Prosecutors can use mitochondrial DNA extracted from hair found in Scott Peterson's boat in their attempt to prove he killed his pregnant wife and dumped her body in San Francisco Bay, a judge ruled Monday.
Mitochondrial DNA has rarely been used as evidence in California court cases and it can't provide a definitive match. The defense challenged the evidence as unreliable because the statistics used to determine the chances of a genetic match are based on a faulty database.
But California's Stanislaus County Superior Court Judge Al Girolami said prosecutors could tell a jury that chances are the mitochondrial DNA from that specific strand of hair could be found in one out of every 112 whites.
The ruling came at the start of the fourth week of the preliminary hearing being held to determine if there's enough evidence against Scott Peterson to warrant a murder trial.
Scott Peterson has pleaded not guilty to killing his wife and unborn son and could face the death penalty if convicted.
Peterson left the courtroom on Monday when testimony turned to the state of his wife's corpse and the remains of their son. Forensic pathologist Dr. Brian Peterson, no relation to the couple, testified that the fetus was in relatively good condition and had only recently broken free from the mother when the two bodies were found in April.
Laci's body, on the other hand, was badly decomposed and her neck, head, forearms and one of her feet were missing. Dr. Peterson said her abdomen was worn away, but that there were no evidence Laci had given birth.
Dr. Peterson said he could find no evidence of wounds and could not determine what weapon may have been used on Laci. He attributed the bad state of her body to exposure to salt water and sea creatures.
Dr. Peterson said his initial measurement of the fetus was the same length as a typical full-term fetus. Scott Peterson's lawyers argued that Laci, who was eight months pregnant when she disappeared, may have given a live birth weeks after she disappeared.
But Dr. Peterson said the prolonged submersion in San Francisco Bay had swollen the body and made his measurement inaccurate. He testified that an anthropologist who examined the body estimated the fetus to be 33 weeks to 38 weeks old. He testified that a doctor who saw Laci on Dec. 23 estimated her pregnancy to be at 32 weeks.
The judge's DNA ruling was expected.
Investigators believed a human hair found in pliers in Peterson's boat came from his wife, who disappeared from their Modesto, Calif., home around Dec. 24 last year. Her body and that of the couple's unborn son washed ashore in San Francisco Bay near Richmond. The site is about 3 miles from where Scott Peterson, 31, said he went fishing on the day of Christmas Eve.
FBI forensic experts extracted mitochondrial DNA from the hair and a criminalist with a state crime laboratory testified last Friday that the hair resembles stands taken from Laci Peterson's brush.
Experts said mitochondrial DNA, a tiny ring-shaped molecule that's much smaller than the more familiar nuclear DNA that reveals genetic makeup, helped identify victims of the Sept. 11, 2001, terror attacks in New York. It can be extracted from hair and bones when little else remains of a body.
An FBI lab expert said mitochondrial DNA testing can be more effective in analyzing DNA when the biological sample is small or degraded, or, as in the Peterson case, when it is a strand of hair.
Experts said mitochondrial DNA from the human body cannot specifically identify an individual. Nor is it as reliable as the more familiar nuclear DNA samples, which can prove an identity based on a person's genetic fingerprint. But mitochondrial DNA, if matched with similar samples from a person's mother or sibling, can show a statistical likelihood of identification and rule out others.
Meanwhile, Peterson's former mistress, Fresno massage therapist Amber Frey, will not testify at the hearing. Her lawyer, Gloria Allred, said prosecutors "obviously made a determination" they have presented enough evidence without Frey.
"The District Attorney has decided not to call her," said Allred, who added that Frey has already been victimized by Peterson's deceit and that making her testify unnecessarily would continue her victimization. Frey has said that Peterson told her he was a widower.