Tinier and tinier quantities of DNA can now be detected – which actually give forensic scientists a problem. Instead of just one person’s DNA at a crime scene, there may be traces of others who have never been there. To what extent can DNA be transferred from place to place, and how can different traces be sifted out?

Dr Alexander Gray from the Leverhulme Research Centre for Forensic Science in Dundee describes the new challenges faced by forensic scientists.