Ricardo G. Federico In The News
A primer on DNA forensics
Deoxyribonucleic acid, “the building block of life,” has become a standard for forensic evidence. The famous “double helix” molecule is unique to each person with the exception of identical twins. DNA is shed at crime scenes through hairs, blood, semen, saliva — virtually any biological matter. It is very stable, meaning even old and degraded samples can be analyzed and compared. Once considered mysterious and suspicious, DNA evidence is now a routine part of the judicial process.
The first forensic use of DNA occurred in England in 1983, when samples from a murder and sexual assault exonerated the main suspect in the case. Three years later, DNA from a second murder was used to link the crimes and convict a different man. In Canada, DNA was first used in 1987 in the case of the “Spandex rapist” who had attacked seven women in Edmonton. But the sample available for testing proved too small to give a reliable result and the suspect was acquitted. (The man was convicted 20 years later when DNA linked him to a separate sexual assault “cold case” in Calgary.)
The first conviction in Canada occurred in Ottawa in 1989, when 32-year-old Paul McNally pleaded guilty to sexually assaulting a 68-year-old woman in her New Edinburgh home. At the time, the Citizen wrote how police used “novel genetic fingerprinting” to link the suspect to the crime, which the judge described as “one of the most repulsive attacks on a woman I’ve ever encountered.”
The victim had identified McNally as her attacker, though he insisted he was innocent. But DNA taken from a sample of McNally’s blood was found to match semen from the attack. After listening to the Crown present its case, McNally switched his plea to guilty and was sentenced to seven years in prison.
Other Ottawa cases
In July 2014, the Sandy Hill Stalker, 35-year-old Martin Dupuis, pleaded guilty to sexual assault and breaking and entering after DNA evidence linked him to one of the crimes.
In January 2013, police charged 34-year-old Clayton Woodman for a bank robbery that had occurred seven years before at the Rideau Centre. Woodman had been ordered to provide a DNA sample after a separate conviction and that sample was eventually linked to the Rideau Centre crime.
In February 2013, police laid first-degree murder charges against Marc Leduc, 56, alleging that DNA evidence linked him to the 2008 death of Pamela Kosmack and the 2011 death of Leanne Lawson.
In 1995, Bill C-104 granted police the right to take DNA samples from those convicted of serious crimes such as murder, sexual assault or robbery. In June 2000, the RCMP established the National DNA Data Bank.
More than 99 per cent of DNA profiles are obtained from blood samples, but authorities can also use cells scraped from an offender’s cheek or hairs plucked from their head. A National DNA Data Bank advisory committee reports to the RCMP commissioner and advises on ethics, privacy matters, changes in legislation and scientific advances.
How the data bank works
If you’re convicted of a serious crime, your DNA profile will be added to more than 300,000 other Canadians whose profiles are in the Crime Offender Index. A second and separate collection – the Crime Scene Index – contains DNA collected from more than 100,000 crime scenes: tufts of hair, spatters of blood, semen, saliva, even human tissue that was scooped, bagged and labelled by crime scene investigators. The samples are labelled with bar codes to keep them anonymous until a match is found. The DNA profiles from the two collections are compared and any matches are sent back to the investigating officer.
A computer network known as CODIS (Combined DNA Index System) links the information and allows different police forces to find and share information. The system can place a suspect at a crime scene, or link multiple crime scenes from different jurisdictions.
Analyzing DNA used to be time-consuming, sometimes taking months to produce a single profile. Improved technology and automation means DNA profiles can now be done in a matter of days and, in the future, the wait could be reduced to just hours. Advances also allow investigators to work with smaller and smaller samples. Twenty-five years ago, technicians needed a quarter-sized drop of blood to extract a DNA profile. Today, investigators can use a sample as small as 40 red blood cells, or just a fraction the size of a head of a pin.
Is DNA infallible?
Hardly, says Toronto defence lawyer Ricardo Federico, who teaches a course on forensic evidence at Osgoode Hall Law School.
“My government lawyer friends say DNA should stand for Do Not Acquit. I say it should stand for Do Not Assume.”
Small sample sizes can mean less reliable results, DNA samples can suffer cross-contamination in laboratories or a suspect may have had a legitimate reason to have been at the crime scene. One study at Boise State University in Idaho had different experts do a blind review of DNA evidence from an actual trial and found they came to different conclusions than the trial expert. That, says Federico, means DNA interpretation can be subjective.
There are also the bizarre exceptions to the idea of unique DNA profiles, more commonly found in TV legal dramas than in real life. Bone marrow recipients will carry their donor’s DNA and DNA “chimeras” carry the DNA of more than one person. Then there was “The Phantom of Heilbronn,” an alleged female serial killer who German police pursued for 15 years. DNA from the phantom was found at 40 crime scenes in Germany and Austria, among them six murders, including the killing of a police officer. The DNA, it turned out, belonged to a woman who worked in the factory that supplied the police with cotton swabs.