Can Your DNA End Up at a Crime Scene Without You?

Dr Kathryn Fox is an Australian medical doctor and bestselling crime writer, best known for her forensic thrillers featuring pathologist Dr Anya Crichton. Drawing on her medical expertise, she crafts gripping, authentic crime fiction and is also a passionate advocate for forensic medicine education and public engagement. Her columns appear every Tuesday and Friday.

Thanks to ever-popular crime podcasts and crime shows, DNA evidence feels like the gold standard. When forensics say they’ve come back with ‘a match’, it’s the smoking gun, the gotcha moment – DNA found at a crime scene proves the person it came from was at the crime. Or does it?

Modern forensic science is a little more complicated than TV would like us to think. Advances in DNA technology mean detection has become increasingly sensitive. It can now detect incredibly small traces – sometimes just a few skin cells left behind after brief contact.

It’s easy to assume that DNA is solid evidence. Better sensitivity should mean more accuracy and reliability. But it also raises two important questions. How is DNA transferred, and how easily?

In recent years, research from forensic laboratories has shown that DNA can be transferred in ways most of us wouldn’t expect. Of course, there are the obvious, such as shaking hands and close physical contact – known as direct transfer – but that isn’t the only way DNA can end up on someone else.

What if you shake someone’s hand and they later touch an object? It’s possible – in some circumstances – for your DNA to be found on that object, even though you’ve never been near it. That’s called indirect, or secondary transfer. And that’s where things get complicated.

Studies now suggest that up to 15 per cent of DNA found on our hands isn’t ours.

DNA from multiple people can often be found on everyday items including door handles, clothing and cash, reflecting how easily genetic material is shared in daily life. Putting our phones down on surfaces in cafes and offices can pick up DNA as well. Some people are more likely to shed their DNA than others and leave more traces behind.

As the day goes on, and the time in between washing hands increases, we seem to collect more. Touch doors, taps, benches, tables, chairs at home and you’re likely to pick up DNA from the people you live with.

Travel to work on public transport, or use a public bathroom? You’ve had ample opportunity to pick up strangers’ DNA on your hands and sleeves, directly or indirectly. Pets have been known to transfer DNA as well.

What does it all mean for solving crimes?

As technology has improved, scientists have been able to generate DNA profiles from increasingly tiny amounts of material – sometimes just a handful of cells.

That’s a significant advance. But the more sensitive the test, the more complex the interpretation becomes.

One of the challenges for forensic scientists and police is not simply identifying whose DNA is present, but understanding how it got there.

DNA doesn’t tell a complete story. Finding someone’s DNA at a location doesn’t answer the most important question:

• How did it get there? Forensic scientists are very aware of this.

They don’t just look at whether DNA is present.
They consider:

how much DNA is there

where it was found

how it may have been transferred

and whether the explanation fits the broader evidence

In other words, DNA is only part of the picture, a piece in the puzzle.

Courts are also becoming more aware of this complexity.

Expert evidence increasingly includes discussion about transfer, persistence, and the limitations of interpretation – not just the presence of a DNA profile.

None of this means DNA evidence is unreliable. Far from it. DNA remains one of the most powerful tools in forensic science. It has helped solve crimes, identify victims, and exonerate the innocent.

DNA is not as simple as it seemed. As technology improves, it’s capable of detecting more than ever, including DNA that may have arrived indirectly or unintentionally.

Sometimes, more information doesn’t make things clearer. It makes them more complicated. For the public, it challenges a long-held belief that DNA provides certainty.

DNA detected at a crime scene doesn’t always explain how it may have arrived there. On the plus side, that means potential new storylines for our favourite podcasts and crime shows.