People can transfer DNA from others after they touch them, potentially complicating a crime scene, new data show.
A handshake is just the beginning. This formal palm-to-palm greeting technique leaves a little DNA behind. That DNA — the code that carries instructions for who you are and how you function — doesn’t stop there.
It could end up transferred to an object that you never touched, two new studies show. If even brief contact with another person or object could spread DNA far and wide, crime scene investigators may need to think carefully when they swab a surface.
Cynthia Cale is a forensic scientist — someone who uses science to solve crimes. She works at the Houston Forensic Science Center in Texas. Previously, she had found that shaking hands for two minutes could transfer one person’s DNA to an object by way of the other person’s hand. But many critics said that two minutes is a very long, awkward handshake.
For her new experiments, she shortened the handshakes to as little as 10 seconds. (That’s still two to five times longer than a typical handshake.) And even that brief contact transfers DNA, her data show.
After people shook hands, one person from each pair picked up a knife. Cale’s team then swabbed the knife handle and tested for DNA. Even after a 10 second handshake, people who never touched the knife were a major source of DNA on the handle some one in every 14 times. Their DNA had been transferred to the knife when the person’s handshaking partner had grasped the handle.
Cale reported her findings at the annual meeting of the American Academy of Forensic Sciences.
When many people touch something
In a separate study, Leann Rizor looked at the transfer of DNA to objects that lots of people might touch. These are things like pitchers or doorknobs. Rizor is a forensic anthropologist — someone who looks at biological remains (such as DNA) to help solve crimes. She did the research at the University of Indianapolis, in Indiana. The last person to touch an object, such as a shared pitcher, often was not who left the most DNA behind, she found.
In her experiment, four university students sat around a table and pretended to pour drinks from an empty pitcher into empty glasses. “We did not want to risk spilling,” Rizor explained. Another 12 students watched this without sitting at the table.
The bystanders could leave the room, talk and move around. The goal was to mimic conditions in a restaurant. As each student at the table handled the pitcher and a plastic cup, researchers swabbed the pitcher’s handle, the cups and the students’ hands for DNA.
Students at the table handled only their own cups and the pitcher. Still, their DNA ended up on the pitcher handle and on each other’s cups — cups other students had never touched. What’s more, DNA from other students in the room showed up on the swabbed objects, too.
Yet none of those observers had touched the students at the table, the pitcher or those cups. Watchers’ DNA may have spread to the cups and pitcher when those people coughed, sneezed or talked, Rizzo now suspects.
The researchers couldn’t determine who handled the pitcher last simply by measuring how much DNA was on the objects. They also could not tell how long someone had touched a pitcher or cup. The data show that DNA can transfer easily in social settings, Rizor notes — and in unpredictable ways. She, too, presented her findings at the American Academy of Forensic Sciences.
It’s likely rare that someone’s DNA will land on an object they’ve never handled, says Mechthild Prinz. She’s a forensic geneticist, someone who studies DNA to solve crimes.
She works at the John Jay College of Criminal Justice in New York City. Left-behind DNA is usually unstable, she says, meaning it will break down with time. While no one can say it never happens, she says “we shouldn’t use [the new findings] to throw the evidence out in every single case.”
Rizor and Cale agree that their new data don’t mean DNA evidence is unreliable. But people investigating crimes must be careful to account for such accidental transfers. When someone’s DNA shows up at crime scenes, he or she could be accused of committing a crime. That can happen even if that person was never there.
A real life example
In fact, that happened to a man named Lukis Anderson. In 2012, a millionaire in San Jose, Calif., died during a robbery. Anderson was charged with the murder. Police found his DNA on the millionaire’s fingernails. There was just one problem. Anderson was unconscious in the hospital during the crime. (Great alibi!) Later, investigators learned what caused the mistake. The paramedics who took Anderson to the hospital were the same ones who tried to save the millionaire’s life. Somehow, the paramedics accidentally transferred Anderson’s DNA to the millionaire’s fingernails.
Some of these instances, Prinz says, may be explained by the fact that some people shed more DNA than others. DNA oozes out of the body with sweat. Someone who sheds a lot of DNA might spread more of their traces — allowing it to be shared more widely. It’s still not clear how often that type of transfer might affect crime-scene investigations, Prinz says. “We’re all still trying to get a handle on how realistic this is.”
