A week ago, attorney James Ross told the jury assembled for the trial of his client, Gregory Hopkins, that he intended to turn the courtroom into a classroom.
On Wednesday, school was in session. The subject: DNA.
For several hours, Mark Perlin, CEO of Cybergenetics and the defense's final witness, discussed the DNA evidence in the 1979 cold case.
At one point in his testimony, before Judge Harry E. Knafelc had him stop, he read aloud to the jury from a statistics book.
For seven days, the Beaver County jury has heard testimony from witnesses about the death of Catherine Janet Walsh on Sept. 1, 1979. Walsh, 23, was discovered dead in her Monaca home, face down on her bed. She wore a nightshirt and her hands were bound behind her back, with a handkerchief pulled tightly around her neck, strangling her.
At the time, there were no visible semen stains nor evidence of recent sexual activity. But in January 2012, Mr. Hopkins, 67, a former Bridgewater councilman, was charged with homicide after DNA technology not available 30 years ago matched him to seminal fluid found on the rope, the nightshirt and the top sheet.
Mr. Hopkins took the stand Tuesday, testifying that he had nothing to do with Walsh's death. His attorney, Mr. Ross, has argued that the DNA should be there because the two had sex in Walsh's room two or three times a few weeks before her death.
During his testimony, Dr. Perlin said Walsh may have perspired, causing an old semen stain Mr. Hopkins left on the back of her nightshirt to dampen and transfer Mr. Hopkins' DNA to the robe tie binding her hands.
"You have the exact conditions that are ideal for semen or DNA transfer," he said, referring to the proximity between the rope tie and the nightshirt.
It was the same scenario that, when questioned last week, forensic pathologist Cyril H. Wecht, a witness for the prosecution, said "I just can't envision it."
Frank Martocci, assistant district attorney for Beaver County, questioned why, if she had perspired, the victim's DNA wasn't found on the back of her nightshirt.
Dr. Perlin referenced what he said was a forensic science adage, that "the absence of evidence is not evidence of absence."
Dr. Perlin's analysis agreed with much of what the state police crime lab found, but he said he also found Mr. Hopkins' DNA on another part of the top sheet, where he also found DNA with a probable match to the victim's estranged husband, Scott Walsh, though he said that match was not as strong as the Hopkins matches. Mr. Walsh testified last week, denying his involvement in his wife's death.
Also, on a bathrobe found beside Walsh, Dr. Perlin said he found unknown, non-sperm DNA that did not match any of the suspects introduced in the trial, evidence that he said should have been pursued in the investigation.
Mr. Martocci, displaying a chart of Dr. Perlin's findings from analyzing DNA results for 18 evidence items, said a handkerchief showed a positive match for Mr. Hopkins. But Dr. Perlin told him he was "cherry-picking" the data.
He said the total distribution of data for the handkerchief showed that it was inconclusive whether DNA from Mr. Hopkins was there.
Closing arguments are set for this morning.
Kaitlynn Riely: firstname.lastname@example.org or 412-263-1707. First Published November 20, 2013 1:45 PM