Credit: Denis Charlet/AFP via Getty
An identical twin commits a crime and leaves their DNA at the scene. Can DNA testing reveal which twin is the culprit?
This question reportedly came up in a case in France last month, where the DNA of twin brothers was found on a gun, but conventional DNA testing could not determine which of the brothers it belonged to. The men are monozygotic twins — the result of a single egg splitting in two after being fertilized by a single sperm — meaning they have the same DNA.
When DNA is found at a crime scene, forensic scientists typically attempt 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 scientist and researcher at Murdoch University in Perth, Australia.
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 any potential matches. Ordinarily, Chapman says, “we can differentiate one person from the next very easily”. But with monozygotic twins, “there just is no difference” in those regions of the genome.
Deep dive
Whole-genome sequencing can make it possible to tell identical twins apart. By analysing the entirety of a person’s genome, scientists can identify differences caused by mutations that occurred after an egg split. These changes are rare, says Chapman. For instance, a 2014 study identified just five genetic changes between a pair of adult twins1.
There have been rare cases in which sequencing of the entire genome has helped courts to distinguish between twins, says Xanthé Weston, a criminologist and researcher at Central Queensland University in Mackay, Australia. But using whole-genome sequencing requires sufficient amount of DNA to analyse, adds Weston.
Other researchers have reported success in distinguishing between twins by sequencing DNA found in the mitochondria, the powerhouses of cells. Compared with nuclear DNA, which is used in genome sequencing and STR analysis, mitochondrial DNA (mtDNA) mutates more often, 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 behaviours — 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 at which methylation commonly differed, they selected and tested various combinations and 5 that were particularly variable. By comparing how much methylation had occurred at these five sites, the team was able to differentiate between 50 of the 54 pairs of twins2. When they repeated the experiment in two adult cohorts, they succeeded in differentiating between 41 of 47 pairs and 105 of 118 pairs of twins.
