Introduction to DNA Profiling

Here we are, the technique considered by many to be a holy grail of forensics. But what is DNA profiling and how does it work? Well first things first, the term DNA profiling refers to the sequencing of an individuals DNA. 

When DNA profiling there are 3 possible outcomes:

1. Exclusion - the sample DNA does not match the suspects, thus ruling them out. This can be used to prove innocence or rule out suspects.

2. Inconclusive - due to damage through contamination or DNA degradation, further information must be sought, and the test must be carried out again.

3 Inclusion - The DNA is a match. However as DNA is not unique the significance of the match is calculated by quantifying the random match probability (Pm)'. The (Pm)' is a statistical test that estimates the probability of two unrelated people sharing the same DNA profile. Where markers are not linked (so inherited independently), the (Pm)' can be estimated by multiplying individual allele frequencies within a sample population. Therefore the more loci that are included in the analysis and the greater their heterozygosity the smaller the value of (Pm)' will be. A smaller (Pm)' means that the DNA being profiled is more likely to have come from the suspect. 

Despite DNA profiling's high levels of specificity there are a few problems with the technique. The first is that DNA will degrade over time, meaning it is harder to produce conclusive results with the test. The second major problem is that DNA from related people is extremely similar so has a much higher (Pm)' value. This means in cases between family members DNA is much less useful.

We'll end the introduction with a quick case study:

   1983, a girl in Narborough, Leicestershire is found dead with sigs of rape. A semen sample recovered from her body was tested and showed that the murderer was blood group A and had an enzyme profile shared by about 10% of the male population. However this was the only evidence recovered, and the investigation made no progress.

   1986, a second girl in the same area, was also found raped and murdered. The semen sample recovered from the body showed the same characteristics as the one from the case 3 years ago. The police established this connection between the 2 cases. The police believed that they were the work of the same man and that he lived in the vicinity. Their investigation led them to Richard Buckland, who was arrested. When interrogated he confessed to the second girls murder but denied having any involvement in the first. Buckland was DNA profiled, as were the semen samples. The girls had in fact been killed by the same man, but this man was not Buckland. 

   Having lost their primary suspect the police tried a different approach. They conducted the first ever mass DNA screening. All men in the vicinity (roughly 5000) had a blood sample or saliva swab taken. From these those exhibiting a matching enzyme profile and blood group where then DNA profiled. However despite all this they had nothing. There were no matches. Once again the investigation hit a wall. 

    Six months later however a woman reported that she had overheard a man claiming to have provided DNA in place of his friend, Colin Pitchfork. Colin Pitchfork was subsequently dragged in, and his DNA profile, unsurprisingly, was a match, inclusive. By 1988 he had been sentenced to life in prison. 

Colin Pitchfork on his arrest

   This case study helps show how DNA evidence alone is rarely enough to find or convict a suspect, but is very effective at excluding suspects. Thanks for reading! I'll be back soon, and we're going to be getting into some harder science! Over and Out!