Identical twins on test: can DNA testing tell them apart?
In a French criminal trial, traditional DNA analysis couldn’t differentiate between twin brothers, but emerging scientific methods could help in such cases
Conventional DNA testing failed to distinguish between twin brothers during a French criminal trial.
Denis Charlet/AFP via Getty
An identical twin commits the crime and leaves his DNA at the scene. Can DNA testing tell which twin is the culprit?
This question was reportedly raised last month in a case in France in which twin brothers’ DNA was found on a gun, but conventional DNA testing could not determine which brother it belonged to. The men are monozygotic twins – the result of a single egg splitting into two after being fertilized by the same sperm – meaning they have the same DNA.
When DNA is found at a crime scene, forensic scientists usually attempt to identify the person it belongs to by using a technique called short tandem repeat (STR) analysis. Brendan Chapman, a forensic scientist and researcher at Murdoch University in Perth, Australia, says the technique uses the polymerase chain reaction (PCR) method to amplify 30 specific regions of the genome that have a lot of genetic variation.
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These amplified regions are sequenced to determine the number of STRs and their base-pairing patterns, which can be compared to genealogy databases or STR profiles of suspects to identify potential matches. Typically, Chapman says, “we can distinguish one person from another very easily”. But with monozygotic twins, there is “no difference” in those areas of the genome.
detailed analysis
Whole-genome sequencing may make it possible to tell identical twins apart. By analyzing the entirety of an individual’s genome, scientists can identify differences caused by mutations that occur after the egg divides. 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 in which whole genome sequencing has helped courts distinguish between twins, says Xanthe Weston, a criminologist and researcher at Central Queensland University in Mackay, Australia. But using whole-genome sequencing requires obtaining sufficient amounts of DNA to analyze, Weston says.
Other researchers have reported success in distinguishing between twins by sequencing the DNA found in the cells’ powerhouse mitochondria. Compared to nuclear DNA, which is used in genome sequencing and STR analysis, mitochondrial DNA (mtDNA) is more frequently mutated, meaning it is more likely to differ between twins.
US courts have accepted mtDNA analysis as evidence since the mid-1990s. But it hasn’t been used in cases involving twins, Weston says.
‘Epigenetic’ changes
Another promising technique considers adding methyl groups to DNA, a process called methylation, which can change the way genes work. These ‘epigenetic’ changes can be caused by factors including a person’s behavior – such as their diet and drinking or smoking habits – and their environment.
Last year, a team of scientists in South Korea sequenced the genomes of 54 pairs of newborn monozygotic twins. After identifying 711 sites where methylation commonly varied, they selected and tested different combinations and 5 that were particularly variable. By comparing how much methylation occurred at these five sites, the team was able to differentiate between 50 of the 54 pairs of twins. When they repeated the experiment in two adult groups, they were able to distinguish between 41 out of 47 pairs of twins and 105 out of 118 pairs of twins.
DNA-methylation analysis has not yet been used to distinguish between twins in court. And Chapman says such analysis may not be able to distinguish between twins who have similar differences due to living similar lifestyles or being exposed to similar environmental factors such as pollution.
Challenges remain
The new techniques are promising, Chapman says, but challenges must be 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 contain very small amounts or are in poor condition.
The analyzes can also be expensive and time-consuming, partly because of the work required to demonstrate in court that the results are conclusive enough to count as evidence. Last year, whole-genome sequencing was accepted for the first time in a US murder trial.
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 why someone’s DNA might be present at a crime scene. She adds, “I would never want anyone to be prosecuted based solely on DNA.”
This article is reproduced with permission and was first published on 5 March 2026.
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