Chris Long of Reno, Nevada received a bone marrow transplant. Four years later, his colleagues at the Sheriff's Office tested his DNA to find out of the transplant had affected his DNA. If a transplant could alter DNA evidence, it could impact their criminal investigations.
The result was shocking: the sperm in Long's semen contained no trace of his own DNA. It all belonged to his donor, a German man. The New York Times explains what happened:
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.
Brittany Chilton, a criminologist, found that chimera DNA had confused other criminal investigators in the past:
And it has misled them, Ms. Chilton learned once she began to research chimerism. 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.
Abirami Chidambaram, who presented the Alaska case in 2005, when she worked for the Alaska State Scientific Crime Detection Laboratory in Anchorage, said she had heard about another disconcerting scenario since then. It involved police investigators who were skeptical of a sexual assault victim’s account because she said there had been one attacker, though DNA analysis showed two. Eventually the police determined that the second profile had come from her bone marrow donor.
So the man is a chimera (he has his own DNA and the donor's DNA in his body). His sperm is still his own DNA, but his semen contains the donor's DNA.
His semen should therefore contain BOTH his DNA and the donor's DNA ... but to make matters a bit more complicated, he's undergone vasectomy so he no longer produces sperm.
Thus his semen only contains only his donor's DNA.
Yeah, complicated!
Similarly, finding donor DNA in Mr Long’s other tissues doesn’t mean he’s a chimera. All organs have a blood supply, so the fact that there’s donor-derived blood cells present is to be expected.