How forensic DNA analysis can falsely link people to crime scenes

When it comes to connecting a suspect to a crime, most people assume that DNA evidence is almost irrefutable. DNA contains a unique fingerprint, so if a suspect’s genetic code matches DNA found at a crime scene, then they had to be 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 the time of the crime to a person who was not involved in the event.

Notably, the study comes with caveats, and forensic scientists are already aware of the test’s limitations, 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 containing genetic material from multiple people. For example, if there is a doorknob at a crime scene, there may be traces of DNA left by all the people who have recently touched that doorknob.

Mixture analysis does not give a simple yes or no as to whether a person was present at the time of the crime. The technique involves computer software that estimates how likely it is that someone’s DNA contributed to a given mixture. In criminal proceedings, such an assessment of probability may be submitted to the jury along with other evidence, pursuant to National Institute of Standards and Technologies (NIST).

Mixtures of DNA are more difficult to interpret than DNA samples left by just one person. Mixtures also become increasingly difficult to interpret as the number of people increases, the amount of DNA from each decreases and the DNA degrades, according to NIST. DNA analysis looks for specific control “markers” in the molecules, so when 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 less genetic diversity, that is, individuals from a demographic group that has very similar DNA.

“Our work shows that we need to be careful in interpreting the results of forensic analysis of certain DNA mixtures,” said the study’s lead author. Rory Rolfsdata scientist at the University of Oregon, told Live Science in an email. “Of course, we need to weigh the strength of the evidence against its reliability. It is possible that a wrongful conviction could have been brought on the basis of a misinterpreted analysis of the DNA mixture.”

To demonstrate this, Rolfs and her colleagues ran computer simulations using data from a genetic database. The data came mainly from a 2016 study which contained pooled data from 250 studies that together included 500,000 people representing 466 population groups from around the world.

The study confirms a flaw in DNA admixture analysis, which is used on samples containing genetic material from multiple people, such as a swab from a doorknob. (Image credit: Rosendo Serrano Valera via Getty Images)

The team used this data to create genetic profiles and DNA mixtures of people of different backgrounds. The probability of a false positive result increased for members of groups with less genetic diversity. This means that their DNA most likely matched the mixture found at the simulated crime scene, even though it was not actually there.

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

“While this number may seem small, we need to interpret it against the number of samples analyzed,” Rolfes said. “For example, if a lab runs 10,000 samples where the POI (person of interest) did not contribute DNA, there is a 1 in 10 chance that one of them will be misinterpreted as having a POI who did contribute.”

By comparison, the false-positive rate jumped to 0.93 percent—closer to a 1 in 100 chance—with mixtures that included six participants that had relatively low genetic diversity.

related: Australia’s ‘Worst Serial Killer’ Freed After Her Children Reveal Deadly Gene Mutations

Is it strange?

The study authors based their study on computer simulations that generated genetic mixtures for analysis. Because of this, they did not identify specific racial or ethnic groups 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 statement released by 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 findings “are not surprising, but it’s nice to see them validated with rigor.” Mark Joblingprofessor of genetics at the University of Leicester in the UK, told Live Science in an email.

“The background of participants with a mixed profile can affect the likelihood of mistakenly including someone in 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.

Denise Syndercombe Courtprofessor of forensic genetics at King’s College London, went further, telling Live Science that the results of the study have already been taken into account and that forensic experts should already be interpreting such evidence properly.

“The authors have identified issues that are well understood and considered by forensic practitioners in their evaluation of mixtures that will be significantly more complex and diverse than those analyzed in this publication,” the Court said in an email. “There does not appear to be any additional message in this publication that the forensic community should be aware of.”

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