One Person, Two Sets of DNA: The Strange Case of the Human Chimera

In Depth
One Person, Two Sets of DNA: The Strange Case of the Human Chimera

In 1953 a woman known only as “Mrs. McK” entered a blood clinic in northern England. She was there to donate: it was a routine trip, a common gesture of goodwill, but the act would permanently alter Mrs. McK’s perception of herself as well as genetic knowledge of what constitutes an individual body. After Mrs. McK donated her blood and perhaps ate a cookie and drank some juice, she sorted herself out, returned home; in all likelihood she believed that her day had been unremarkable. And for her, it had been. But the piece of herself that she had left at the clinic—that bag of blood meant for a stranger—would have a dizzying journey.

Mrs. McK’s blood was sent to a lab where it would, of course, be screened for blood-borne diseases and cleared for transmission into another body. It was in the lab that the local clinic doctor found something puzzling about the donation—the blood seemed to be a mixture of two types. The doctor double-checked to make sure that the sample was indeed from a single person, then double-checked to confirm that it been handled and transported according to procedure. Stumped as to why Mrs. McK’s sample could contain two separate blood types, the local doctor sent the specimen to Robert Race and Ruth Sanger, specialists at the Medical Research Council Blood Group Unit in London.

Sanger and Race also puzzled over the sample: How could one person carry two separate blood types? As far as both of these experts knew, that was biologically impossible. But then it occurred to Race that only a few years ago a colleague had published a paper about fraternal twin cows who also carried two separate blood types; the twin cows both carried each other’s blood in their separate bodies. Race determined that Mrs. McK must be a twin, and proposed that the twin’s blood had infused into her body during gestation where it continued to circulate decades later.

Race made these determinations with just a small piece of Mrs. McK, writing an entire history of a woman from her blood, allowing her genetics alone to map an entire life. He was right: later, Mrs. McK confirmed that she had indeed been a twin. Her brother had died at the age of three, some thirty years prior.

According to scholar Aryn Martin, Mrs. McK was the first identified human chimera. In particular, she was a blood group chimera, an often undetectable “condition” that is generally found among twins and in those who absorbed a twin while still in utero. Before Mrs. McK, chimerism had been observed and even induced in lesser mammals and plants, the results often of human intervention: gene splicing to create newer, better variations. But human chimeras were thought to be non-existent. But Mrs. McK was a discovery, and her very existence produced a kind of existential angst.

In a letter to the renowned immunologist Peter Medawar, Race wrote of Mrs. McK’s “truly begotten blood,” a turn of phrase nearly overburdened by its religious overtones of creation. “Isn’t it extraordinary,” Race continued “to be able to group a person who has been dead for 30 years!” In turn, Medawar wrote in his essay The Uniqueness of the Individual, “There is no telling how long Mrs. McK will remain a chimera, but she has now been so for twenty-eight years; probably, in the long run her twin brother’s red blood cells will slowly disappear, and so pay back the still outstanding balance of his mortality.”

“The still outstanding balance of his morality”—it’s a string of words both coldly observational and yet deeply reflective of the contentious relationships between individuality, life, and genetics. Almost thirty years after a little boy’s death, Medawar conjures up a ghost, raises Lazarus from the dead because his genetic material continued to pump through the body of his living sister. But who was Mrs. McK if she is not simply herself? Where did her body begin and the body of her long-lost twin end?


Chimera D’Arezzo, c. 400 BCE. Creative Commons

The answer, simply, is that Mrs. McK was a chimera.

The original Chimera was born of a terrifying family tree: the child of “grim Echinda,” the half-nymph, half-snake mother of monsters. The Chimera counted the Gorgon and Lernean Hydra among its monstrous siblings, and it supposedly roamed the countryside of Asia Minor, terrorizing the Lycians, bringing volcanos, shipwrecks and “snorting out breath of the terrible flame of bright fire.” Homer described the Chimera in the Illiad as, “a thing of immortal make…not human, lion-fronted and snake behind, a goat in the middle.” The Chimera and its subsequent incarnations was destined to be a composite, born as it was to a mother made of clashing biology herself.

Echidna, that “speckled skin” monster, Hesiod wrote in Theogony, “a nymph who dies not nor grows old all of her days,” assembled her children in her womb like a Surrealist montage. The Chimera and its siblings was a fusion of animalistic body parts, arranged by the already disparate parts inherited from its mother, stitched together by a biology whose only consistency was its lack of such. It’s telling that one of the most terrifying vision the ancients Greeks could evoke was made from unrelated pieces of many animals, a creature who had no singular identity, who eluded easy classification in the animal kingdom. A goat or a lion are, ostensibly, not frightening in and of themselves; but cobble them together and suddenly the creature evades description, is absolutely petrifying.

Mrs. McK functioned in the same way. The detection of her chimerism and the theoretical resurrection of her dead brother produced a kind of person that had been previously known only in fanciful images of deformed humans, largely from the Renaissance: bodies that carried on their person the parts of a twin that hadn’t fully separated or been absorbed in utero. As a descriptor, chimera had largely been relegated to the laboratories of botanists and experimental geneticists.

Why did scientists use such a grand term for their splicing and dicing? The chimera suggests the sublime, and perhaps, simultaneously, an acknowledgment of the fear and surprise they associated with over their own capabilities. In 1984 British geneticists produced a chimeric “geep” created by combining embryos from both goat and sheep. The sterile, lonely geep lived until adulthood. And in the twenty first century, scientists have produced a number of rabbits whose blood contains human cells. The descriptions of chimeric animals, carefully created by mixing parental cells, are bland and often dry:a Margaret Atwood dystopia rendered in the scientist’s adopted language of neutrality and observation.

But if geeps and chimeric mice and human-infused rabbits are the vanguard of stem cell research, there’s still reassurance to be found in their rooted animality. Individuality—or, at least, knowledge of the self as unique and autonomous—is likely something lower mammals have never concerned themselves with, or if they have, it’s not a concept they are able to articulate. For us, individuality remains the most human of human concerns.


Fortunion Liceti and Gerardus Blasius, De monstris, 1668. Padua. Wellcome Library, London.

Most human chimeras were, at one time, twins. Current theories posits that genetic chimeras develop spontaneously when fraternal twin embryos fuse or when one twin absorbs the other. The absorption process is called Vanishing Twin Syndrome, a haunting phrase to describe the ingestion process. There are also smaller degrees of chimerism—microchimerism—that can arise from organ transplants and blood exchanged by twins in utero. Very rarely, the DNA of a child lost in utero can turn a mother into a chimera, perhaps a manifestation of grief; in a defiant refusal to let go, her body will absorb its tissue, retaining her loss and invisibly altering her body into a kind of living memorial.

Human chimeras, unlike their laboratory-produced counterparts, make us question what is natural for an individual human being. For all of our postmodern jargon about what constitutes an individual and the understanding that identities are flexible and performed, some deep part of us acknowledges that genetics are fixed. We allow that, with the exception of identicals, it’s genetics make you uniquely you. It’s the ultimate out for the postmodernist, a secret hideaway of the most human ideology: I am who I am who I am.

And genetic material is a proxy for the unique individual. We have forged a link between genomes and individuals, a single genetic system matches with a single body. The genetic trace of a single, unmatchable body is common enough that its phenomenon is reduced to the spectacle of prime time television. CSI, Law and Order, pick your procedural drama: all trade in the dramatic identification between one body, one unique DNA profile. Miniscule flecks of your person—skin cells, saliva, and semen—are definitive pieces of a puzzle. It’s a concept of the individual employed as easily by television writers as it is by the state. For the state to identify familial bonds, it reduces an individual to the genetic traces of a miniscule bodily deposit.

Take the case of Lydia Fairchild, a pregnant woman who was asked by the state to prove the paternity of her two children after applying for public assistance. DNA tests confirmed that the father was who Fairchild claimed it was—but that she was not the childrens’ mother.

Fairchild was accused of illegal surrogacy and welfare fraud. The court threatened to remove her children from her. The Washington state court also ordered that an immediate DNA test be taken of her child at birth, and the results also did not match Fairchild’s DNA. She was saved by lucky timing: an article published in the New England Journal of Medicine, outlining the case of another chimeric woman. Fairchild’s lawyer demanded that she be screened, and testing eventually confirmed that she was a tetragametic chimera, meaning that she carried two strands of DNA, the result of two sperm implanting with two eggs.

Fairchild’s father later said on a Discovery Channel documentary, I Am My Own Twin, “I believed my daughter, but at the same time I also believed the law had something against her. I believed that somewhere, she did something, she was in trouble or something. I have always had faith in DNA.” This speaks broadly to the faith we have in genetics and the determination of humans to find an unshakable ground of individuality, as well as the idea that conventionally conceiving children grants you a kind of biological immortality. The natural biology of Lydia Fairchild negates this essential concept of passing on your individual self, through DNA.


Willem Piso, Engraving of an Animal, Part Camel, Park Red Deer, 1658. Wellcome Library, London.

The eventual outcome of Fairchild’s case was determined by a woman named Karen. In 2002, Karen was looking for a potential kidney donor and, like most loyal children, her three sons immediately offered up their organs, but none were a match. “Your sons’ blood does not match your blood,” Karen recalled her doctor telling her, “And that’s an impossibility. So they couldn’t be your children.” Bowled over by the realization that her three sons—children that she had carried and birthed—were not her children, Karen sought more answers from her physician. Dr. Margot Kruskall explained to Karen that she was a tetragametic chimera. “In other words, she was destined originally to be fraternal twins.” Kruskall later told NPR. Karen had, unbeknownst to anyone, a vanished twin, another half who had disappeared at an early stage of development, only to be absorbed by her living sister. And so Karen carried two distinct cell lines.

Kruskall’s choice of words feels ominous. To “destined originally” is for your identity to be formed before consciousness—for the self to be determined by biology. And certainly Karen herself felt that way. After learning that she was a chimera, she felt as though her bond to her children had been interrupted: “Telling my sons about this was the hardest part because I felt that part of me hadn’t passed on to them. I thought, “Oh, I wonder if they’ll really feel that I’m not quite their real mother somehow because the genes that I should’ve given to them, I didn’t give to them.”

In myth, Echidna, like Karen, like Lydia, could not control the parts of herself her children would inherit. Echidna might have looked at her children, found little of herself; she might have seen mismatched body parts and yet found them beautiful anyway. Maybe then, Karen’s expression of the foreignness of her body—the hint that she has no idea who she is (“not quite their real mother”) because her invisible set of genomes are not what she believed them to be—is what’s most fascinating about chimeras.

If the chimera still terrifies, it’s because the idea shakes an idea of ourselves to its core. That your body is not unique, that it might not even be yours but rather usurped by a ghostly twin who vanished long before any semblance of consciousness. That in utero, your tissue absorbed a twin, you ate your other half alive, ingested a person who overtook your blood stream, a person who secretly injected its DNA into yours—silently, sneakily, without anyone knowing that this body even existed.

There is a verse in the Old Testament Psalms, a tender poem written to praise God’s perfect creation: “For you created my inmost being; you knit me together in my mother’s womb. I will give thanks to You, for I am fearfully and wonderfully made.” To knit something that is both fearfully and wonderfully made—to find and join disparate parts, arm to body to neck, forming a textured whole. What is that other than a chimera?

Previously: A Brief History of Anti-Vaxxers

Read More:

Stefany Anne Goldberg, “Double Trouble” at The Smart Set

Ian Hacking, “Making Up People,” at the London Review of Books

Aryn Martin “The Chimera of Liberal Individualism: How Cells Became Selves in Human Clinical Genetics,” Osiris, 2007

Image via Tara Jacoby

Inline Feedbacks
View all comments
Share Tweet Submit Pin