The Washington state couple (whose names have been withheld for privacy reasons) initially thought that their child, conceived in vitro, had been created with a stranger's sperm.
What else could explain their discovery last year that their newborn son's blood type didn't match either of theirs? Or the subsequent paternity test confirming that the man of the pair was not the child's biological father? The mother became pregnant through artificial insemination, so it only made sense that the fertility clinic had made a terrible mistake and used the wrong sperm sample.
Except staff at the clinic told them that the 34-year-old father was the only white man who had donated sperm at the facility on the day the child, who appears white, was conceived, Buzzfeed reports.
In some ways, though, the couple was right. Their child did bear the DNA of another man - but he wasn't quite a stranger, at least in biological terms.
The DNA matched that of the man's unborn twin, essentially making the man the biological uncle of his own wife's child.
The couple was understandably confused when they received this explanation from genetic testing company 23andMe and Barry Starr, a Stanford University geneticist who runs the "Ask a Geneticist" blog on which the new parents first posed their query.
The familial definitions are a matter of genetic makeup. Starr told TIME: "It just leapt out at me: uncle. If it was a parent-child relationship, you would see 50 percent of the DNA related. If it is an uncle to a niece or nephew, it's 25 percent related. This man and his son are 25 percent related."
According to Starr's research into the matter, the baby's mysterious origins are a product of the genetic phenomenon of "chimerism," in which one fetus that dies early in pregnancy is subsequently "absorbed" by the remaining sibling. This process, called "vanishing twin phenomenon," is believed to be much more common than people think: a 1998 study found that one out of every eight single births begins as a twin or other multiple at conception.
The vanishing twin effect occurs when, instead of producing twins, two zygotes fuse into one. Starr believes that this is what happened to the Washington man, who ostensibly absorbed some of his fraternal twin's cells while in the womb, becoming a "chimera" - a spectral blend - of himself and his unborn brother.
The term comes from Greek mythology, which tells the story of a fire-breathing hybrid creature, the Chimera (or "Chimaera"), comprised of a goat's head coming out of a lion's back and a dragon's head coming out of its tail.
The name today is generally used to refer to the fantastical and imaginary, though, as it turns out, chimeras in biological science are both astonishing and real.
Previous chimera cases have been found in mothers who retained some of the cells of their children. Per the Daily Beast, chimeras can also arise through blood transfusions or organ transplants.
The Washington couple's case is extraordinary because the cells that the man took from his twin were germline cells - cells with the capacity to grow into eggs or sperm, TIME reports. Thus, the man's sperm took on a mixture of his DNA and his brother's, with his immune system recognizing both as "the self" because the absorption of cells occurred while the man was still inside the womb.
As a result, 90 percent of the man's sperm is comprised of his own DNA, and 10 percent is that of his twin. The man has another child whose DNA does match his.
"What is interesting is to explore in what way this affects the person's feeling of identity," Columbia University stem cell scientist Dieter Egli told TIME. "These are questions that are more important as we start to use cell therapies and cell transplantations as well."
In the U.S., over 382,000 legal paternity tests are ordered annually, usually with the most unexpected results pointing to infidelity. In this couple's case, however, their test results yielded even more perplexing questions about the man's relationship to his DNA, his unborn twin (who is also a part of him) and the child to whom he is biologically an uncle even though he or she was in many ways created with his sperm.
The implications of this conclusion are slightly eerie, as if to say that the man's unborn sibling continues to haunt his body - and now lives on through a child who shares more physical traits with the twin who never took a single breath than with him.
But while all this sounds like a salacious romance-turned-science-fiction plot, most people just aren't aware of the chimeras among them, or even if they might be chimeras themselves.
The man in this case has two-toned skin, a common mark of chimerism. As a child, he was so embarrassed by the marked difference in shades on his body that he told people he was a burn victim, according to Medical Daily.
Other chimeras may have two different eye colors, or even two sets of sex organs.
The medical curiosity is perhaps even stranger when it manifests in a woman, as Lydia Fairchild (also of Washington state) learned in 2003 after a test revealed that she was not her children's biological mother.
The results were inexplicable, the Daily Beast details, as Fairchild insisted that she had given birth to all her kids. She was applying for public assistance at the time, and the state accused her of fraud and threatened to have her children removed by child services.
The court only believed her after a court officer was present at the birth of her third child, after which an immediate DNA test revealed that Fairchild wasn't the biological mother of that newborn, either. But, after her case was championed by an attorney determined to get to the bottom of her medical mystery, it was discovered that Fairchild was a chimera who carried the DNA of her demised twin. And it was that maternal DNA that had been passed onto the three children she had borne.
Starr's statement notes that while chimerism causing confused paternity tests have historically been quite unusual, it may become increasingly common as more people use fertility clinics to conceive. These assisted reproductive technologies raise the chances of one pregnancy resulting in multiple births - or, sometimes, one birth that happens to contain a chimeric, unborn parent of the living sibling's future child.