“Three months after his bone marrow transplant, Chris Long of Reno, Nevada, learned that the DNA in his blood had changed. It had all been replaced by the DNA of his donor, a German man he had exchanged just a handful of messages with.

He’d been encouraged to test his blood by a colleague at the sheriff’s office, where he worked. She had an inkling this might happen. It’s the goal of the procedure, after all: weak blood is replaced by healthy blood, and with it, the DNA it contains.

But four years after his lifesaving procedure, it was not only Long’s blood that was affected. Swabs of his lips and cheeks contained his DNA – but also that of his donor. Even more surprising to Long and other colleagues at the crime lab, all of the DNA in his semen belonged to his donor. “I thought that it was pretty incredible that I can disappear and someone else can appear,” he said.

Long had become a chimera, the technical term for the rare person with two sets of DNA. The word takes its name from a fire-breathing creature in Greek mythology composed of lion, goat and serpent parts. Doctors and forensic scientists have long known that certain medical procedures turn people into chimeras, but where exactly a donor’s DNA shows up – beyond blood – has rarely been studied with criminal applications in mind.

Tens of thousands of people get bone marrow transplants every year for blood cancers and other blood diseases including leukaemia, lymphoma and sickle cell anaemia. Although it’s unlikely that any of them would end up as the perpetrator or victim of a crime, the idea that they could intrigued Long’s colleagues at the Washoe County Sheriff’s Department, who have been using their (totally innocent) colleague in IT as a bit of a human guinea pig.

The implications of Long’s case, which was presented at an international forensic science conference in September, have now captured the interest of DNA analysts far beyond Nevada.

The average doctor does not need to know where a donor’s DNA will present itself within a patient. That’s because this type of chimerism is not likely to be harmful. Nor should it change a person. “Their brain and their personality should remain the same,” said Dr Andrew Rezvani, medical director of the inpatient blood and marrow transplant unit at Stanford University Medical Centre.

He added that patients also sometimes ask him what it means for a man to have a woman’s chromosomes in their bloodstream or vice versa. “It doesn’t matter,” he said.

But for a forensic scientist, it’s a different story. The assumption among criminal investigators as they gather DNA evidence from a crime scene is that each victim and each perpetrator leaves behind a single identifying code – not two, including that of a fellow who is 10 years younger and lives thousands of miles away. And so Renee Romero, who ran the crime lab at the Washoe County sheriff’s office, saw an opportunity when her friend and colleague told her that his doctor had found a suitable match on a donor website and he would be undergoing a bone marrow transplant.

“We need to swab the heck out of you before you have this procedure to see how this DNA takes over your body,” she recalled telling him.

Long agreed. He welcomed an intriguing distraction from his diagnosis of acute myeloid leukaemia and myelodysplastic syndromes, both of which impair the production of healthy blood cells.

At the time, he said, “I didn’t even know if I would live.”

Four years later, with Long in remission and back at work, Romero’s experiment persisted, aided by her crime lab colleagues. Within four months of the procedure, Long’s blood had been replaced by his donor’s blood. Swabs collected from his lip, cheek and tongue showed that these also contained his donor’s DNA, with the percentages rising and falling over the years. Of the samples collected, only his chest and head hair were unaffected. The most unexpected part was that four years after the procedure, the DNA in his semen had been entirely replaced by his donor’s.

“We were kind of shocked that Chris was no longer present at all,” said Darby Stienmetz, a criminalist at the Washoe County sheriff’s office.

If another patient responded similarly to a transplant and that person went on to commit a crime, it could mislead investigators, said Brittney Chilton, a criminalist at the sheriff’s office forensic science division.”