DNA evidence connects to suspect in Surrey murder trial, jury hears

DNA expert Janny Lau testified at the second-degree murder trial of Adam Troy Mann Thursday in New Westminster Supreme Court.

Mann is accused of fatally stabbing Surrey’s Tori Dunn in June of 2024 during an alleged home invasion in Surrey’s Port Kells neighbourhood. He listened quietly throughout the day’s proceedings, as did many of Dunn’s family and friends, including her boyfriend at the time of her death with whom she shared the home, Trafford Wace.

A forensic biologist who works in the RCMP National Forensics Laboratory, Lau testified about how she tested evidence exhibits connected to the case — a bloody kitchen knife, a hoodie and a swab from a cellphone — against known samples of Dunn and Mann’s DNA, after showing the court a Power Point presentation explaining how DNA testing works.

“My job is to interpret … the DNA profile, determine how many people are in that sample from the known samples, and then perform any preparation and measurements or inclusion to provide statistical analysis and to provide the analysis,” she said. “There are very specific locations that we’re looking in the DNA. Those locations are called short tandem repeat regions, and what they are, are the sequences that actually have sequences that will repeat itself many times.”

Lau told the court blood found on the blade of a large kitchen knife found in the neighbourhood near the Port Kells home showed it was a highly probable match to Dunn; Wace testified earlier in the trial he noticed a kitchen knife missing from their home after the alleged home invasion.

“The DNA typing profile obtained in area AA, the centre blade, matches that of the known sample (from) Tori Dunn,” Lau said, adding that the likelihood the blood came from someone other than Dunn was one in 290 quintillion.

Court heard a quintillion is a one followed by 18 zeros.

The knife handle, after testing, confirmed the presence of DNA from three different people, Lau said.

When that DNA was tested against the known DNA sample taken from Mann, Lau said results showed it was 2.7 quadrillion times more likely one of the DNA profiles on the handle came from Mann, rather than from a random member of the population.

She confirmed to that a quadrillion is one followed by 15 zeros.

Lau also testified about a grey “hoodie” sweatshirt — something Wace previously testified he saw the suspect wearing — found outside near Dunn’s house, which she said had two DNA profiles on the sleeve that were likely matches to Dunn and Mann.

A swab taken from Dunn’s iPhone, also located outdoors in Dunn’s neighbourhood, was also a highly likely match to the known DNA sample from Mann, Lau told the court. Results showed it was 12 to 17 billion times more likely the DNA sample came from Mann, rather than from a random, unknown member of the population.

Upon cross-examination, defence attorney Paul McMurray focused on the databases used by the RCMP in DNA analysis.

Lau testified that, in 2024, the RCMP relied on the Canadian Caucasian population database. The federal police force has since switched to using a stratified population database that includes several different ethnicities.

Lau re-analyzed the samples using the stratified population database with all of the evidence exhibits collected, court heard, and found similar results to the original results that used the Caucasian database.

Asked if she was concerned about the minor numerical differences from using the different databases, Lau responded she was not.

Although the results were “numerically slightly different,” Lau pointed to the “very strong” verbal likelihood ratio in each instance, noting “‘very strong’ is strongest verbal scale there is,” with a one million per cent probability.

McMurray wondered if any of the databases used had similar genetic backgrounds to Mann, who court heard is half African-American and half Indigenous.

“If you don’t have any samples from persons of a similar genetic background, you’re comparing apples to oranges,” he said.

Lau responded any DNA sample’s characteristics would “be observed within the mixture.”

“I’m calculating how often I would see someone in the population with this profile,” she explained.

One more witness is expected to testify on Friday (Oct. 16).

B.C. Supreme Court Justice David Layton is presiding over the trial, which is expected to last about three weeks. It started on Oct. 5.