When your semen carries another man's DNA

Excerpts from an absolutely fascinating report in the New York Times:

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 It had all been replaced by the DNA of his donor, a German man he had exchanged just a handful of messages with...

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...

Mr. 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...

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...

In 2004, investigators in Alaska uploaded a DNA profile extracted from semen to a criminal DNA database. It matched a potential suspect. But there was a problem: The man had been in prison at the time of the assault. It turned out that he had received a bone marrow transplant. The donor, his brother, was eventually convicted...

In 2008, he was trying to identify the victim of a traffic accident for the National Forensic Service in Seoul, South Korea. Blood showed that the individual was female. But the body appeared to be male, which was confirmed by DNA in a kidney, but not in the spleen or the lung, which contained male and female DNA. Eventually, he figured out that the victim had received a bone marrow transplant from his daughter.

More worth reading at the link.