DNA or fingerprints: which is the best?

THE latest in our series of Friday science features looks at the forensic world of fingerprints and DNA with Graham Williams, lecturer in forensic science at University Campus Oldham’s degree partner, the University of Huddersfield

DNA evidence was responsible for identifying the offender in many recent high-profile cases. But it is easy to overlook the other evidence types — especially finger mark evidence, more commonly known as fingerprints.

Fingerprints have been used for hundreds, even thousands of years, and have been responsible for identifying or eliminating many suspects since the establishment of the Fingerprints Bureau at Scotland Yard in 1901.

So why do we hardly ever hear of high-profile suspects being caught by fingerprint evidence? Is it because all criminals are wise to fingerprint evidence and take precautions?

It is certainly true that most people are aware of fingerprint evidence and, if they were preparing to commit a crime, would wear gloves.

However, a lot of crimes are opportunistic and therefore may be carried out without gloves. Our fingerprint experts are still busy and suspects are continually identified.

I suspect that one reason why we do not hear of these cases is because the identification of a suspect via fingerprints can be carried out very rapidly. This means that by the time the public become aware of such a case, the suspect has already been arrested.

In contrast, DNA evidence takes some time to process and therefore it can be some time before a suspect is identified. So when we hear of such a case it becomes much more interesting because we do not know who did it.

It is not a case of DNA evidence supplanting fingerprint evidence; in fact it would be a very poor investigator who ignores fingerprint evidence in favour of DNA evidence.

This raises the question, which of the two types is the stronger evidence?

Answer: Fingerprints — and I say this as a DNA expert.

Obtaining full matching fingerprints is said to be a conclusive match, whereas getting full matching DNA profiles is said to have a match probability of one in a billion.

This means that DNA evidence, even at its strongest, still has room for doubt, whereas full matching fingerprints do not.

We also consider the nature of the transfer of evidence. If I were to touch a smooth surface such as a wall, I would deposit DNA and leave some fingerprints behind. This is called direct, or primary, transfer.

However, if someone was to come along and wipe that wall with a cloth, it would remove my DNA on to the cloth and wipe the fingerprint off.

If that person then uses that cloth to wipe the door handle, my DNA can then be transferred on to that door handle.

Therefore, my DNA could be recovered from that handle without me ever coming into contact with it. This is indirect, or secondary, transfer.

In this example, my DNA is transferred, but my fingerprint is not. This means that if my fingerprint is found on a surface, then I must have touched that surface; whereas, if my DNA is found on a surface, then I may have come into contact with that surface or it got there by secondary transfer.

It can be seen that not only is the probative value of the fingerprint much higher than DNA, but it is also much more significant within the context of a crime.

If fingerprints are so good, then what is the point of DNA evidence?

Not every touch or contact leaves a fingerprint; sometimes the surface of interest is ridged or rough, thus rendering a fingerprint unusable. Fingerprints can also be smeared.

An incomplete fingerprint can still be used but may not give a conclusive match, while DNA can also be obtained from body fluids left behind at a scene. In such cases, fingerprint evidence would not be useful, but DNA would be.

So, given all this, should we choose fingerprint evidence or DNA evidence? The answer is that we should use both. There will be times when DNA is the better evidence, there will be times when a fingerprint is the better evidence and there will be times when both can be used together.

With a good forensic strategy in place, DNA evidence and fingerprint evidence can complement each other very well.