How forensic DNA analysis can falsely link people to crime scenes

When it comes to linking a suspect to a crime, most people assume that DNA evidence is almost irrefutable. DNA carries a unique fingerprint, so if a suspect’s genetic code is matched with DNA found at a crime scene, then it must have been there – right?

A new study highlights why it’s not always that simple.

The study, published online Sept. 28 in the journal iSciencefound that a common DNA test used in forensics is more likely to produce “false positive” results for certain groups of people. False positives, in this context, would incorrectly match DNA found at a crime to a person who was not involved in the event.

Notably, however, the study comes with caveats, and forensic scientists are already aware of the limitations of this test, experts told Live Science.

What the study found

The new research focuses on “DNA admixture analysis,” which is used to identify suspects from a DNA sample that contains the genetic material of multiple people. For example, if there is a doorknob at a crime scene, there could be traces of DNA left by all the people who have recently touched that doorknob.

Admixture analysis does not give a simple “yes” or “no” as to whether a person was present for a crime. The technique involves computer software that estimates how likely someone’s DNA is to contribute to a given mixture. As part of a criminal trial, that probability estimate could then go to a jury, along with other evidence, according to National Institute of Standards and Technology (NIST).

Mixtures of DNA are more difficult to interpret than DNA samples left behind by a single person. Mixtures also become increasingly difficult to interpret as the number of people increases, the amount of DNA in each decreases and the DNA degrades, according to NIST. DNA analyzes look for specific “markers” in the molecules, so as the DNA degrades, these markers are lost.

In addition to these limitations, researchers have now confirmed that DNA admixture analysis is less accurate for people with lower genetic diversity—that is, individuals in a demographic group who share very similar DNA.

“Our work shows that we need to be cautious in interpreting the results of forensic analysis of certain DNA mixtures,” lead author of the study. Rori Rohlfsa data scientist at the University of Oregon, told Live Science in an email. “Certainly we have to weigh the strength of the evidence against its reliability. A wrongful conviction may have been made on the basis of a misinterpreted DNA admixture analysis.”

To demonstrate this, Rohlfs and her colleagues ran computer simulations using data from a genetic database. The data comes mainly from a 2016 study which had aggregated data from 250 studies that collectively included 500,000 people representing 466 populations worldwide.

The study confirms a shortcoming in DNA admixture analysis, which is used on samples containing genetic material from more than one person, such as a swab from a door handle. (Image credit: Rosendo Serrano Valera via Getty Images)

The team used this data to generate genetic profiles and DNA mixtures, including people of different ancestries. The probability of a false positive increased for members of groups with lower genetic diversity. That means their DNA was more likely to match the mixture found at the mock crime scene when it wasn’t actually there.

The problem got worse when the number of contributors in the mix increased. For about 40 percent of the three-person mixes, the false-positive rate was about 1 in 100,000 samples, Rohlfs explained.

“Although this number may seem small, we have to interpret it in comparison to the number of samples analyzed,” said Rohlfs. “For example, if a lab analyzes 10,000 samples where the POI (person of interest) did not contribute DNA, then there is a 1 in 10 chance that one of them will be misinterpreted as the POI contributing.”

By comparison, the false positive rate rose to 0.93%—approaching a 1 in 100 chance—with mixtures involving six contributors that all had relatively low genetic diversity.

Related: Australia’s ‘worst serial killer’ freed after her children’s deadly genetic mutations come to light

Is this surprising?

The study authors based their study on computer modeling that generated genetic mixtures for analysis. For this reason, they did not identify specific racial or ethnic groups that are most likely to be affected; they were simply demonstrating a general principle.

Groups that may have relatively low genetic diversity include indigenous and Pacific Islander groups, according to a release from the University of Oregon.

Live Science asked two genetics professors who were not involved in the study to give their thoughts on its findings.

Overall, the study’s results are “unsurprising, but it’s good to see this rigorously confirmed.” Mark Joblingprofessor of genetics at the University of Leicester in the UK, told Live Science in an email.

“Contributor ancestry to a mix profile can affect the chance of wrongly including someone as a contributor to the mix,” Jobling said. That chance could be reduced by raising the statistical threshold at which some of these results are considered useful, he added.

Court Denise Syndercombeprofessor of forensic genetics at King’s College London, went further, telling Live Science that the study’s findings are already being considered and that forensic scientists should already be interpreting such evidence appropriately.

“The authors have identified issues that are well understood and considered by forensic practitioners in admixture evaluation that will be significantly more complex and varied than those discussed in this publication,” Court said in an email. “There do not appear to be any additional messages from this publication that the forensic community needs to be aware of.”

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