For the first time in the US, an identical twin has been convicted of a crime based on DNA analysis.
On 21 August, a jury found Russell Marubbio, 54, guilty of a sexual assault that occurred in Woodbridge, Virginia, in 1987.
Virginia-based Parabon NanoLabs ran an analysis on the DNA sample found at the scene nearly four decades ago. Genealogist CeCe Moore then traced the perpetrator’s family tree and seemed to hit a wall when the sample led to identical twins.
Usually, that would be the end of using DNA to solve the case. Identical twins have the same DNA – almost. There are sometimes a few cells that split and evolve differently.
“On average, twins will have eight of those types of differences across the whole 3 billion bases” of the genome, said Ellen Greytak, the director of bioinformatics at Parabon.
The scientists were able to detect some of these mutations in the sample, compared to cheek swabs from the Marubbio brothers, leading to the conviction on Friday.
“This landmark case marks the first successful admission and application of this specific technique in a US court to overcome challenges in DNA identification of identical twins,” Amy Ashworth, the commonwealth’s attorney for Prince William county, Virginia, wrote in a statement.
On 19 December 1987, a 50-year-old cashier at a Chevron gas station in Woodbridge, took a bathroom break and was followed by a young man. He restrained her and sexually assaulted her at knifepoint.
The assailant left semen at the crime scene, but his DNA never matched with any offenders listed in the law enforcement database known as the Combined DNA Index System (Codis).
In 2019, Giannina Pinedo, a master detective with 22 years of experience, transferred to the cold case unit in Prince William county. She began looking through unsolved cases where DNA samples were present at the crime scene, alongside Colleen Grantham, another master detective with 23 years of experience.
Scientific and technological advances in DNA sequencing and genealogical databases are now yielding breakthroughs in cases that long ago went cold. The Golden State Killer, for example, was arrested in 2018 after genetic genealogy led to a suspect, Joseph James DeAngelo, who pleaded guilty in 2020.
The two Virginia detectives worked with several labs in different states. They submitted the DNA evidence to Parabon in August 2022. The company put together a sketch, based on genetics, of what the perpetrator might look like – and Moore, their chief genetic genealogist, tracked down possible suspects.
The detectives worked with special agents in Florida, where the Marubbio twins lived, to obtain cheek swabs from the brothers for genetic sequencing.
Parabon scientists then did whole-genome sequencing to uncover the smallest genetic differences between the men.
“These are somatic mutations,” said Claire Glynn, a professor and the executive director of the Henry C Lee college of criminal justice and forensic sciences at the University of New Haven. “A better word, I think, for the public is acquired mutations, where you can differentiate between identical twins.”
She thinks of it as similar to punctuation in a sentence. The “words” don’t change – the arrangements of A, T, G and C are the same in both twins – but sometimes there are commas in one sample where another has periods.
“We would see the mutations from very early on in life,” Glynn said. They develop in utero, after the embryo splits in two.
“In terms of the science, that’s well studied and well understood,” she said. “The science behind it is absolutely robust. It’s simply that it’s not that common that we would encounter this in cases.”
There was a case in Massachusetts in 2017 where a similar analysis was performed, but the judge on that case said the evidence was not admissible.
In 2022, an identical twin was convicted of sexual assault in the Netherlands using similar genetic analysis.
The rare genetic differences between identical twins may not show up at every place in the genome, Parabon’s Greytak said.
“Only certain cells in the twin will have that mutation and other cells won’t. So they’re like a mosaic.”
The scientists employed “ultra-deep sequencing”, looking for any differences in the twins’ samples, Greytak said. “It’s really a small amount, but it’s a very distinctive, clear difference.”
Those differences matched with the semen sample left at the crime scene, and Marubbio was convicted of violent abduction and rape.
“Occasionally, you get these identical twin defense cases. And it used to be, they would give up,” said David Kaye, a professor emeritus of law at Arizona State University. “They would say: ‘Well, all right, the DNA is not going to help us.’ Now, it is possible, and it will.”
Commercial genealogy has also expanded rapidly in recent years with the advent of inexpensive home testing and online databases. Law enforcement are permitted to search two databases, GEDMatch and FamilyTreeDNA, only among users who have opted into law enforcement use.
In this case, Moore traced two second cousins back to a common ancestor in the 1860s, and then built a family tree forward.
“Two of our matches were second cousins once removed to each other, and that’s really optimal for us. If someone is related, but not too related, it points to just one branch of their family tree that we need to look at,” she said.
Moore hit an early snag when none of the people in one branch seemed to match; eventually, she realized that one of the matches had accidentally switched their DNA kit with their spouse.
“When we looked at the spouse’s tree, that’s where we found the connection right away. It jumped out at us,” Moore said.
She also looked at geographic location – someone who was reasonably close to the scene of the crime. Investigative genealogists will search for a host of records: birth, death, and marriage certificates; census entries; information from graveyards; newspaper clippings.
In this case, Moore said: “We were able to find some very compelling information in high school yearbooks.”
Once possible suspects are identified, genealogists like Moore can inform detectives of the lead.
“It’s what I call a highly scientific tip,” Moore said. “They still have to do their full investigation, their due diligence, just like they would if I called the tip into Crime Stoppers.”
Forensic investigative genetic genealogy, or Figg, “gets us to a potential person of interest”, Glynn said.
Sometimes law enforcement will then collect DNA from the suspect surreptitiously – looking through trash, if local laws allow it; collecting dishes or utensils from restaurants; locating discarded coffee cups or cigarettes.
If those samples match the crime scene DNA, then detectives move forward with the case. That often involves an arrest and a cheek swab to see if that sample also matches.
The number of identical twins is “higher than people usually think”, Kaye said. “It’s not a common situation in the criminal investigative area, but it certainly occurs. So I would expect there would be more cases,” where analyses like these are performed to differentiate identical twins.
And identical twins are “increasingly common because of fertility treatments”, Moore said.
Understanding the genetic differences between identical twins has long been a goal of researchers, Glynn said.
With the Marubbio analysis, “to actually have it used in court and have it accepted by the judge is a huge step forward”, Moore said.
These kinds of analyses are becoming more common in cold cases as well as active cases, and more law enforcement are being trained on conducting genealogical research, Glynn said.
The process can be expensive – especially when the crime-scene samples were heavily degraded by time or the environment.
“But it can also be moderately cost effective,” Glynn said, especially compared to the resources spent on cold cases that stretch over decades.
“If we invest and spend the money on these advanced methods that then identify and allow us to apprehend perpetrators of violent crimes, the impact on society – you can’t put a value on that, because if a violent serial offender is apprehended, no further people can be harmed,” she said.
