Explainer's Amanda Knox roundup.

Explainer's Amanda Knox roundup.

Explainer's Amanda Knox roundup.

Answers to your questions about the news.
Dec. 7 2009 6:03 PM

Why Did Amanda Knox Get an Extra Year in Jail?

Can DNA evidence really yield a false match? And other questions about the Italian murder trial.

Amanda Knox. Click image to expand.
Amanda Knox

American college student Amanda Knox was convicted of the 2007 murder of her British housemate Meredith Kercher by an Italian court on Saturday. Her sensational trial has raised a number of questions.

Knox was sentenced to 26 years in prison. Her ex-boyfriend, Raffaele Sollecito, got 25 years. And Rudy Guede, an Ivorian-born drifter, was sentenced to 30 years in an earlier trial. Why the different numbers?

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As in the United States, sentencing in an Italian murder trial depends on various aggravating and mitigating factors. In the trial of Knox and Sollecito, the panel weighed the cruelty of their crime against the fact that neither had a criminal record. In the view of the empanelled judges and laypersons, these balanced out so each received the minimum sentence of 21 years on the murder charge. The other charges—sexual assault, staging a break-in to mislead investigators, transporting the murder weapon, and theft of Kercher's money and cell phone—accounted for another four years. And then Knox got an extra year tacked on for falsely accusing an innocent man of the crime. (She'd fingered the owner of the bar she worked at in the early days of the investigation, although she later claimed the accusation was made under duress.)

In theory, Guede was due a lighter sentence, since he'd agreed to an "accelerated trial" without live testimony. On the other hand, he had a history of drug-dealing and petty crime. So his judge sentenced him to life imprisonment for the murder—with the penalty being discounted to 30 years on account of the speedy trial. (It didn't help Guede any that he was never accused of handling the murder weapon. In Italy, the triggerman is no more culpable for a crime than his accomplices.)

These numbers may change. In a few months, the panel for Knox and Sollecito's trial will submit a report describing how it reached its conclusions, and the defense lawyers can appeal the decision. Guede's appeal is already under way. All of the sentences could be reduced if higher courts balance the aggravating and mitigating factors differently.

Investigators found Kercher's and Knox's DNA on a kitchen knife, but attorneys for the American argued that the evidence was contaminated and that there wasn't enough of it to make a conclusive link. Can contaminated DNA evidence really give a false match?

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Yes, especially when you're dealing with tiny amounts. Most laboratories will not analyze DNA samples under 500,000 picograms—less than the amount left behind in a fingerprint. The first analysis in the Knox case appears to have failed because the sample was too small, and the second round of tests yielded a match that was so weak that American crime labs would have rejected the result.

It's very common for police or lab technicians to accidentally transfer a few thousand picograms of DNA between objects. (Evidence from different cases has even managed to get mixed up in forensic crime labs.) For example, if the police had touched something in Sollecito's apartment—which was thoroughly colonized with Knox's DNA from dead skin cells, dandruff, coughs, and sneezes—prior to handling the knife, that might have been enough to provide a false match. The surface on which the knife was found could also have transferred Knox's DNA to the weapon. Even opening the duvet that was stained with Kercher's dried blood could have sent DNA airborne and onto other pieces of evidence in the laboratory.

Testing also cannot determine how long DNA has been present on an object, so investigators have no way of knowing if the traces of DNA on the knife were from normal kitchen use.

Investigators made the DNA match in spite of the fact that the kitchen knife had been scrubbed with bleach. Is it really so hard to get your genetic material off of a knife?

Yes. Bits of DNA can hide in the crevices of a wooden handle or in the ridges of a serrated blade. Bleach might work, but you'd have to be very, very thorough in your cleaning. (For more on how to scrub DNA, read this Explainer from 2007.)

Got a question about today's news? Ask the Explainer.

Explainer thanks Dan Krane of Wright State University, Stefano Maffei of the University of Parma, William Pizzi of the University of Colorado Law School, and Albert Scherr of Franklin Pierce Law Center.

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Brian Palmer covers science and medicine for Slate.