DNA evidence – is it infallible?

Last week a Vietnamese woman was arrested in Queensland in a very high profile case that captured news headlines across Australia and even the world. She was charged with being responsible for contaminating strawberries with sewing needles, causing mass chaos in the Australian fruit and vegetable industry.  We won’t go into the details of this case because it’s readily available in other news sources but the reason we mention it is because this story has prompted a lot of people to ask us about the evidence that was mentioned in news reports.  In particular, they asked us about DNA evidence and how this woman could possibly defend the charges, given that her DNA was found inside of a punnet of strawberries in Victoria in which a needle was found.

As soon as most people hear the term ‘DNA’, they will automatically think that the evidence is extremely strong. Most people will only have ever been exposed to the concept of DNA evidence from watching television shows or movies, which usually show that as soon as a person’s DNA is found at a crime scene then they must be guilty because DNA is unique to each person, like a fingerprint.  However, while most of us have heard of DNA evidence only few of us will understand what it actually is, how it works and how it can be used to prove that somebody is innocent or guilty of a criminal offence.

What is DNA?

To those who are not familiar, DNA is a genetic blueprint which is unique to each individual. DNA is contained within every single body cell of a human being and is made up of tiny pairs of molecules.  DNA was first used in criminal investigations in 1984 to solve a very violent crime in the United Kingdom and it has been an extremely important law enforcement tool ever since.

DNA can exist in a person’s skin, hair, saliva, blood or any other body cell, provided that there is enough sample collected. Whilst it is easiest to extract DNA from an actual piece of skin, hair, drop of blood or saliva, it is also possible to extract DNA from surfaces or objects that a person has come into contact with.  For example, if a person took a shower and then dried themselves off with a towel, there is a very good chance that the towel will contain skin cells (not visible to the eye) which contain DNA.

Potential problems with DNA evidence

As soon as most people learn that the Vietnamese woman’s DNA was found inside of a punnet of strawberries that contained a needle in Victoria, given that she lived in Queensland, they would assume that she must’ve been responsible for inserting the needle. How else would her DNA possibly get inside of that punnet? Well it’s not that simple.

When obtaining a DNA profile, what firstly needs to be considered is whether the sample is a ‘full profile’ or a ‘mixed profile’. When a sample of DNA is obtained, it can further be determined how many actual ‘contributors’ there were to the DNA, meaning how many people’s DNA the sample contains.  If a person wore a hat, and they were the only person ever to have worn or handled that hat, then if tested it would likely contain a ‘full profile’, meaning that it contained the DNA of one person only.  However, that person wore the hat and then let their friend borrow their hat, once tested there is a good chance that the result would be a ‘mixed profile’ of both people.

This then leads to something known as ‘touch transfer’ of DNA. If person A is driving a car they would likely leave their DNA on the steering wheel.  If person B then drives the same car it is possible that they would collect person A’s DNA on their hands.  If person B then went and wiped their hands on a towel it is very possible that, in addition to leaving their DNA on the towel, they would also deposit person A’s DNA on the towel.  Therefore, person A’s DNA might be located on a towel that they’ve never even seen!

It is necessary to consider all of these things before any weight can be attributed to DNA evidence.

Can DNA testing be flawed?

In Queensland all forensic testing is undertaken at the John Tonge Centre. They are made up of scientific and medical experts and their work is relied upon to convict and acquit people of very serious criminal offences so you can imagine how strict their processes must be.

We once had a client who was charged with a very serious crime. His DNA was located somewhere that he couldn’t explain and so it appeared to be a very strong case.  The only possible option for us was to analyse the actual testing procedure to see how it was conducted and to see if there were any errors in how it was conducted.  In this case we obtained the full DNA testing records, which were about 300 pages.  What we discovered in a very small part of the records was something that is extremely rare.  The records showed that during the DNA testing process there was an ‘anomaly’ (error).  While testing, one of the tips of the ‘pipettes’, which is a laboratory tool used to measure and transport tiny amounts of liquid, fell off and potentially came into contact with other pieces of laboratory equipment.  This meant that the testing process was potentially contaminated and raised doubt about our client’s guilt.

DNA evidence, unlike on television and movies, is not perfect. It, like any other type of evidence, is still susceptible to error and can not always be used by itself to conclusively prove the innocence or guilt of a defendant.

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