Three months after his bone marrow transplant, Chris Long of Reno, Nev., 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 It had all been replaced by the DNA of his donor, a German man he had exchanged just a handful of messages with.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 Mr. 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 Mr. 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.