Identical twins on trial: can DNA testing tell them apart?
In a recent French criminal trial, a perplexing question arose: can DNA testing differentiate between twin brothers? The case involved twin brothers whose DNA was found on a gun, but traditional DNA analysis failed to determine which brother the DNA belonged to. These brothers are monozygotic twins, meaning they share the same DNA as a result of a single fertilized egg splitting into two.
Forensic scientists typically rely on a technique called short tandem repeat (STR) analysis to identify individuals based on their DNA. This method amplifies specific regions of the genome with genetic variation and compares them to databases or suspect profiles. However, in the case of monozygotic twins, where there is no genetic difference in these regions, traditional DNA testing falls short.
Whole-genome sequencing offers a potential solution to this dilemma. By analyzing the entire genome, scientists can identify rare mutations that occurred after the egg split, allowing them to differentiate between identical twins. While this approach has been successful in some cases, it requires a significant amount of DNA for analysis.
Another promising technique involves examining DNA methylation, which can alter gene function. Variations in DNA methylation patterns can differentiate between twins, as evidenced by a study in which scientists successfully distinguished between pairs of twins based on methylation differences.
Despite these advancements, there are challenges to implementing these techniques in forensic investigations. Obtaining sufficient DNA samples from crime scenes, the cost of analysis, and the need to validate results for courtroom use are all hurdles that need to be overcome.
It is essential to combine DNA evidence with other forms of evidence, such as fingerprints, when prosecuting individuals based on DNA alone. While advancements in DNA analysis offer new possibilities in forensic science, a holistic approach to evidence gathering and analysis remains crucial in ensuring justice is served.
This article was originally published on March 5, 2026, and is reproduced with permission from Nature magazine.
