September 10 – Innocent, I tell ya!

Today’s factismal: DNA evidence can only be used to prove innocence, and never proves guilt.

We’ve all seen it on TV a zillion times by now: some investigator picks up a hair or piece of bubble gum and sends it off to the lab to be analyzed. Ten minutes later, the lab tech comes out and says “It’s a match! We’ve got him!” The only problem with that (as with most science shown on television shows) is that it is completely wrong.

In order to understand why, we need to step back in time some 29 years. A scientist by the name of Alec Jeffreys is trying to understand why some diseases tend to run in families. If a father has heart disease, then his sons are likely to have it as well. If a mother has diabetes, then her children probably will get it too. But why? Jeffreys was convinced that the answer to the question lies in the genetic code, so he made x-ray photographs of crystallized DNA (at the time, that was the only way to do it) that had been donated by his technician’s family. Though he thought that the images were a mess, he did notice that there were some surprising similarities between the images. He quickly realized that he had created a “fingerprint” of the DNA that could be used to tell if a person was related to another one.

His first case involved a boy from Ghana; the father insisted that he wasn’t related to the boy. Jeffreys ran his test and produced similar patterns, showing that the boy and the father were related. In his next case, Jeffreys was able to show that the main suspect in a series of rapes wasn’t related to whomever did the crime but that another suspect was; he had used DNA fingerprinting to show someone’s innocence.

Today the science of DNA fingerprinting has advanced considerably. Instead of looking at x-ray photographs of crystallized DNA, researchers sequence the DNA to identify the actual order of the units that make up the DNA. This test is a huge advance because it is faster, costs less, and uses less sample material than Jeffreys’ original method. But, like Jeffreys’ original method, it cannot show that the DNA came from a specific person; instead, all it can do is show that the DNA donor and the suspect are related – and a negative test proves innocence. Thus, a DNA test can never prove guilt, only innocence.

Of course, researchers are still trying to pin down the genetic components of disease. If you’d like to help them, then why not join the Personal Genome Project?

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