27 January 2023

DNA evidence is not the silver bullet crime shows would have you believe

| Katrina Condie
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Lawyer Andrew Byrnes from Andrew Byrnes Law Group says DNA evidence is just one of many factors that must be considered in a criminal case. Photo: Michelle Kroll.

Fans of shows like CSI might believe DNA evidence is the key to solving criminal cases but, in reality, there are many factors that can result in DNA samples being thrown out of court.

Canberra criminal lawyer Andrew Byrnes from Andrew Byrnes Law Group says while DNA evidence is a powerful tool in a prosecution, it is not necessarily the silver bullet people think it is.

“When you watch crime dramas or listen to crime podcasts about serial killers, it can sometimes come down to DNA samples leading to the conviction of the killer. But, in real life, DNA is not necessarily the smoking gun you might think it is,” he said.

Andrew says sometimes DNA evidence can come undone at a criminal trial because it’s not necessarily possible to determine how DNA came to be somewhere or how long it has been in that location. DNA can also be contaminated or degraded over time.

He explained that DNA reports talk more about it in the sense of probability rather than it being like a fingerprint. It’s not necessarily possible to determine how long DNA has been on an item, and where there is more than one trace, it is impossible to determine the order in which they arrived there.

“There are many factors that can result in the exclusion of DNA from a case, including transference,” he said.

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“For example, if I was at Mooseheads one night out in town and I gave my mate a bottle of beer I was holding, my DNA may go onto the bottle from my skin shedding a little bit on contact with that bottle. My DNA could then be transferred from the bottle onto his hand.

“Let’s say later that night, my mate assaults someone and police take a DNA sample. It could end up being that my DNA that is found on the alleged victim because of transference. Just because my DNA is on the victim, it doesn’t, of itself, prove that I was the attacker.”

Andrew says where and how the DNA is collected can also play a huge part in whether it can be submitted as evidence.

“If a crime is committed in a shopping centre or prison, for example, these places can – unpleasant though it may sound – have all sorts of bodily fluids floating around and a DNA sample could have DNA from all sorts of ordinary people going about their daily business,” he explained.

“There are many ways DNA can show up at a crime scene or on someone’s clothing that isn’t necessarily associated with the guilt of the accused person.”

When DNA is moved from one sample to another, it can sometimes be like a chain reaction where DNA gets moved across multiple sites so that the owner never actually comes into contact with the DNA’s eventual location.

Forensic DNA evidence has sometimes led to wrongful convictions if the DNA is damaged and only produces a partial profile, which will match with many people.

While DNA detection is becoming more sensitive, so is the likelihood of secondary transfer of DNA sources.

“Evidence, particularly related to the collection and handling of DNA samples or exhibits from which they are taken, should not be accepted at trial uncritically,” Andrew explained.

In one recent ACT court case, the accused was acquitted after their legal team established the reasonable possibility of DNA evidence contamination.

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In another case, DNA admissibility with a 99.9 per cent estimation of belonging to the accused was rejected due to how it was presented in court. The DNA was excluded as its value was outweighed by its potential to prejudice.

Andrew said DNA evidence was ruled inadmissible in a 2015 Canberra rape case because of a clear possibility of contamination. The underwear on which DNA was found may have been from the process of secondary transfer.

In yet another case, a man accused of raping a girl who had passed out in a nightclub toilet after being drugged was wrongly convicted on the basis of DNA evidence and spent 16 months in jail. In this case, there was no evidence besides DNA to indicate the accused’s involvement.

Andrew says often too much reliance is placed on DNA evidence and it must be considered in the context of all the evidence presented by the prosecution.

“Like all evidence, DNA can be used, misused and abused,” he said.

DNA evidence is often submitted in serious cases such as murder, manslaughter, rape or assault, and while cases can be defended, Andrew says that having a good criminal lawyer will help you increase your chances of successfully beating a charge or lessening the penalties.

If you need help navigating the legal system after being charged with a criminal offence, contact the criminal defence lawyers at Andrew Byrnes Law Group.


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Wasn’t the basis of the governments new vehicle theft (or entering) based purely on DNA evidence?

Yes. I believe that was more to do with someone’s DNA being in a car does not equate to them driving a car.

Don’t get too wrapped up on Mr Byrnes and his attempt at explaining Locard’s exchange principle. That was definitely more a problem in the US with touch DNA and with condensation on bottles and glasses not much of an issue hanging out drinking with a mate.

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