In late 2012, a man raped six women in the south of France. DNA evidence led officials to two suspects, a pair of twins. The victims recognized the men, but couldn’t tell them apart; since the twins’ DNA is identical, officials didn’t have a way to figure out which one of them to prosecute.
Forensics specialists have a few ways to tell twins apart, such as testing sperm or using identifying markings like tattoos or scars, but these techniques are very limited. Now a team of researchers has developed a new way to differentiate twins’ DNA by identifying parts of it that have changed over time because of environmental factors.
Over time, factors like diet and smoking can change how our DNA is expressed, which is called epigenetics. These environmental factors often cause certain chemicals that are part of the methyl group to attach to the DNA. But the methylation does more than just change how the DNA is expressed—it changes the DNA’s melting point. The researchers took cheek swabs of five pairs of identical twins, extracted the DNA, and then identified spots in the DNA with particularly indicative types of methyl chemicals had bonded. The researchers then calculated the melting point of the DNA and found that the methylation changes the melting point ever so slightly, allowing the researchers to distinguish one twin from another.
The researchers acknowledge that this technique might not work with twins that are too young to have undergone much epigenetic change, or those who have lived in such similar environments their entire lives that their DNA methylation would be exactly the same. Still, this technique is much faster and cheaper than existing methods, which might help officials effectively solve more crimes.