Expert says DNA test unreliable
11/04/2003 8:25 AM
11/19/2007 2:05 PM
A defense expert on Monday blasted a DNA test that the FBI used on a hair found in Scott Peterson's boat, saying the technique can produce false results that are then compared against a flawed database.
In a day of testimony marked by sharp exchanges with a prosecutor, the defense expert also said mitochondrial DNA testing was susceptible to contamination because of the small sample amounts and the procedures used.
"When I sneeze, my DNA really does go into the air," said Wil-liam Shields, a professor at State University of New York at Syracuse.
"Contamination is the biggest danger to doing appropriate and reliable DNA work," Shields testified in Stanislaus County Superior Court.
The testimony came on the fourth day of Peterson's preliminary hearing on double-murder charges.
The 31-year-old fertilizer salesman from Modesto is accused of murdering his pregnant wife, Laci, and unborn son, Conner. Prosecutors are seeking the death penalty.
The human hair, found attached to a pair of needle-nose pliers, could be key physical evidence linking Laci Peterson to her husband's boat.
Prosecutors have contended in court papers that Peterson's body was in the Modesto warehouse that her husband used in his work and also in his boat. In April, passers-by found her body along the eastern shore of San Francisco Bay; prosecutors are likely to try to show that Peterson ferried her body into the water and dumped it.
But Shields testified Monday that the hair from the boat could have come from one in any nine Caucasians.
That stood in stark contrast to testimony from FBI analyst Constance L. Fisher, who said last week that the hair could not have come from Scott Peterson and may have come from his wife. Fisher said the hair could have come from one in any 112 Caucasians.
That discrepancy was due to flawed FBI calculations and an unreliable database, Shields said.
"I guarantee you," Shields said, "the way it is presented (by authorities) is biased against the defendant -- and it's wrong."
The defense is trying to show the testing technique is unreliable and evidence derived from it should be excluded from court. Judge Al Girolami is not expected to rule on the issue until after FBI scientist Dr. Bruce Budowle testifies next week.
Shields testified that FBI techniques do not sufficiently take into account evolving genetic science and could wrongly conclude that a DNA sample could not have come from a specific individual.
Under cross-examination by Senior Deputy District Attorney Dave Harris, Shields conceded that the FBI results, if accurate and not contaminated, would rule out the hair as Scott Peterson's.
Mitochondrial DNA is widely believed to be inherited maternally, meaning family members along the same maternal line will have the same mitochondrial DNA.
Unlike nuclear DNA, mitochondrial DNA cannot be used as a unique identifier, but it can be used to rule out that a sample came from a particular person.
But those results can be thrown off if a sample is contaminated by a person somewhere along the line touching the sample or a test tube, Shields said.
In regard to a 1995 FBI study to determine contamination levels that would not impact results, he called the work "a very bad experiment without statistical validity" because only five samples were tested.
Under pointed questioning by Harris, Shields acknowledged that he had never personally extracted DNA to do a mitochondrial DNA test. He said he garnered his expertise from reading reports done by others, viewing work in other labs and some of his own lab work.
Shields, who wore a tie apparently depicting a molecular structure, also said he would not testify as to whether mitochondrial DNA was "generally accepted" among the scientific community -- calling that a legal term.
Under California law, evidence from a novel scientific technique cannot be admitted in court unless a judge determines it is generally accepted by the relevant scientific community.
Shields' refusal to use the term helped spark touchy exchanges with Harris as the two bickered over scientific terms and definitions.
"You would agree that mitochondrial DNA is something that comes from basic biology?" Harris asked.
"No," Shields said. "It comes from mitochondria."
Harris, his voice calm, would regularly follow one of Shields' responses with, "Now, let's go back and start this again."
Shields grew increasingly agitated with Harris in a string of exchanges, finally saying: "You've been doing this all along. Stop misrepresenting what I'm saying."
At that point, Girolami asked Shields to "calm down" as whispers and quick smiles swept through a courtroom that, for the first time since the hearing began, had more than a dozen empty seats in it.
"I'm sorry, your honor," Shields said. "I'm sorry."
Monday's proceedings were broken off at several points as lead defense attorney Mark Geragos took calls regarding a Los Angeles murder trial he is handling.
The jury in that case is deadlocked on all but one of the charges after deliberating for six days, Geragos said outside court.
Girolami agreed to allow Geragos to appear today in Los Angeles to address a question from the jury.
In Geragos' absence, veteran Modesto attorney Kirk McAllister is set to resume questioning police Detective Jon Evers.
Peterson, who appeared in court in a tan suit, pale blue shirt and light tie, only spoke when Girolami asked if he agreed to have the hearing prolonged to accommodate Geragos.
"In essence, yes," he said.
Bee staff writer John Coté can be reached at 578-2394 or email@example.com.
Bee staff writer Garth Stapley can be reached at 578-2390 or firstname.lastname@example.org.
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