DNA tests leave little doubt on identities

April 20, 2003 

Though the official declaration must come from the coroner's office, technicians said the DNA tests that identified the bodies of Laci Peterson and her unborn son are unquestionably accurate.

Using DNA taken from the shin bone of the female body and from a blood sample taken from Scott Peterson by law enforcement officials, serologists in the state forensics lab in Richmond determined that the infant was the child of the female and Scott Peterson, Lockyer said. He said the statistic was 1 to 18 billion, meaning that only one out of every 18 billion people also could positively test as the parents of the child.

The world's population is roughly 6.3 billion.

Using cheek swabs from Laci Peterson's parents, Sharon Rocha and Dennis Rocha, the lab determined there was a 1 in 1.9 billion chance that Sharon and Dennis Rocha were not the parents of the adult female.

John Tonkyn, supervisor of the state's Missing Persons DNA project, explained that the DNA samples taken from the remains also were compared with random DNA samples to see if there were positive matches there. There were none.

"Law code assumes paternity when the code exceeds the hundreds, and this is in the billions," California Attorney General Bill Lockyer said. "There is no question in our mind the unidentified female is Laci Peterson and the unidentified infant is the child of Scott and Laci Peterson."

Though the bodies discovered on the shore near Richmond on April 13 and 14 were badly decomposed, there was enough tissue to do the tests, technicians said Friday.

Scott Peterson was arrested earlier Friday and faces two counts of murder in the deaths of his wife and unborn son.

The body of the baby was found April 13 on the rocky shoreline near Richmond. On April 14, the remains of the adult woman were found about a mile away.

Technicians at the state crime lab in Ripon explained the process for identifying DNA earlier this year.

After a sample is collected, the DNA is replicated into larger copies of itself using a process called polymerase chain reaction. Then the DNA is broken up into pieces, sorted according to size, and compared with other samples as well as control samples.

The results are printed out in a form that looks like a long bar code. The thickness of the bars indicates the presence of certain sequences in the DNA strands, sequences that are passed from parents to their children.

Bee staff writer Patrick Giblin can be reached at 578-2347 or pgiblin@modbee.com.

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