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 couldn’t 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 can’t differentiate between DNA belonging to identical twins
He’s 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 wasn’t the only person whose DNA matched that semen. His twin, Dwight, was also a match. Traditional forensic methods can’t 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 investigators that 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 suspect’s 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 doubt---based 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 what’s 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 the genetic letters (called base pairs) comprising the 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.
The sequence mutations are random, so it’s incredibly unlikely they’d be the same in both twins---and it’s those discrepancies that can be used to pin a crime on a twin.
In a proof-of-principle study, Rolf and his colleagues analyzed sequences from a pair of twins and one of their kids. Scientists could positively identify which twin was the child’s father, based on five single base pair differences present in the father and son, but not in the uncle. They published the work earlier this year in Forensic Science International: Genetics; it’s this paper that caught the Suffolk District Attorney’s attention.
The office sent evidence to Eurofins for analysis. After the results came back, the McNair was indicted and arraigned for the crimes in September.
“At arraignment, the assigned prosecutor cited the Eurofins test results and said Dwayne McNair was ‘two billion times more likely’ than his twin to have been the source of the crime scene DNA,” says Suffolk DA spokesperson Jake Wark.
Now, the question is: Will the genetic test be admissible in court? It would be the first time it's been used in the United States. A preliminary hearing has been scheduled for Jan. 12, 2015, after which a judge will decide whether evidence from the test is admissible.
Some experts seem to think it will hold up.
“It is scientifically sound and reliable, has a high probability of success, is based on standard, generally accepted forensic DNA sequencing technology, and has an infinitesimally low risk of error if proper laboratory practices are followed,” said Bruce Budowle, in an affidavit to the court. Budowle, who is now at the University of North Texas Health Science Center, once led the FBI’s DNA typing laboratory.
Yet some scientists are a bit wary. “I think it’s an interesting idea,” says computational biologist Yaniv Erlich of the Massachusetts Institute of Technology. The work published by the Eurofins team is accurate, he says, but is only based on one pair of twins. In an ideal world, Erlich would test the method using dozens of twin pairs, while simulating the small amounts of DNA that might be found at crime scenes.
He’s also concerned about the test’s applicability to different tissue types, and blood in particular (which is not an issue in the McNair case). In 2011, Erlich took a close look at the blood of identical twins who shared a placenta during development; early on, these twins are also sharing blood – even as adults, each twin has blood cells with DNA from the other twin.
“Think about one twin being Coca-Cola, the other twin is Sprite,” Erlich said. Their blood will be a mix of Coke and Sprite. “It’s not 50-50,” he said, “but it could be 20-80.”
In other words, while the Eurofins method might not work as well with blood samples left at crime scenes. But when it comes to saliva, skin, or semen, it could mean that identical twins are about to lose their genetic get-out-of-jail-free card.
