DNA Test That Distinguishes Identical Twins May Be Used in Court for First Time

In 2004, two young women were abducted at gunpoint while walking home near Boston at night. The crimes happened eight days apart, but the pattern was the same: The women were shoved into a car by two men, pistol-whipped, driven to a different location, and raped. While collecting her clothes, the second victim managed to grab the condom one of the men had worn; she hid it in her pocket, and turned it in as evidence.

One of the two men involved pleaded guilty to the attacks in 2012. The other remained at large. Police had a suspect, but they couldnt pin the crime on him due to a twist of genetic fate: He had an identical twin brother, and DNA from the condom matched both siblings. But now, a decade after the assaults, scientists have developed a genetic test that can distinguish between identical twins, and it may be used in court for the first time in this case.

The second suspect is 33-year-old Dwayne McNair. In September, McNair was arraigned on eight counts of aggravated rape and two counts of armed robbery, stemming from the two sexual assaults.

Traditional forensic methods cant differentiate between DNA belonging to identical twins

Hes been a suspect in the crimes since 2007. According to court documents, a standard genetic test linked him to semen collected from the second attack back in 2008. That would ordinarily be enough to justify charges, but Dwayne wasnt the only person whose DNA matched that semen. His twin, Dwight, was also a match. Traditional forensic methods cant differentiate between DNA belonging to identical twins, and without a clear way to establish whether Dwayne or Dwight had left the semen at the scene, police had no probable cause to make an arrest in 2008.

But in 2012, the other man involved in the assaults told investigatorsthat Dwayne had been his partner in the crimes. And earlier this year, prosecutors learned of a new forensic genetics test claiming to differentiate between biological samples belonging to identical twins. According to the Suffolk County District Attorney the test points to Dwayne, not Dwight, as the perpetrator of the 2004 assaults.

Normally, forensic tests work by extracting and amplifying regions of DNA collected from a crime scene. Then, investigators look for a match between the evidence and a suspects genetic sequence. Ordinarily, this kind of testing is sufficient: Most humans vary from one another enough for investigators to easily identify whether a suspect left blood, skin, hair, semen, or something else at a crime scene.

This is not true with identical twins. Grown from the same, single fertilized egg, monozygotic twins have nearly identical genomes. So, for decades, twins committing crimes had a relatively easy way to establish doubtbased on DNA evidence alone, their identical sibling would be equally as likely to have deposited whatever genetic material might have been left at a crime scene.

Maybe not anymore.

Using whats known as ultra-deep, next-generation sequencing, a team in Germany has developed a test that claims to reliably identify which twin a biological sample belongs to. The test works by taking a close look at thegenetic letters (called base pairs) comprisingthe 3 billion-base-pair human genome. Because mutations randomly occur during development, even genetically identical twins will vary at a handful of locations, says Burkhard Rolf, a forensic scientist at Eurofins Scientific, the company that developed the test.

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DNA Test That Distinguishes Identical Twins May Be Used in Court for First Time