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Secondary DNA Transfer: Little Known Phenomenon That Puts You at a Crime Scene You’ve Never Visited and Places a Murder Weapon You’ve Never Touched in Your Hand

by Miles Dyson

In 2008, European authorities were hot on the trail of a highly prolific serial killer and burglar. The “Phantom of Heilbronn” robbed jewelers, burglarized caravans, and murdered multiple people, including a law enforcement officer.

The Phantom left forensic evidence all over the continent. His DNA was found at 40 crime scenes in Germany, France, and Austria. Police offered a large reward for his capture and spent an estimated 16,000 hours working the case. But there was one small problem with the case.

He was a she, and she wasn’t a criminal mastermind. The Phantom of Heilbronn was an elderly Polish factory worker who unwittingly contaminated the forensic swabs she manufactured. While she worked, she transferred her DNA onto the swabs. Investigators later transferred her DNA to the scene of a slew of unrelated crimes.

The Phantom of Heilbronn debacle is a prime example of the dangers of DNA contamination. Inside the Cell author Erin Murphy draws a distinction between DNA contamination and DNA transfer. Contamination, Murphy argues, can be avoided or limited by employing best practices. DNA transfer, however, is a much more insidious problem, because it is utterly unavoidable. In two minutes, Murphy says, the average person sheds enough skin cells to cover a football field.

Murphy profiled a horrific murder in order to illustrate the unique problems that skin-cell DNA transfer presents to forensic investigators. In 2009, a Yale graduate student was found dead in a mechanical chase area behind a wall in a scientific laboratory. DNA obtained from the victim’s body and clothing revealed two unique genetic profiles. One person was identified as a co-worker from the lab; the other was a local convicted felon.

Unsurprisingly, investigators looked at the convicted felon first. That turned out to be a literal dead end—the man had died two years before the murder. So, investigators wondered, how was his DNA found on the victim, including in the waistband of her underwear? Secondary DNA transfer.

The convicted felon also happened to be in construction and had worked on a job behind the lab wall years earlier. He shed skin and sweat cells back there, and because the chase was closed to traffic and adjacent to the tightly controlled laboratory environment, his cells (and DNA) were still there when the victim crashed through the small space. A DNA transfer event took place, and a man was implicated in a crime that he could only have committed as a member of the undead.

The dead man also was very likely to have been what scientist studying DNA transfer would call a “good” shedder. Some people are naturally prodigious shedders of biological material. Those with flaky, sweaty, or diseased skin are thought to be good shedders. Does this matter?

It did to David Butler, an English cabdriver accused of a murder that he said he did not commit. Investigators targeted Butler when they found his DNA under the victim’s fingernails six years after she was killed. Butler denied knowing the woman and said the killer may have ridden in his cab and picked up his DNA on the day of the murder.

Butler was, in fact, a good shedder. He suffered from a severe skin condition that earned him the nickname “Flaky.” The authorities didn’t buy Butler’s explanation, despite having no other evidence tying him to the crime. The jury did, and he was acquitted of the murder after spending eight months in jail.

“DNA has become the magic bullet for the police,” Butler said after he was acquitted. “They thought it was my DNA, ergo it must be me.”


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