By Noelle Phelan

DNA testing has frequently been used to assist law enforcement in solving crimes. Where DNA evidence is left behind at the scene of a crime, DNA profiling of the evidence can potentially be used to locate the perpetrator. The DNA evidence at the scene of the crime can be matched to the perpetrator either via the presence of their DNA in the system or by a sample obtained from a suspect, either voluntarily or by warrant. However, a problem arises when the suspect in question is a twin, specifically an identical twin. Identical twins have the same DNA; therefore finding DNA evidence only narrows it down to one of the two twins, but does not distinguish which of the twins may have committed the crime in question. This was the circumstance in a relatively recent rape case which occurred in Boston.

In 2004, two women were brutally beaten and raped by an unknown attacker. The victim of the second assault managed to take from the scene of the crime the used condom that one of the attackers had used during the sexual assault. She turned this evidence into the police, which provided them with a sample of DNA from the unknown rapist. Several years later, the police obtained a tip regarding one Dwayne McNair, and tailed him to work where they obtained his discarded cigarette butt. The DNA on the tip of the cigarette matched the DNA from the condom that the victim had turned over to the Boston Police Department following her assault. The police believed they had their man, but a problem arose when the BPD discovered that Dwayne McNair had not only just a twin, but an identical twin.

Both of the twins were ordered by the court to submit a DNA sample, and both twins complied. The issue was not resolved however because the DNA testing of the twins’ samples indicated that they were truly identical twins. The testing could not differentiate one twin’s DNA from the other’s and therefore could not show which twin had left the DNA at the scene of the crime. This left the DA’s office in the untenable position of being able to prove that one of the twins had committed the rape, but unable to prove which of the twins had done it. The majority of the scientific world had accepted that it was impossible to tell the DNA of identical twins apart. While many scientists agree that not every gene in identical twins are exactly the same, the general consensus was that there was no way to determine the slight differences that may arise between the genes of identical twins.

In 2014, the Suffolk DA’s office, led by prosecutor David Deakin was ready to proceed on the trial against Dwayne McNair, relying on the DNA evidence to show that it was one of the twins who had committed the rape, and the testimony of the other assailant in the crime who was identified by a national DNA database, and who claimed that it was in fact Dwayne that had been the other rapist. The case was not as solid as they would have liked, as there was ambiguity as to whether the witness could in fact determine which of the twins was actually present at the scene of the crime. Just before the trial was slated to start, prosecutor Deakin learned about a new technological advancement by a German company which claimed to be able to successfully distinguish one identical twin’s DNA from the other’s.

The company called Eurofins, applied massive parallel sequencing to the DNA of identical twins which allowed them to map the genome of each twin. This technology enabled the scientists at Eurofins to see the entire genetic code of a person, including any mutations which may have occurred. While identical twins have virtually the same DNA, not every single cell on every single gene is exactly the same. There may be mutations in one twin’s DNA which do not appear in the DNA of the other twin. Eurofins first tested their technology on a pair of identical twin brothers and mapped out the genetic code of each of the twins. When they compared the two genomes, they discovered that one of the twins had five mutations that were not demonstrated in the DNA of the other. Eurofins then obtained the DNA of a child of one of the twins, but they did not know which twin was the father and which was the uncle. When they mapped the child’s genetic code, they determined that the child had the same five mutations as one of the twins, correctly indicating that twin was the father of the child and the other twin was the uncle.

Prosecutor Deakin realized that this could be the solution to his problem when it came to the DNA of identical twins. The Suffolk DA hired Eurofins to test the DNA of both of the McNair twins, along with the DNA which was obtained at the crime scene. When the genome sequencing was completed, there were nine differences out of 3 billion in the genetic code of the McNair twins from the DNA found at the crime scene. Eurofins then looked to the DNA sequencing from the DNA found in the condom of the suspect and compared that with the DNA samples from the McNair twins. The analysts at Eurofins found seven locations where the DNA of Dwayne McNair’s saliva matched with that of the suspect, and two locations where his twin Dwight’s DNA matched. They also determined that two of the mutations which were observed in Dwayne’s DNA were also present in the DNA obtained from the crime scene, and no mutations were observed in his twin’s sample. Based on this evidence, the scientists at Eurofins determined that Dwayne McNair was two billion times more likely than his twin to be the rapist.

This new technology obviously has massive implications for the field of criminal law. While the rate of instances where a defense is raised because of the ambiguity of DNA in identical twins is incredibly low, this technology potentially eradicates that defense entirely. In addition, this type of DNA testing and mapping will provide better and more accurate results than the DNA testing which is currently being utilized by the courts, as it will show down definitively down to the gene who the DNA in question belongs to. The field of DNA evidence and testing has been growing and advancing by leaps and bounds over the last few decades, and this technology will provide the impetus for this field to continue to advance and make conviction of criminals easier and more exact.

The problem with this type of DNA testing is with its reliability. While the scientific data seems to show that there is enough proof that this sequencing works and is accurate and reliable, this is the first company to suggest that there is a possibility of differentiating between identical twins based on their genetic code or genome. This technology has never been admitted in a court of law before, and thus the defense raised a Daubert hearing to challenge the validity of the technology and the expert testimony regarding the results of the testing which will be used in court against Dwayne McNair. In order for the prosecution to be able to use this type of testing, both in this case and in future cases, a judge will have to make a ruling regarding whether or not this technology, and the testimony inextricably linked with it, is the product of reliable scientific principles and methods, and that it applies to the facts of the case.

If the judge rules that it does meet this standard, then the prosecution will be allowed to use it in their case against Dwayne McNair. If the judge rules that this standard is not met, then prosecutor Deakin will be forced to rely on the DNA testing already at hand, and the word of the other assailant in this case to obtain a conviction against Dwayne McNair.

Disclaimer: The views expressed in this blog are the views of the author alone and do not represent the views of JHTL or Suffolk University Law School.