How can a man who was never born father a son? When the ghost of his genes lives on in the DNA of his brother, genetics researchers have found.

A 34-year-old U.S. man is the first-ever reported case of a paternity test fooled by a human “chimera” — someone with extra genes absorbed from a nascent twin lost in early pregnancy.

About 1 in 8 single childbirths are thought to have started as multiple pregnancies. Cells from these miscarried siblings are sometimes absorbed in the womb by a surviving twin — but are only rarely discovered by surprises such as the paternity-test puzzle.

“Even geneticists are blown away by this,” Barry Starr, a geneticist at Stanford University, told BuzzFeed News.

Last year, a Washington couple came to Starr, who answers the “Ask a Geneticist” questions on the website of the Tech Museum of Innovation in San Jose, California, looking for help with what appeared to be a mistake at a fertility clinic. The answer to their mystery points to a possible genetic loophole in standard paternity testing, Starr said — “one where we have no idea how big the problem is.”

In June 2014, the parents (who have chosen to remain unnamed because of concerns for their privacy and confidentiality of medical records) had a son with the help of fertility clinic procedures. The boy was born healthy, but strangely, his blood type didn’t match that of his parents.

An at-home paternity test suggested an explanation: The man wasn’t actually the father of the child.

“You can imagine the parents were pretty upset,” said Starr, whose colleagues have presented the case at two scientific meetings this month, most recently the International Symposium on Human Identification meeting. “They thought the clinic had used the wrong sperm.”

The parents hired a lawyer and sought a more precise paternity test from an accredited lab. Just like the at-home test, the new analysis relied on skin cells from a cheek swab to check the father’s genes against the child’s. Again, the test came back negative for paternity.

Accredited U.S. labs perform nearly 400,000 paternity tests every year in legal, immigration, and criminal cases, with more than 99% of them relying on cheek swab samples. About 24% find that the man is not the father of the child.

Concerned that the fertility clinic had made a mistake, the Washington couple approached them with the results of the paternity test. But the fertility clinic said that the 34-year-old father was the only white man to donate sperm at the facility on the day their son was conceived, and the child looked white.

That was when the couple approached Starr, who suggested they test the father and son with a direct-to-consumer genetic ancestry test sold by the startup firm 23andMe. The results of those tests came back late last year. Bizarrely, their results said that the man was his son’s uncle.

“That was kind of a eureka moment,” said Starr. At that point, he realized they might be dealing with a chimera.

“Chimera reports are very rare, but they are real.”