When a man received paternity test results showing that the son he and his wife conceived via IVF was not his biological child, the new dad was understandably concerned.
He had no idea that investigations would later reveal he was actually his baby's uncle - and that the infant's biological father was the man's own twin brother, who had never been born.
The 34-year-old American, who is choosing to remain anonymous, is thought to be the first "human chimera" to have the condition revealed by unexpected paternity test results.
A chimera is an organism with at least two genetically distinct types of cells. It occurs when someone was originally going to be born a twin, but two fertilised eggs fuse in the mother's womb, becoming one foetus which carries two distinctly separate strands of DNA.
In chimeras, their twin lives on microscopically somewhere inside the body as DNA. They are essentially their own twin, all in one body.
It is possible for that absorbed twin's DNA to then be passed on to the chimera's children - which is what happened in this family's case.
According to BuzzFeed News, the man's case came to light when he and his wife learnt their son's blood type did not match either of theirs. The Washington couple, whose baby was born in June last year, then had a paternity test carried out. It revealed the man was not the baby's biological father.
"You can imagine the parents were pretty upset," Stanford University geneticist Barry Starr told BuzzFeed. "They thought the clinic had used the wrong sperm."
However, the clinic denied any wrongdoing, saying that the man in question was the only white man to have given sperm at the facility on the day the boy was conceived, and the baby appeared to be white.
In an attempt to solve the mystery, Dr Starr organised for the couple and their son to undergo a genetic ancestry test.
Standard paternity tests only test for fatherhood, but genetic heritage tests look at hundreds of thousands of markers and can establish genetic genealogies for entire families.
Everybody was surprised when the genetic heritage test indicated the man who thought he was the boy's father was actually the baby's biological uncle.
It was then that Dr Starr realised the unusual test results were possibly caused by the fact the man was a chimera.
"That was kind of a eureka moment," he said. "Chimera reports are very rare but they are real."
Further testing revealed that 10 per cent of the sperm produced by the man did, in fact, genetically match his son, confirming the man's status as a chimera.
Another well-documented chimera case is that of US woman Lydia Fairchild.
In 2002, authorities instigated court action to take Ms Fairchild's children away from her after tests revealed she was not their biological mother.
The then-pregnant mother-of-two was seeking financial support from the government following the breakdown of her relationship with her children's father. In order to get the support the former couple were ordered to undertake DNA testing to prove they were the children's parents.
While the tests proved that her ex-partner was indeed the children's father, they also showed Ms Fairchild was not their biological mother.
Ms Fairchild only managed to stop the court action and keep custody of her children after her lawyer stumbled across a earlier case of chimerism which made him question if his client could be a chimera.
First genetic material from Fairchild's own mother was tested, which proved a link between the children and their grandmother. A cervical smear from Ms Fairchild later showed DNA cells matching all three of her children.
It was then confirmed that Fairchild was indeed a chimera and her "invisible twin" lived on only in her ovaries. Her twin, who never lived, was her children's biological mother.