Identical twins: Can DNA distinguish which twin dunnit?

The real life murder trial of 33-year-old brothers, Jérémy and Samuel Youmbi in France has revived the age old dilemma: which twin pulled the trigger? After reviewing the DNA on the assault rifle used to commit the double murder, one investigator is quoted as saying, "Only their mother can tell them apart."

In response to this trial, Nature published an article on scientific advances to help distinguish the DNA of identical twins when both are accused of the same crime. Rarely does Nature extend a helping hand to crime writers in such a specific way, so of course, I'm thrilled to summarize it.

I understand that people may yawn at the mere mention of DNA but this is eye opening rather than yawn worthy. Perhaps the lede of the DNA story will interest you; being born with the same DNA doesn't mean it stays the same because life changes us. This is true for characters in novels and also for DNA. In a novel we call it a character arch, in DNA we call it mutations.

Identical twins have the same DNA because they come from a single egg splitting in two after being fertilized by a single sperm.

Forensic scientists typically attempt to identify the person DNA belongs to using a technique that essentially samples parts of the DNA, concentrating on certain areas that distinguish people easily. This is called short tandem repeat (STR) analysis, using the polymerase chain reaction (PCR) method to amplify up to 30 specific regions of the genome that have a lot of genetic variation. Normally, this easily differentiates one person from another but identical twins are born with the same DNA in these regions.

With twins, analyzing the entirety of a person’s genome allows scientists to identify differences caused by mutations that occurred after an egg split. The problem is that these mutations are rare, random and also require finding enough DNA to do this analysis.

Another option is sequencing DNA found in the mitochondria, the powerhouses of cells. Mitochondrial DNA (mtDNA) mutates more often, meaning it is more likely to differ between twins.

A final and promising option is where life really changes DNA: methyl groups. These are the basis of the often discussed ‘epigenetic’ changes caused by factors including a person’s behaviours — such as their diet and drinking or smoking habits — and their environment. These changes add methyl groups to DNA, a process called methylation, which can change how genes work.

These techniques are in their infancy and require more research to bring them into court as a reliable way to distinguish between identical twins. However their promise lies in the fascinating fact that the DNA we're born with changes over the course of our lives.