Can Science Solve France's Twin Rape Mystery?

Twins Even identical twins aren't exactly identical Dreamstime

A pair of identical twins is currently being detained in the French city of Marseille for six rapes committed in the past six months. Victims have identified the twins’ face, and DNA evidence from one of the crime scenes confirmed that at least one of them is guilty. But because the twins, identified only as Yohan and Elwin, share almost the same DNA, prosecutors say they have no way of determining which brother is the rapist, or if they were both involved. Shouldn't science be able to solve this kind of mystery?

Theoretically, yes.

Monozygotic twins, such as Yohan and Elwin, are created from one egg fertilized by one sperm. Until recently, it was believed that their DNA was 100% identical. But rare mutations occur as the fetuses develop, so while both genomes start out the same, they end up being different.

The problem is that those mutations can’t be found in all tissues, which means that looking for genetic differences between twins is like trying to find a needle in a haystack.

In a similar rape case involving identical twins in Michigan in 2004, a forensic laboratory called Orchid Cellmark tried to find DNA mutations that might reveal which brother’s semen was the one found at the crime scene. At the time, scientists targeted about 500,000 common genetic variations that might differ between brothers. (Those are called single nucleotide polymorphysms, SNPs, and are genetic variations present in at least 1 percent of the population.) But they weren’t able to identify any differences with certainty, and prosecutors didn’t convict anyone.

Research on identical twins has progressed since then, but it usually focuses on the origins of diseases (looking at the genetic differences between one twin with Parkinson’s and one without, for instance), not on distinguishing twins’ genomes for rape trials.

A major breakthrough occurred in 2008, when geneticists at the University of Alabama found more genetic variations between twins than previously thought. They reached that conclusion by looking not at SNPs, but at Copy Number Variations, a type of mutation that few researchers had studied at the time. The team led by Carl Bruder and Jan Dumanski compared 19 pairs of identical twins, and found CNVs (which happen when genes are deleted or inserted in the DNA) in all the twins, meaning that they were all distinguishable.

The rub: the technique for distinguishing between identical twins’ DNA is not ready to be used in court.This fall, researchers from McGill University in Montreal also announced they were able to distinguish between identical twins’ DNA, this time by looking at “single base mutations,” mutations where a single letter in the genome is changed to another letter. “We have produced evidence that in some sets of identical twins, the identity of each twin can be discerned through DNA testing,” Brent Richards, an assistant professor of medicine and human genetics at McGill, told Popular Science.