all of the cells in that individual have the same genotype.

    True mosaicism results when chromosomes fail to segregate properly during embryonic cell division. The individual has some cells with a typical chromosome number, and some cells with an atypical chromosome number (i.e. an aneuploidy).

  28. Chris Burke is not a chimera. Knowing him, but without ever personally asking him (because it ws a neither here nor there question…it did not matter, in other words), I do believe he has Mosaic Down Syndrome. MDS is different than Trisomy 21 Down Syndrome in many ways. Chris is very smart about politics and movies and he is in a band with twins. He is a nice man and a bit different than his acted role of Corky.
    Chimerism, like twins, can be inherited.
    I have a son, diagnosed with mosaicism 50%, but I believe it is chimerism; Dad ppears to be one also. At first, I thought I was going crazy (and frankly others did also) because kept thinking my husband was a twin and playing tricks on me. Long story short, when my son was born, he started what they call “shape shifting” also. Others saw it as well;he is like a chameleon. Anyways, I really startedto wonder about myself UNTIL I caught it on film with someone else taking the photos. We did a remake of a portrait for my son and compared the two which were taken in the same month. WOW!!! He looks like TWO DIFFERENT PEOPLE!!! One looks like Trisomy 21 and the other looks just like a normal kid. Weird, but the full moon has an effect on him (and my husband) as does mood and environment (kinda like a chameleon). I asked the geneticist to test for chimerism, but he (at that time) said it was not and was just Mosaic Down Syndrome 50%. Husband has not been tested due to lack of insurance and adequate $$$$ because those testsare too expensive for our budget.I am pretty darn sure they are both chimerics, especially after all this and then I found out twins are common in his family. I wish there was a way I could post the pictures of my son, but no upload button here.

  29. would i be a chimera if one half of my eye is greenish yellow and the rest is blue, i have a extra wisdom tooth and index wrinkle on my pinky finger???? just wondering

  30. hi everyone,i was pregnant with fraternal twins and at about 15weeks they said one had vanished,months later my son was born and i think hes a chimera,he has 2 totally different faces,one side is smaller than the other,his ears dnt match at all,head on both sides is a different shape and so are his eyes…anybody else have a baby or themselves have this going on? hes only 4 months old now…im wondering if he grows out of this or its permenant…thanks

  31. I am a chimera with 2 hearts that work independently of each other and 3 sets of teeth. I have been growing new body parts for the last 7 yrs and wes the full moon can make great changes in my body at times like in peru. my body in having problems working with all the changes.
    thanks for being there katrena

  32. I think that my daughter might be a chimera. I watch the program a few years back on Discovery/TLC (can’t remember) and at the time was shocked that alot of the things they talked about was things I had noticed on my daughter. She has a diffenate line in skin color down her torso one side being darker than the other. She’s been told by a doctor that one leg is shorter than the other, one butt cheek is higher the other is wider. She’s always been very athletic and has very strong body perspiration; she has to use mens deorderant, womens just doesn’t do it. She is now pregnant she’s having some difficulty. She is past due and she isn’t dilating. They told her that her cervix is not placed right; The baby is way to high and 93 percent chance she won’t deliver on her own that she will have to have a c-section. Does any of this sound like she could be a chimera.

  33. Sounds like most of you have medical students’ syndrome… You believe you or someone you know has every single thing you read about. Genetics are complex. Just because someone has more than one thing abnormal about them or a few “symptoms” of chimerism doesn’t mean they are a chimera. It is more likely that they have some other sort of genetic mutation that is far more common. A single mutation can have many effects.
    #1 – There is no way to know what “the most common type” of chimerism is when we have no idea how often it occurs and it has only recently come into light.
    And Barr bodies do not make a female a chimera. All the cells contain the same genome, but one of the two x chromosomes is condensed. This condensation of the chromosome prevents the genes it carries from being transcribed but they are retained in every cell of that females body (aside from the germline). A chimera must hold two different genomes, which is obviously not occurring in females.
    Most of these comments suggesting chimerism are dealing with pigmentation in the eyes or skin. This is most likely due to the silencing of a gene, and this phenomenon is called position effect variegation. The genome is identical in all the cells, but some cells fail to express certain genes because the DNA for that gene in those particular cells is too tightly wound to be transcribed.

  34. i was told by a friend that i am a chimera because i have different colored eyes. it has been bothering me to find out more so anyone that has any suggestions of links or any basic information about chimerism please email me at thanks ~stella~

  35. On April 24, 2010, we welcomed a beautiful baby into our family. 2 hours after he was born that he has Chimerism. I have exhausted resources trying to find parents who have been where we are. We’re facing surgeries and a lot of doctors appointments. As a mom I’m scared of the uncertainty of my baby’s future. If any knows who I could contact PLEASE HELP. I’ve been told by our specialist our baby may only be the 2nd known case with a split gender chimerism.

    1. We have a beautiful baby girl that was born in June, 2009. We didn’t know anything was unusual when she was born. We were told she had to be taken to MCG Medical Hospital as we were getting ready to bring her home. She hadn’t had any problems, but they told us they weren’t sure if we had a boy or a girl. We were shocked, and the docter had terrible bed-side manners. There were no doctors at the hospital that she was delivered at that had seen a case of chimera first hand. We have been through 4 major procedures to determine if there could be testicular tissue mixed with her ovarian tissue, and there is on one side, at least. I was hesitant to respond, because I feel our daughter will be just fine. However, I quess people are just in-humane. We have been searching sites to find out more information on chimera, and people are just rude on their responses to the condition. I went to the extent of using a different e-mail, because people act like this is a joking matter. Our daughter is just perfect for us, and we will keep searching to find the best options to save her good ovary and keep her healthy. It looks like it will be a long wrong road, but I believe she will be just fine.

  36. I have a brown “wedge” in only one of my blue eyes which I read somewhere is one sign of Chimerism. Is this so? Is there any relationship with Chimeism and emotional/mental disorders or is this an “unknown” area of science? i.e. could the two different sets of DNA have mental effects?

  37. i have been seeking info about my weird eyes & I came across this article…My eyes are both two different colors, one is almost all blue with like a quarter brown, the other half blue half brown, my torso has a “line” where my upper half is a lghter skin tone than my bottom half…guess i might be a chimera…

  38. all this sounds close to what me and my son have.
    the only thing with us is our eyes change the different colors. it happens every month. they will change three colors.

  39. I have just completed a DNA paternity test where it came back negative, everybody who knew about the test is convinced the result is wrong. Including the other putTIVE father.

    My family has had a high volume of twins and i have been told if this is the case the chance of me being a chimera are higher in comparison to somebody from a family with no twins.

    Is it possible i am a chimera my mum is an unidentical twin, does this also make the chances higher?

  40. My scores that matched were

    D16S539 – 5.35
    D18S51 – 1.52
    D21S11 – 2.18
    D3S1358 1.05
    D5S818 – 1.30
    D8S1179 – 2.41

    All the other parts came back 0.01 or <0.01.

    It is impossible for me too be related to the child but the above shows the scores , do these results indicate a relationship

  41. Just because you have two different sets of eye color or hair color does not mean you are a chimera. Silencing genes and barr bodies could be responsible. Whoever said that they didn’t discuss barr bodies; there was a reason for that. A barr body does not make a you a chimera because the X chromosomes come from the same zygote. In order to be a chimera you have to have chromosomes from at least two different zygotes. Not every calico cat is a chimera it simply depends on which X chromosome is inactive due to the barr body. Seriously some of you need to research before you just post whatever thought you feel like and try to pass it off as if you know what you are talking about or what the article is talking about.

  42. I’ve noticed there are some people that are seemingly very knowledgeable about this subject that have posted comments here and I have a few questions that I’m hoping someone can answer.
    First here is the back story:

    I do know that I started out in a twin pregnancy but ended up as a single birth due to a Vanishing Twin. This wasn’t discovered until I was 12 years old, in 1982. I had become very ill, and actually came very close to not surviving. I underwent emergency surgery to remove a mass found on my ovary through ultra sound. Days later when my doctor came to my hospital room to delivery the results of the pathology report he explained that the tumor was called a Teratoma/Dermoid tumor and that it was benign and that it consisted of hair, teeth, calcium, bone fragments and other various types of things. I was of course completely freaked out and thought what he was describing sounded like the grossest thing I could even imagine. He then asked my mother if she had been aware that she was carrying twins during her pregnancy with me. She said No, but that twins run in her family and that her father, my grandfather was a twin. Even at my young age I suddenly understood so many things that had confused me. Oddly, or maybe understandably, the whole twin issue was just sort of overshadowed by the fact that I had just nearly died and was now recovering from major surgery. Over the years when ever it was discussed it was always referred to as a tumor that “could have possibly been what started out as a twin, maybe”. I knew though. Looking back, I always had known. I’ve even wondered if there was a possibility of something more profound concerning my early childhood imaginary friend, who was my constant companion until one day when I wasn’t interacting with her my mother asked me where she was and I just flatly said, “she left.” And that was it, she had vanished.

    So, that’s the story, and here are my questions:

    1. Is it possible to be a Chimera if the vanished twin is contained within one of these tumors?

    2. Does anyone have any information on if there have been any studies of Chimera playing a role in Auto Immune Disorders such as lupus. (which I was diagnosed with at age 15)

    3. If a woman is a Chimera and she has a child can that child also be a Chimera regardless of if the pregnancy included a vanishing twin event…in other words can one be a Chimera simply by inheriting the trait from ones Chimera mother?

    Thanks in advance for any answers.

  43. oh my gosh. i am so surprised. when i went through these comments i realized one sign mentioned and remembered i have a line down my arm showing different skin tones… am i a chimera or doesn’t this necessarily mean i am a chimera? someone please answer before i go crazy jumping to conclusions!

  44. 2 years ago I had to have tests done after they discovered I only had 1 kidney…after further tests I was told I have 2 sets of the whole reproductive system…2 wombs 4 overys etc etc all mirrored next to each other…in fact I could carry in both wombs at the same time…amazing
    I was not told anything else in fact after all the tests I was just left to feel like a strange freak but all healthy and working fine…it was not till I spoke to a friend that they mentioned chimerism and I found this site…Horray I now have a name for my freakism…lol
    I would love to speak to anyone who knows about this…..thanks

  45. I have a friend who had two sets of female reproductive stuff. Twins run in her family. She’s had two very normal healthy pregnancies but she was told it might have been possible for both to get preggers. Everything else is normal.

  46. I have an eye that is half brown and half blue. And one of my ear lobes is attached and the other is unattached.


  47. I am in fact a true chimera. Up until 2003 the medical professionals I dealt with assumed I was some unknown type of genetic related intersexual (they had no clue what I was until this point). It wasn’t until 2003 that they figured out what I actual am. They did a complete DNA profile on me –they assumed me to be a mosaic type of hermaphrodite until this test confirmed that I actually have 92 chromosomes. My chimerism is extremely rare because one set is male and the other set is female. Generally chimera’s are two same sexed embryos that co-mingle and infuse into a single body.

    In my case, I am a set of differently sexed twins which expressed physically as a true hermaphrodite. My DNA reads as 46XY + 46XX. The report says “True hermaphrodism by way of chimerism.”

    You have no idea what sort of medical mess this creates from such complicated genetic soup. Be thankful you are a single set of normative 46 X?…because the alternatives are not a blessing. A case of Trisomy 21 means down’s syndrome and that is only one small quirk in an otherwise normal set of DNA. Imagine all the health disorders that happen when you have 46 extra chromosomes giving your body competing messages. I have struggled with all sorts of life-long serious and life-threatening issues as a result of this one condition. I will never have a normal life on any level as a result of a birth defect that I have no control over. I can’t see how anyone would see that as a blessing or an advantage.

    It is a mere assumption that being chimera means that you have a redundant system that gives health advantages. The truth is my XX cells often attack my XY cells as foreign invaders which causes deathly health related drama in my life! It is literally a war of the sexes a genetic case of sibling rivalry. But seriously, the idea that the tissues tolerate each other is somewhat of a myth. Some do but some do not and as a result a person like myself suffers much and is far higher risk of cancers and auto immune disorders. The blood seems to have cross immunity but not all cells in the body react that way.

    There is so little know of my specific condition…with only 40 known of cases in the world that are documented –it is not as if there is a huge historical repository of information. Maybe chimerism is more common and undetected with a same-sexed twin sets…it would be virtually impossible to know this without wide spread DNA testing…but they have only know human chimerism was possible since 2000. They stumbled onto this by accident due to abnormal tests with fertility clinics doing IVF. A case like mine would be more evident because of the expression of non-normative external organs..but in cases of same-sexed chimerism…it could be very common.

    I suspect that people like myself tend to die before they ever have a chance to be born because of the many very complicated health issues involved. The reason cases like Lydia and Karen make the news is that it is fascinating in an “Oh that is so frikkin’ freaky cool sort of way”. But someone like me– well my situation is not so glamorous and so nobody wants to speak about it or advertise it. I certainly don’t see the Opera show knocking on my door, or the doctors, or Barbara Walters….or Dr. Phil…and yet there are less than 8 cases out of 40 world wide of differently-sexed chimerism (hermaphrodite expression). I guess this is just as well the world isn’t ready and would be far to judgmental and I do not need the freak label applied to me life is already to hard as is. I am not sure a talk show could do me justice or give me any dignity for the diversity that is in my DNA…not sure it could be handled gracefully.

  48. Anon to #66

    Thanks so much for the information. Our daughter was diagnosed with split gender chimera in 2009. (a few days after birth) We have had to make decisions to remove testicular tissue in hopes that her body will not have hormones fighting against one another and to prevent cancer. We have decided not to operate on any external organs, because we feel she’s perfect and it should be her decision. I don’t feel any two people are alike, but I’m not sure if a slight difference in her external organs will be upsetting for her. Would you mind telling me your age and how you were told about the condition? I want to make the right decisions for our child, but there is so much that is unknown. I have found very little information and a lot fewer cases than you. I think people may think this condition sounds scary, but we just have to do the best with what the Lord gives us. Our daughter is now 2, and she’s been very healthy. She seems to be a girly-girl, but I don’t know what she wills feel like inside as she grows up. She has that 46xxxy make-up, and I quess we will just have to wait and see.

Comments are closed.