Can DNA testing separate identical twins at trial?

Identical twins on trial: can DNA testing tell them apart?

By Rachel Fieldhouse & Nature magazine

An identical twin commits a crime and leaves his DNA at the crime scene. Can DNA testing reveal which twin is the culprit?

This question reportedly came up in a case in France last month, where DNA from twin brothers was found on a gun, but conventional DNA testing could not determine which of the brothers it belonged to. The men are identical twins – the result of a single egg splitting in two after being fertilized by a single sperm – meaning they share the same DNA.

When DNA is found at a crime scene, forensic scientists usually try to identify the person it belongs to using a technique called short tandem repeat (STR) analysis. The technique uses the polymerase chain reaction (PCR) method to amplify up to 30 specific regions of the genome that have a lot of genetic variation, says Brendan Chapman, a forensic pathologist and researcher at Murdoch University in Perth, Australia.

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These amplified regions are sequenced to determine the number of STRs and their base pair patterns, which can then be compared to genealogy databases or STR profiles of suspects to identify potential matches. Usually, says Chapman, “we can distinguish one person from the next very easily”. But with identical twins, “there’s just no difference” in those areas of the genome.

Deep dive

Whole-genome sequencing can make it possible to tell identical twins apart. By analyzing a person’s entire genome, researchers can identify differences caused by mutations that arose after an egg split. These changes are rare, says Chapman. For example, a 2014 study identified only five genetic changes in a pair of adult twins.

There have been rare cases where whole-genome sequencing has helped courts distinguish between twins, says Xanthé Weston, a criminologist and researcher at Central Queensland University in Mackay, Australia. But using whole-genome sequencing requires obtaining a sufficient amount of DNA to analyze, Weston adds.

Other researchers have reported success in distinguishing twins by sequencing the DNA found in the mitochondria, the cell’s powerhouses. Compared to nuclear DNA, which is used in genome sequencing and STR analysis, mitochondrial DNA (mtDNA) mutates more frequently, meaning it is more likely to differ between twins.

US courts have admitted mtDNA analysis as evidence since the mid-1990s. But it has not been used in cases involving twins, says Weston.

‘Epigenetic’ changes

Another promising technique looks at the addition of methyl groups to DNA, a process called methylation, which can change how genes work. These “epigenetic” changes can be caused by factors including a person’s behavior – such as diet and drinking or smoking habits – and the environment.

Last year, a team of researchers in South Korea sequenced the genomes of 54 pairs of newborn identical twins. After identifying 711 sites where methylation was commonly different, they selected and tested different combinations and 5 that were particularly variable. By comparing how much methylation had occurred at these five sites, the team was able to distinguish between 50 of the 54 twin pairs. When they repeated the experiment in two adult cohorts, they succeeded in distinguishing between 41 of 47 pairs and 105 of 118 twin pairs.

DNA methylation analysis has not yet been used to distinguish between twins in court. And Chapman notes that this type of analysis may not be able to distinguish between twins who have similar variations caused by leading similar lifestyles or being exposed to similar environmental factors such as pollution.

Challenges remain

The new techniques are promising, Chapman adds, but there are challenges to overcome before they can be used in forensic investigations. For example, they all require relatively large amounts of DNA, but samples from crime scenes often include only small amounts or are in poor condition.

Analyzes can also be expensive and time-consuming, partly because of the work needed to demonstrate to a courtroom that the results are conclusive enough to count as evidence. Last year, whole-genome sequencing was admitted in a US murder case for the first time.

Weston says it’s important to combine DNA evidence with other types of evidence, such as fingerprints, when prosecuting someone because there are many reasons someone’s DNA might be present at a crime scene. “I would never want to see anyone prosecuted on the basis of DNA alone,” she adds.

This article is reproduced with permission and var first published on March 5, 2026.

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