The defence called on its own DNA expert Friday in the murder trial for the death of Candace Derksen.

Mark Edward Grant faces a charge for first-degree murder. He has pleaded not guilty.

Grant's lawyer, Saul Simmonds, continued his argument against the reliability of DNA evidence in the case.

"From our perspective, the evidence so far should leave you with responsible doubt…the DNA evidence is lacking," Simmonds told the jury.

Dr. John Waye, a genetics professor from McMaster University in Hamilton, Ont., took the stand for the defence on Friday.

Waye reviewed DNA work done during the investigation into Derksen's death by a lab called Molecular World in Thunder Bay.

"It's flawed. It doesn't follow scientific method," said Waye.

The Crown has argued Grant's DNA and hairs were found at the crime scene.

Last week, a DNA expert from Molecular World testified for the Crown that there was only a one-in-50-million chance the DNA sample came from someone other than Grant.

But Waye told the jury Friday the DNA work done was biased against Grant as it ignored data that excluded him. Because the DNA sample was old and degraded by the time it was submitted to Molecular World in 2007, the company used only 10 out of 15 markers that are usually present in a DNA profile, court heard.

"You're looking at the profile of Mr. Grant being a contributor and you're covering up all the evidence that disagrees with that," Waye testified. "Once you're faced with data you have to deal with it. You can't just ignore it."

During cross-examination of Waye on Friday, the Crown asked him if he can say Molecular World's DNA findings are wrong. Waye testified there is no evidence to say there are right, adding that in science you can say anything, but it has to be based on observation.

The defence couldn't get their expert to conduct their own DNA examination as the sample taken from the crime scene was used up during earlier testing.

Derksen, 13, disappeared while walking home from school in November 1984. Her body was found in a shed, weeks later.

Grant was arrested in May 2007 and charged with first-degree murder for Derksen's death after a cold-case police unit began investigating.

On Thursday, the Crown wrapped up presenting its case.

The trial is continuing with the defence presenting its case in Winnipeg.

- with a report from CTV's Stacey Ashley