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How Reliable Is DNA In a Criminal Case?


If you watch any TV dramas, you’ll know that DNA is considered to be the Holy Grail of criminal evidence.  A piece of it in your trial can quickly turn even the most supportive jurors towards a guilty verdict.

But is the reverence given to this science justified?

How reliable is DNA in a criminal case?

DNA was first used 31 years ago in the case of Dawn Ashworth, a schoolgirl who was raped and murdered, and has since grown to be believed by many to be indisputable evidence.

The truth, however, is more complex.

While it is possible on occasion to secure a very clear DNA match, as our knowledge of DNA and our ability to find even tiny traces of it improve, it actually raises alarming questions about what the presence of DNA is actually evidence of.

For example, technology that allows the recovery of tiny traces of DNA means that such traces can be difficult to identify as being from skin cells, saliva or some other body fluid, as well as samples actually containing material from several people.

To further complicate things, it must be remembered that not all people leave DNA in the same way.  Some people leave DNA and others, well, don’t.  This depends on several factors such as how long it has been since the person washed their hands.  In some cases, a person may touch an item and leave no DNA, but leave the DNA of a person with whom they have close physical contact.

This new study has also shown that DNA can be transferred from an item of clothing to another item during laundry in a washing machine.  This discovery is relevant for many cases, including the murder of Meredith Kercher where DNA evidence implicating the boyfriend of the victim’s flatmate was found on the victim’s bra strap.  It is easy to imagine how many opportunities there would be for two females sharing a home to transfer the DNA of a boyfriend’s to the other person.

New research shows than in 218 successful appeal cases in England and Wales between 2010 and 2016, DNA evidence had been misleading.  The relevance, validity or usefulness of DNA evidence has been challenged in these chases.

So, how reliable is DNA?  We think the UK government’s chief scientific adviser into forensic science has summed it up well:

“our ability to analyse may outstrip our ability to interpret”

If you have concerns about an ongoing case or a conviction using DNA evidence, call our expert team now on 01623 397200.




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