What is DNA?
DNA is short for deoxyribonucleic acid. DNA is a chemical found in the centre of every living cell in our bodies. DNA contains all the information your body needs to grow and develop. Our DNA determines the colour of our eyes, our hair colour, even whether we are tall or short. We inherit half of our DNA from our mother and half from our father. Our DNA is unique to each of us, with the exception of identical twins.
The DNA in a person's body is the same in every cell of every type in the human body, except for red blood cells which do not contain it. It is possible to obtain a DNA profile from samples such as blood (from white blood cells), saliva, semen and hair roots, and from surfaces or objects that someone has touched or been in contact with.
How is DNA useful to the Police?
In Forensic Science, the process of analysing DNA is referred to as DNA profiling. In Scotland twenty one regions of DNA, known as Short Tandem Repeats (STRs) are analysed, each of which can vary considerably between unrelated people. We also look at further regions which indicates the gender of the source. By using these DNA regions together it is easy to tell DNA profiles from unrelated individuals apart. We can compare the DNA profile from a reference sample provided by a person, such as a swab from the inside of their mouth, with DNA profiles obtained from body fluids or objects found at crime scenes.
DNA Profiling in Support of Criminal Justice
Forensic Services SPA use DNA to provide four key services to the Police, Crown Office and Procurators Fiscal.
- Cases with a known suspect: comparisons can be made between reference samples and crime sample evidence material.
- Cases without a known suspect: the Scottish and UK DNA databases can be used to identify matches between a crime sample DNA profile and a person held on the database.
- Criminal Paternity testing in cases of rape or incest where a child has been conceived.
- Body identification, such as in missing person enquiries.
DNA profiling is used in solving crimes ranging from volume crime such as housebreaking and car crime to serious crimes like assaults, murder and rape. The forensic scientists will look for suitable samples at a crime scene and will also exam items such as weapons or clothing where DNA may be present. If a person’s reference DNA profile differs from that of the crime sample then he or she can be excluded as a contributor to that sample. If, however, the DNA profiles obtained from the crime and reference samples match then this person may have contributed DNA to the sample. We then evaluate the DNA profiling result considering how likely the DNA result is if the person of interest is source of the DNA or body fluid or if another individual unrelated to them is the source.
The ability to tell individuals apart, and to provide statistical weight to the findings, is why DNA has become so useful in criminal investigations. Evidence from DNA profiling contributes to many cases. It provides leads for investigations, can help exonerate individuals in some cases and can help juries assess the likely guilt or innocence of accused persons.
Forensic Services has invested in the latest advancements in DNA profiling meaning that in recent years we have increased and improved the results we can report to the Police, Crown Office and Procurators Fiscal. Y-STR DNA profiling is a form of DNA profiling which only targets male DNA and is now used routinely by scientists in sexual crimes with increasing success. Adopting new DNA interpretation software means we can now routinely report complex DNA profiles that previous would have been unsuitable. SPA Forensic Services will continue to explore novel techniques with the aim of enhancing our DNA services.
Crimes without suspects: The Scottish DNA Database
When a suspect is arrested the Police have the right to take a DNA sample from them. The sample is usually a mouth swab. All such samples are analysed and the profiles stored on the Scottish DNA Database in Dundee. They are also sent to the UK National DNA Database in Birmingham.
The DNA database can help to solve undetected cases where there is no suspect. For crimes without a suspect, DNA profiles obtained from objects or body fluids found at crime scenes can also be sent to the DNA Databases, to see if the profile matches any of the DNA profiles from arrested people who are subsequently convicted. Finding a match between samples from crimes and samples taken from convicted people is used to help solve volume and serious crime.
DNA profiling can be used to help determine whether or not a particular man is likely to be the child’s biological father. The same process can also help determine whether a child has been conceived as a result of familial rape or incest.
Where other means of identifying a body are not successful, or inconclusive, DNA profiling can be used. Comparing the DNA profile from a body to DNA profiles from parents, or from personal items, can help confirm the identity of the body.
Did you know?
DNA profiling is a well-established field of Forensic Science. It was first used in a criminal investigation in the UK in the 1980's, during the investigation of two murders in Leicestershire. The DNA profiles obtained from samples from the victims showed that both murders had been carried out by the same individual. The DNA profiles obtained from theses samples did not match that of the prime suspect at the time and exonerated them. Leicestershire Constabulary then carried out the world's first intelligence-led DNA screening. All adult males in three villages - a total of 5,000 men - were asked to volunteer and provide blood or saliva samples. A local baker, Colin Pitchfork was arrested and his DNA profile matched with the semen from both murders. In 1988 he was sentenced to life imprisonment for the two murders.