How weak DNA evidence tracked down and then rescued Amanda Knox | Techilink

If you watch crime dramas, you’ll get the impression that DNA evidence is an airtight case. And if you Doing If you have that impression, you might be confused about the internationally famous case of American Amanda Knox, convicted of murdering her British roommate in Perugia, Italy in 2007. After all, the prosecution’s case was based on DNA evidence. evidence; Knox’s genetic fingerprints were found by Italian police on the handle of a kitchen knife, which also had the victim’s DNA on the blade.

But not all DNA evidence is created equal — and Knox walked free from an Italian prison last week after scientists destroyed the forensic evidence against her as completely unreliable. How did the DNA analysis go so wrong?

To understand the issues with the Knox case, we drew on the extensive real-world genetics experience of the Ars scientific staff and spoke with Dr. Lawrence Kobilinsky of the John Jay College of Criminal Justice in New York. Kobilinski has seen the DNA test results of the Knox case and helped us work through the reasons why DNA evidence isn’t always as watertight as it sometimes appears on TV.

DNA analysis amplifies a small piece of DNA into millions of copies, but this amplification process can lead to problems if not carefully managed. The results of this process are not self-explanatory – interpretation is always required – and the interpretation of DNA analysis became a decisive problem for Amanda Knox. Ultimately, terrible crime scene management and an unwarranted certainty about DNA evidence on the alleged murder weapon led to a murder conviction that collapsed on appeal.

The Knox case

Amanda Knox was a 20-year-old American citizen who lived in Perugia, Italy and shared an apartment with several other women. One of them, Briton Meredith Kercher, was murdered on November 1, 2007, her body found naked in her locked bedroom, with a fatal knife wound to the neck. Knox claimed to have spent the night with her boyfriend in another building and only returned in time to help discover Kercher’s body.

Although Perugia resident Rudy Guede was charged with rape and murder, Knox and her boyfriend, Raffaele Sollecito, were also eventually charged in the case. A witness claimed that the couple had been near the apartment on the night of the murder, and that DNA evidence (on a knife belonging to Sollecito and on Kercher’s bra) allegedly linked them to the crime. Amid a swarm of media attention, Knox and her boyfriend were eventually convicted of murder.

Then came the appeal. The witness who allegedly saw the duo turned out to be a heroin addict who gave inconsistent statements. That shifted the focus from witness statements to the DNA evidence, which was eventually evaluated by two experts from the Universita di Roma.

The experts were not kind to the evidence. The bra clasp, it turned out, had been on the floor for more than six weeks after the murder before being secured and processed; photos show it was moved between the murder and final collection. The closure was the only DNA evidence that placed Sollecito at the scene of the crime; no DNA has put Knox at the scene.

The alleged murder weapon, a long kitchen knife, was found in Sollecito’s home, in his kitchen knife drawer. The knife contained little DNA and according to the experts, the local authorities had not properly performed the tests to compensate.

Basically, there were problems with all the DNA evidence used in the trial. Without a witness or reliable DNA evidence, Knox’s conviction was overturned on October 3, she was released and she immediately returned to the US.

Obtaining DNA evidence

To understand what went wrong with the DNA evidence here, we need to look at the techniques that help generate that evidence. (The discussion gets a bit technical, but it’s important to understand the reasons why this evidence was rejected.)

The modern use of forensic DNA is based on a technique called the polymerase chain reaction (PCR), which won inventor Kary Mullis half of the 1993 Nobel Prize in Chemistry. PCR repeatedly amplifies specific stretches of DNA. Scientists start by designing two short stretches of DNA called “primers” that flank a particular genetic sequence of interest. These primers then enable a protein called polymerase to copy the intermediate DNA sequence, making two identical copies from a single source. A cycle of temperature changes can reset the system and each cycle doubles the number of identical molecules present. The result: fast, exponential copying of a single DNA molecule. (For more, read our previous in-depth report on PCR.)

This exponential growth theoretically allows a single DNA molecule to be amplified into an entire population of identical molecules, making it trivial to detect. In practice, Kobilinsky said PCR has allowed definitive identification of the source of DNA samples less than 100 picograms (10^-12 of a gram) of DNA. (That’s the weight of about 100 bacteria.)

However, this extreme sensitivity creates its own problems. “You have to be extra careful not to contaminate the sample or equipment,” Kobilinsky said, since just a little bit of contaminating DNA is enough to generate a false positive from a sample that otherwise lacks the relevant DNA sequence. That was a danger here: The DNA from the bra clasp, which was eventually used to place Sollecito (and by induction, Knox) ​​at the scene, had been sitting for weeks in an apartment that Knox had lived in and Sollecito had visited.

PCR also tends to generate artifacts. Although the primers are highly specific to a particular DNA sequence, there is a large population of primers in any reaction. This increases the likelihood of a rare event such as the amplification of a DNA sequence mismatch. If something strange happens early enough in the amplification process, it’s even possible for an artifact to become the primary product of a PCR reaction, with confusing results.

The more times you cycle a response, the more likely you are to reinforce something spurious. Kobilinsky laid down strict rules for the number of cycles performed in a forensic PCR reaction: 28 cycles under standard conditions and 31 cycles for “highly sensitive” tests, used when available amounts of DNA are very small.

There are ways to check for many of these issues – running reactions without any DNA sample to test for contamination, using known positive samples, etc. All of these increase the reliability of the evidence by identifying the tests that cannot be trusted . But these checks emphasize the point: DNA evidence alone is not as conclusive as often thought. And other issues came into play when the knife was tested.