Judges, juries and other members of the criminal justice system rely upon DNA evidence to decide guilt, innocence and sentences for incarceration—and even death—every day. Yet most people would be shocked to learn that new technologies have made DNA evidence less reliable than it was in the past. For instance, recent advances in forensic science use smaller DNA samples, introducing more subjectivity into the identification process than ever before.
Risks of Small Samples
Twenty years ago, available technology could only reliably test DNA from fresh semen, blood stains, or other large tissue samples. According to Christopher Phillips, a researcher in forensic genetics at the University of Santiago de Compostela in Spain, such a test is considered substantially reliable for purposes of forensic evidence.
However, advances in technology now require as few as 25 to 30 cells for DNA testing. Then, scientists use the polymerase chain reaction to replicate the DNA in the lab to create a testable size sample. This process is often used with what’s known as “touch” or “trace” DNA when the sample is collected from fingerprints on a doorknob or other objects. Touch DNA can even be transferred when in an evidence bag with other items awaiting testing or when handled by techs.
Top geneticists such as Greg Hampikian, professor of biology and criminal justice at Boise State University, are concerned about the risk of false positives when such small samples transfer from one surface to another. Hampikian—who is also head of the Idaho Innocence Project—has worked on cases such as the Amanda Knox trial, where Knox and her boyfriend were eventually acquitted because the DNA samples on key pieces of evidence could have been transferred from various surfaces during the testing period or were too small for sufficient testing.
“There’s something called ‘accidental transfer’ or ‘secondary transfer,'” Phillips said. “The DNA on a weapon might come from the person who actually touched the object or the person who shook hands with the person who touched the object.”
For example, a 2015 study asked people to shake hands with another person for two minutes. Then, each person handled their own knife. DNA from both people was located on 85 percent of the knives, and even more DNA from the secondary source was present on 20 percent of the knives.
Moreover, based on the relative flakiness of their skin, some people leave more DNA than others from casual contact with a person or an object. Others may leave no DNA at all. Studies indicate factors such as how recently someone washed their hands or which hand someone used to touch something can impact the amount of DNA left behind.
“In different scenarios, some people leave DNA and some people don’t,” according to Ruth Morgan, the director of the Centre for the Forensic Sciences at University College London. “We’ve had some experiments where the person whose DNA we were looking for left either a partial profile or not really a viable profile – but there was other DNA [from a person] who we were able to identify as a close partner who hadn’t touched the item; they hadn’t been in the lab.”
This process of creating a DNA profile to identify someone is complex. But rather than mapping the whole genome, forensic labs focus on 20 locations, or loci—according to the FBI CODIS database standard—where short tandem repeats (STRs) occur. Typically, more than half will be the same as close relatives and several will match strangers, but the likelihood of sharing all of them with a non-relative is less than one in one trillion.
Over time, however, DNA breaks down, weakening the loci within samples and increasing the difficulty of a 100 percent match. Add that to the risks of touch DNA traveling when our skin naturally sheds—perhaps even to places we’ve never been—or lingering for months until a crime occurs, and the possibilities for misidentification appear indefinite.
According to the United Kingdom’s chief scientific advisor of forensic science, “our ability to analyse may outstrip our ability to interpret” these substances that are detectable at minute levels.
Yet forensic science seems destined to repeat its mistakes with the technology on the horizon. While experts like Hampikian are expressing concern about the use of small samples, the next generation of forensic robots seem poised to generate a DNA sample from a single cell.
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