Genetic evidence is one of the many forms of evidence the prosecution holds against Anthony B. Mertz, now on trial for the murder of Shannon McNamara.

During testimony Monday, expert witness Jennifer Lu, a scientist at the Illinois State forensics lab in Springfield, said the likelihood is 1.8 billion to one that DNA from McNamara’s fingernail scrapings matched Mertz’s DNA.

Alan Friedman, forensic scientist for Helix Biotech Inc. of Wisconsin, described genetic evidence as transfer evidence.

“DNA is biological fluid transferred from the perpetrator to the victim or the victim’s property or from the victim to the perpetrator or the perpetrator’s property,” he said.

During a hearing last year, Coles County Circuit Judge Dale Cini approved the prosecution’s use of genetic evidence for the Mertz case.

Both fingernail scrapings and a knife containing what appeared to be blood could be used in attaining DNA and therefore possibly finding a match.

When a piece of evidence like a bloody knife is found, Friedman said, the blood is identified as human blood and then profiled to see if it matches with the victim. Then, if evidence of the perpetrator can be found on the knife as well, through fingerprints or the perpetrators own blood, or the knife is found on the suspect’s property, the evidence against the suspect is stronger.

In the Mertz case, the prosecution has put greater focus on using the genetic evidence found in fingernail scrapings than evidence found on the knife.

“Often, in the course of an assault, the victim will scratch the perpetrator, resulting in dermal tissue or blood being embedded under the victim’s fingernails,” Friedman said.

In these cases, he said, it is very likely the victim’s genetic profile will be found as well as the perpetrator’s.

Either the nails will be clipped or the tissue will be scraped from beneath the nails.

Once the sample is taken, the lab analyst will extract DNA from the sample.

“This is done through a widespread Polymerase Chain Reaction,” Friedman said. “It acts as a molecular photocopy machine.”

This “machine” then works over again, doubling each time for approximately 30 cycles, forming over 1 billion copies.

Analysts then examine regions of the genetic profile, comparing the 13 regions that vary greatest between people.

“The likelihood of finding the same profile in different people is much less than one in a trillion,” Friedman said.

Once a match is determined, it becomes a reliable information source in trial because of that one in more than one trillion chance.

The lab analyst is generally the one to testify in trial.

“A lab analyst is considered qualified as an expert witness,” Friedman said.

The analyst testifies answering basic questions.

“They’ll ask the analyst if he or she received the items, recognize the evidence, found a successful DNA profile, compared the reference samples, and if a conclusion was determined,” Friedman said.

DNA doesn’t tell guilt or innocence, Friedman said.

In the Mertz case, evidence of blood was found on items in McNamara and Mertz’s apartments as well as in a dumpster near Mertz’s apartment complex. Blood found on the handle of a box cutter and a shoe box from Mertz’s apartment has been admitted to evidence. The bloody knife and paper bag found in the dumpster were also admitted Friday.

These samples were all positively identified as human blood and submitted for DNA testing. The evidence will be used by the prosecution.

“The attorneys must still see how the genetic evidence fits into the puzzle,” he added.

The prosecution must prove guilt beyond a reasonable doubt.

“Like all physical evidence, DNA evidence is circumstantial,” Friedman said. “It can tell you who it came from but not where or when.”