How does DNA profiling help solve crimes?

DNA - the hereditary material in humans and almost all other organisms - matching that of Uwe Böhnhardt, a dead member of a neo-Nazi cell, has been detected on the recently discovered remains of a girl who disappeared 15 years ago in Bavaria. Experts are now trying to determine how the DNA ended up on a tiny scrap of cloth under the girl's body. A German forensic geneticist told DW how the method works, and why it's key to helping solve crimes.

DW: DNA profiling, the magical technology to help solve crimes - what exactly does this form of testing tell us?

Professor Lutz Roewer: It only reveals the match, that is, the conformity of a biological trace with a person - nothing more. There is no correlation to how the trace got there, or when. People expect a lot from DNA fingerprinting, but it really is only an identification analysis.

Is it better than an actual fingerprint, which is unique after all?

Yes, because you're not comparing curves and arches but highly stable, quantifiable patterns from our body. Experience tells us that everyone has his or her own fingerprint, but you can't prove it's true. With DNA we're sure, because the frequency of characteristics can be defined. That's why it has become the gold standard of all identification methods. 

How accurate, how reliable, is it?

Very accurate. It's limited only in cases where you have a partial profile due to the quantity or quality of the probe. In that case there's a certain probability that one or two other people in the world might have that same partial profile. Which is why a number of DNA sequences are needed for the probes. If we can successfully analyze them all, there's no doubt about the match.

How much material do you need and what do you mean by 'quality?'

In theory, all you need is one single cell. All body cells except for red blood cells contain DNA. Cells are exhaled, people shed skin cells and even bones have cells that contain DNA - it's everywhere in our bodies. The quality depends on the probe. Profiling creates a DNA pattern that is basically made of numbers that encode individual DNA sequences. If these sequences are destroyed or damaged by time, the environment or bacteria, that particular spot can't be determined, which means the evidence isn't as definite.

How long is DNA traceable?

The oldest probe examined in our lab was a bone from the 10th century, and the DNA looked like a fresh trace. You should never expect not to find DNA because a probe is old.

In the current case, one of the theories was that DNA traces from Böhnhardt's body contaminated the traces found close to Peggy's remains - in a lab. Can DNA transfer to another probe?

You have to anticipate that happening. Labs have installed a lot of precautionary measures, increasingly making them safe sites of analysis, for instance with air locks. But computer monitoring is even more important, all profiles are compared against all the other sample probes. Matches pop up automatically.

If - like in the present case - the profile from a completely different case pops up, the two might be linked, but you also have to check for possible contamination. After all, individual cells can be transmitted through the air, too. It happens to all the labs, that's why they keep special files to eliminate people who actually work at the lab. That's the main source of contamination.

Is DNA fingerprinting always used in criminal investigations?

If there's biological material, yes. Not just in murder cases, but in burglaries and bicycle theft cases, too. The profiles are compared to others stored in a data bank. The German DNA data bank, which includes material from convicted criminals as well as profiles from missing persons, was set up in 1998. Forensic fingerprinting was invented in Britain, and we've been using the method in Germany since the late 1980s. Our institute alone works on several thousand cases every year. The number of murder cases is relatively low, and forensic labs nowadays also work on evidence surrounding more trivial offenses like property crimes. 

Professor Lutz Roewer heads the Institute of Legal and forensic Sciences at the Charite-Universitätsmedizin Berlin clinic in the German capital. Charite is one of the largest university hospitals in Europe, and one of the oldest in Germany.