Question: If someone has been murdered and you want to find out who has killed them, how do you use DNA to determine who committed the crime?

Mark Hill answered on 25 Mar 2011:

Hello xxgreggxx,

I think that the lab work falls more within the expertise of Kat, Niamh and Jodie. However, I will try and help.

At the scene sterile swabs are used over likely DNA sample surfaces, where samples are expected to be. These are known as latent samples. They are individually packaged and labelled, to avoid cross-contamination. They are then sent to the froensic lab for analysis.

I understand that DNA samples are compared with known held records, from people convicted of a crime. The DNA is made up of long chain combinations of paired molecules of one of four different nucleotide acids – Adenine, Guanine, Cytosine and Thymine. The unique way in which they pair up and the orders of those pairs give us an opportunity to compare different samples.

Initially, just after DNA evidence entered forensic science, single locus probe (SLP) comparison, at one point in a DNA string. This progressed into short tandem repeat (STR) comparison, which uses several preselected sites on a DNA string, for comparison.

The result of a comparison test is never conclusive, but is expressed as a probability of the test sample NOT being that of the suspect – usually in being 1 in 3 billion chance of the sample not being that of the suspect.

I hope that this helps answer your question.

Good question, Thank you.

Mark.