DNA has become the gold standard for evidence in criminal cases. It has a high reputation for accurately identifying people who had some contact with the scene of a crime and results in many convictions since jurors give great weight to DNA evidence. According to Katie Worth, a “2008 series of studies by researchers at the University of Nevada, Yale and Claremont McKenna College found that jurors rated DNA evidence as 95 percent accurate and 94 percent persuasive of a suspect’s guilt.”

But more importantly, it has resulted in many convicted people being able to prove their innocence when their DNA did not match that found at the scene. But as with all forms of evidence, one must be wary of depending too much on just one piece, however reliable one may think it. Worth reports on a 2012 case where a man was wrongly convicted of a murder because his DNA was found on the fingernails of the victim, even though he had had no contact with him, a phenomenon called ‘secondary DNA transfer’.

When the DNA results came back, even Lukis Anderson thought he might have committed the murder.

“I drink a lot,” he remembers telling public defender Kelley Kulick as they sat in a plain interview room at the Santa Clara County, California, jail. Sometimes he blacked out, so it was possible he did something he didn’t remember. “Maybe I did do it.”
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Before he was charged with murder, Anderson was a 26-year-old homeless alcoholic with a long rap sheet who spent his days hustling for change in downtown San Jose. The murder victim, Raveesh Kumra, was a 66-year-old investor who lived in Monte Sereno, a Silicon Valley enclave 10 miles and many socioeconomic rungs away.
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Three-and-a-half weeks later, the police arrested Anderson. His DNA had been found on Raveesh’s fingernails. They believed the men struggled as Anderson tied up his victim. They charged him with murder. Kulick was appointed to his case.

As they looked at the DNA results, Anderson tried to make sense of a crime he had no memory of committing.

“Nah, nah, nah. I don’t do things like that,” he recalls telling her. “But maybe I did.”

But he hadn’t and how his DNA ended up on the victim makes for a fascinating story, starting with a paper by Australian forensic scientist Roland van Oorschot who back in 1997 first showed that DNA traces could be found on objects touched by people. But while that took the world of criminal investigation by storm, people tended to overlook another thing in the paper: that the DNA of people could appear on objects that had not been touched by them.

In one of his lab’s experiments, for instance, volunteers sat at a table and shared a jug of juice. After 20 minutes of chatting and sipping, swabs were deployed on their hands, the chairs, the table, the jug and the juice glasses, then tested for genetic material. Although the volunteers never touched each other, 50 percent wound up with another’s DNA on their hand. A third of the glasses bore the DNA of volunteers who did not touch or drink from them.

Then there was the foreign DNA — profiles that didn’t match any of the juice drinkers. It turned up on about half of the chairs and glasses, and all over the participants’ hands and the table. The only explanation: The participants unwittingly brought with them alien genes, perhaps from the lover they kissed that morning, the stranger with whom they had shared a bus grip or the barista who handed them an afternoon latte.

This is a problem that has got worse, paradoxically, with improved technology.

Forensic DNA has other flaws: Complex mixtures of many DNA profiles can be wrongly interpreted, certainty statistics are often wildly miscalculated, and DNA analysis robots have sometimes been stretched past the limits of their sensitivity.

But as advances in technology are solving some of these problems, they have actually made the problem of DNA transfer worse. Each new generation of forensic tools is more sensitive; labs today can identify people with DNA from just a handful of cells. A handful of cells can easily migrate.

Some people are prodigious shedders of their DNA, leaving them all over the place and on other people while others move through the DNA world almost invisibly. Furthermore, since poor and minority community members tend to end up disproportionately in DNA databases, their DNA is likely to be picked up in database searches even if they were not involved in a crime, purely as a result of it ‘traveling’. Another of the sources of DNA contamination consist of the very tools that police investigators use to collect DNA evidence.

The lead police investigator Erin Lunsford was able to figure out how Anderson’s DNA had got to the crime scene.

It was Lunsford who figured it out in the end.

He was reading through Anderson’s medical records and paused on the names of the ambulance paramedics who picked up Anderson from his repose on the sidewalk outside S&S Market. He had seen them before.

He pulled up the Kumra case files. Sure enough, there were the names again: Three hours after picking up Anderson, the two paramedics had responded to the Kumra mansion, where they checked Raveesh’s vitals.

The prosecutors, defense attorney and police agree that somehow, the paramedics must have moved Anderson’s DNA from San Jose to Monte Sereno. Santa Clara County District Attorney Jeff Rosen has postulated that a pulse oximeter slipped over both patients’ fingers may have been the culprit; Kulick thinks it could have been their uniforms or another piece of equipment. It may never be known for sure.

Anderson was lucky that his public defender looked into his case closely and found that he had a strong alibi for that night, even though he himself was too drunk to remember what had happened, and that the chief police investigator was willing to look more carefully into the matter rather than try and close it quickly.

The lesson is that one should never depend on just one piece of evidence but always seek corroboration, especially when the stakes are so high.