Equivalent twins on trial: can DNA testing inform them aside?
In a French legal trial, standard DNA evaluation couldn’t distinguish between twin brothers, however rising scientific strategies might assist in such instances
Typical DNA testing failed to differentiate between twin brothers throughout a French legal trial.
Denis Charlet/AFP through Getty
An equivalent twin commits a criminal offense and leaves their DNA on the scene. Can DNA testing reveal which twin is the wrongdoer?
This query reportedly got here up in a case in France final month, wherein the DNA of dual brothers was discovered on a gun, however standard DNA testing couldn’t decide which of the brothers it belonged to. The lads are monozygotic twins — the results of a single egg splitting in two after being fertilized by a single sperm — that means they’ve the identical DNA.
When DNA is discovered at a criminal offense scene, forensic scientists sometimes try to determine the individual it belongs to utilizing a method known as brief tandem repeat (STR) evaluation. The approach makes use of the polymerase chain response (PCR) technique to amplify as much as 30 particular areas of the genome which have quite a lot of genetic variation, says Brendan Chapman, a forensic scientist and researcher at Murdoch College in Perth, Australia.
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These amplified areas are sequenced to find out the variety of STRs and their base-pair patterns, which might then be in contrast towards family tree databases or STR profiles of suspects to determine potential matches. Ordinarily, Chapman says, “we are able to differentiate one individual from the subsequent very simply”. However with monozygotic twins, “there simply is not any distinction” in these areas of the genome.
Deep dive
Entire-genome sequencing could make it doable to inform equivalent twins aside. By analysing everything of an individual’s genome, scientists can determine variations brought on by mutations that occurred after an egg break up. These modifications are uncommon, says Chapman. As an illustration, a 2014 examine recognized simply 5 genetic modifications in a pair of grownup twins.
There have been uncommon instances wherein sequencing of the complete genome has helped courts to differentiate between twins, says Xanthé Weston, a criminologist and researcher at Central Queensland College in Mackay, Australia. However utilizing whole-genome sequencing requires acquiring a enough quantity of DNA to analyse, provides Weston.
Different researchers have reported success in distinguishing between twins by sequencing DNA discovered within the mitochondria, the powerhouses of cells. In contrast with nuclear DNA, which is utilized in genome sequencing and STR evaluation, mitochondrial DNA (mtDNA) mutates extra typically, that means it’s extra more likely to differ between twins.
US courts have admitted mtDNA evaluation as proof for the reason that mid-Nineties. Nevertheless it has not been utilized in instances involving twins, says Weston.
‘Epigenetic’ modifications
One other promising approach seems on the addition of methyl teams to DNA, a course of known as methylation, which might change how genes work. These ‘epigenetic’ modifications might be brought on by elements together with an individual’s behaviours — similar to their weight loss program and ingesting or smoking habits — and their atmosphere.
Final yr, a workforce of scientists in South Korea sequenced the genomes of 54 pairs of new child monozygotic twins. After figuring out 711 websites at which methylation generally differed, they chose and examined numerous combos and 5 that have been significantly variable. By evaluating how a lot methylation had occurred at these 5 websites, the workforce was capable of differentiate between 50 of the 54 pairs of twins. Once they repeated the experiment in two grownup cohorts, they succeeded in differentiating between 41 of 47 pairs and 105 of 118 pairs of twins.
DNA-methylation evaluation has not but been used to differentiate between twins in court docket. And Chapman notes that this sort of evaluation won’t be capable of distinguish between twins who’ve comparable variations brought on by main comparable life or being uncovered to comparable environmental elements similar to air pollution.
Challenges stay
The brand new strategies are promising, Chapman provides, however there are challenges to beat earlier than they can be utilized in forensic investigations. As an illustration, all of them require comparatively giant quantities of DNA, however samples from crime scenes typically comprise solely small quantities or are in poor situation.
Analyses may also be costly and time-consuming, partly due to the work wanted to exhibit to a courtroom that the outcomes are conclusive sufficient to be counted as proof. Final yr, whole-genome sequencing was admitted in a US homicide trial for the primary time.
Weston says that you will need to mix DNA proof with different varieties of proof, similar to fingerprints, when prosecuting somebody, as a result of there are numerous the explanation why somebody’s DNA is perhaps current at a criminal offense scene. “I might by no means need to see any individual prosecuted on the premise of DNA alone,” she provides.
This text is reproduced with permission and was first printed on March 5, 2026.
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