After a bone marrow transplant, a man with leukemia found that his donor’s DNA traveled to unexpected parts of his body. A crime lab is now studying the case. 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 It had all been replaced by the DNA of his donor, a German man he had exchanged just a handful of messages with.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...