I need this information for my new novel, but thought I’d share… by The Society of Procrastinating Linguists
(i) A stain containing DNA is found at a crime scene. It is estimated that only one person in a million will have a DNA profile that matches that of the stain.
(ii) The defendant has a DNA profile that matches the stain.
(iii) Ergo, there is a million to one probability in favour of the defendant having left the stain.
In fact, in a country with 60 million people who were potentially present at the crime scene, there is only 1% chance, if we disregard all circumstances, for that suspect to be the person present. In a world of 6 billion the chances diminish quickly. Of course such DNA evidence could be coupled with circumstantial evidence to inform the jury for better judgment.
DNA case R -v- Doheny & Adams ( EWCA Crim 728)