Can DNA testing inform an identical twins on trial aside?

An identical twins on trial: can DNA testing inform them aside?

By Rachel Fieldhouse & Nature journal

An an identical twin commits against the law and leaves their DNA on the scene. Can DNA testing reveal which twin is the perpetrator?

This query reportedly got here up in a case in France final month, through which 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 boys are monozygotic twins — the results of a single egg splitting in two after being fertilized by a single sperm — which means they’ve the identical DNA.

When DNA is discovered at against the law scene, forensic scientists sometimes try and establish the individual it belongs to utilizing a way referred to as quick tandem repeat (STR) evaluation. The method makes use of the polymerase chain response (PCR) methodology 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.

On supporting science journalism

If you happen to’re having fun with this text, contemplate supporting our award-winning journalism by subscribing. By buying a subscription you’re serving to to make sure the way forward for impactful tales in regards to the discoveries and concepts shaping our world at the moment.

These amplified areas are sequenced to find out the variety of STRs and their base-pair patterns, which might then be in contrast in opposition to family tree databases or STR profiles of suspects to establish potential matches. Ordinarily, Chapman says, “we will differentiate one individual from the subsequent very simply”. However with monozygotic twins, “there simply isn’t any distinction” in these areas of the genome.

Deep dive

Complete-genome sequencing could make it potential to inform an identical twins aside. By analysing everything of an individual’s genome, scientists can establish 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 circumstances through which sequencing of the whole genome has helped courts to tell apart between twins, says Xanthé Weston, a criminologist and researcher at Central Queensland College in Mackay, Australia. However utilizing whole-genome sequencing requires acquiring a ample 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, which means it’s extra more likely to differ between twins.

US courts have admitted mtDNA evaluation as proof for the reason that mid-Nineties. But it surely has not been utilized in circumstances involving twins, says Weston.

‘Epigenetic’ modifications

One other promising method seems on the addition of methyl teams to DNA, a course of referred to as methylation, which might change how genes work. These ‘epigenetic’ modifications may be brought on by components together with an individual’s behaviours — corresponding to their weight-reduction plan and consuming or smoking habits — and their surroundings.

Final 12 months, a crew 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 varied combos and 5 that have been significantly variable. By evaluating how a lot methylation had occurred at these 5 websites, the crew 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 tell apart between twins in court docket. And Chapman notes that this sort of evaluation won’t have the ability to distinguish between twins who’ve related variations brought on by main related life or being uncovered to related environmental components corresponding to air pollution.

Challenges stay

The brand new methods 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 massive quantities of DNA, however samples from crime scenes typically comprise solely small quantities or are in poor situation.

Analyses will also be costly and time-consuming, partly due to the work wanted to reveal to a courtroom that the outcomes are conclusive sufficient to be counted as proof. Final 12 months, 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 forms of proof, corresponding to fingerprints, when prosecuting somebody, as a result of there are a lot of the reason why somebody’s DNA is likely to be current at against the law scene. “I might by no means wish to see someone prosecuted on the idea of DNA alone,” she provides.

This text is reproduced with permission and was first printed on March 5, 2026.

It’s Time to Stand Up for Science

If you happen to loved this text, I’d wish to ask on your assist. Scientific American has served as an advocate for science and business for 180 years, and proper now could be the most crucial second in that two-century historical past.

I’ve been a Scientific American subscriber since I used to be 12 years outdated, and it helped form the way in which I have a look at the world. SciAm at all times educates and delights me, and evokes a way of awe for our huge, stunning universe. I hope it does that for you, too.

If you happen to subscribe to Scientific American, you assist be certain that our protection is centered on significant analysis and discovery; that we now have the sources to report on the selections that threaten labs throughout the U.S.; and that we assist each budding and dealing scientists at a time when the worth of science itself too typically goes unrecognized.

In return, you get important information, fascinating podcasts, sensible infographics, can’t-miss newsletters, must-watch movies, difficult video games, and the science world’s greatest writing and reporting. You’ll be able to even present somebody a subscription.

There has by no means been a extra essential time for us to face up and present why science issues. I hope you’ll assist us in that mission.