Peterson Defense Challenges DNA Analysis

AP ^ | 11/3/2003 | BRIAN MELLEY

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MODESTO, Calif. - Defense attorneys in the Laci Peterson murder case Monday challenged the type of DNA analysis done on a hair found in Scott Peterson's boat, saying the technique is too unreliable to be used in court.

Prosecutors believe the hair, found in a pair of pliers in the boat, could be from Peterson's wife, Laci, the mother-to-be who disappeared last December. An FBI crime lab supervisor testified during the preliminary hearing last week that mitochondrial DNA from the hair matched a gene swab taken earlier this year from Laci Peterson's mother, Sharon Rocha.

Defense witness William Shields, a biology professor from the State University of New York at Syracuse, testified Monday that while mitochondrial DNA testing can be useful, it is not as precise as other types of DNA testing.

Mitochondrial DNA cannot specifically identify an individual, but if compared with samples taken from a family member, it can show the statistical likelihood that a hair or other tissue came from a certain person.

Scott Peterson, 31, told police he last saw his wife on the morning before Christmas as he left to go fishing near Berkeley. He told them he returned to their Modesto home late that afternoon, shortly before family members reported Laci Peterson missing.

The bodies of Laci Peterson and her son washed ashore along the San Francisco Bay in April, about three miles from where her husband said he was fishing.

Peterson, a former fertilizer salesman, is charged with murder in the deaths of his 27-year-old wife and the baby boy she was carrying. The preliminary hearing is to determine if he will stand trial.

There is no evidence Laci Peterson was ever in the boat before her death, and prosecutors are expected to show that she did not even know about the vessel.

Mitochondrial DNA _ a molecule that is much smaller than the more familiar nuclear DNA that is used to reveal a person's genetic makeup _ helped identify victims of the World Trade Center attack. It can be extracted from hair and bones when little else remains of a body.

Peterson attorney Mark Geragos has branded mitochondrial DNA evidence unreliable.

Testifying for the defense, Shields said he was "appalled" at the sample size used by the FBI. Shields said there can be mutations or contamination of mitochondrial DNA that could skew results.

Shields, who wore a necktie imprinted with the DNA double helix, said the FBI had also used a faulty statistical method that skewed the probability the hair could have belonged to someone other than Laci Peterson. The FBI expert said the DNA extracted from the hair is found in 1 in 112 white people; Shields said it would be found in about 1 in 10.

On cross-examination, Shields said he gets as much as 60 percent of his income from testifying for the defense in criminal trials and rarely works for prosecutors. He also said he had never extracted mitochondrial DNA for forensic purposes and is not a forensic scientist.

The hearing is being held because the technique has rarely been used in California courts. The prosecution must clear three hurdles to use the evidence. It must show that: the technique is accepted by forensic scientists, the witness is an expert in the technique, and the testing and analysis were done properly.

The hearing resumes Tuesday with the defense cross-examining a police witness who testified last week. The proceedings are expected to be brief because Geragos has to return to Los Angeles for another trial.