DNA Profiles for Forensic Use
Module: DNA Profiling
Each of the chromosomes in your cells contains sections of non-coding DNA — DNA that does not code for a protein. Non-coding DNA contains areas called short tandem repeats (STRs), made up of repeats of short base sequences, such as CATG in the sequence CATGCATGCATG.
If the DNA of two people was analysed for 10 different STRs on different chromosomes, there is only one chance in a million that they would have the same number of repeats in all of these STRs. Identical twins are the only exception — they have identical DNA and identical STRs.
If a crime suspect's DNA profile for 10 STRs matches the STR profile of a sample found at the crime scene, there is a very high probability that both lots of DNA are from the same person. However, if the profiles differ for even one STR, this cannot be assumed.
DNA is used as evidence in court, but it is considered ‘circumstantial' evidence, and can only be used as proof with other supporting evidence. However, it has proven useful in establishing the innocence of suspects.
Is DNA evidence infallible? Watch a story from the ABC's Catalyst program and decide for yourself
As Seen on TV
Just like our favourite forensic science TV dramas, forensic scientists can help solve crimes by analysing DNA samples from crime scenes and comparing them to DNA samples from victims and suspects.
However, in reality, forensic techniques are a bit more complicated and take much longer than the ten minutes seen on TV — more like two weeks, in fact. And, in a real forensic investigation, specialists would be performing specific tasks, rather than one person doing everything.
Lawyers and forensic scientists have noted that many people have a distorted view of how forensic science is really used in criminal cases, and believe that all forensic evidence is infallible.
Forensic Profiling in Australia
Forensic profiling began in Australia in 1988, and has assisted police in thousands of investigations since. The Australian Government has set up a DNA database of known criminals called CrimTrac. The database contains profiles from samples collected at crime scenes, convicted offenders and unidentified bodies.
According to the Victoria Police, it takes approximately 2 weeks to obtain a full DNA profile from a tissue sample. This can be sped up in urgent cases; however, there are many samples to process, and several samples collected at each crime scene.
In Australia , forensic DNA testing is done at government laboratories, as well as at some universities and private organisations.
For more information about DNA extraction and interpretation, go to the Victoria Police website, click here.
DNA Testing of Prison Inmates
A number of crimes remain unsolved due to lack of evidence. The majority of crimes are committed by only a few people. Of these few, there are a number of repeat offenders. The United States , England and New Zealand have central DNA databases containing the profiles of all previously convicted people. This allows for a quick identification if they reoffend.
Australia's CrimTrac central database is also likely to help solve some unsolved crimes. At the moment, the states and territories have different laws regarding access to DNA profiles of suspects or charged persons. This issue has a concerns surrounding it, such as whether taking DNA samples from prison inmates is a breach of legal rights.
For more information on CrimTrac, click here.
Disaster Victim Identification
It is often difficult to identify victims after disasters such as bombing or fires. Forensic scientists are called in to identify the DNA obtained from body parts or teeth.
During the aftermath of the 2002 Bali bombing, relatives of victims were asked to arrange collection of DNA samples from personal items such as toothbrushes or combs. So far, of the 221 missing or deceased in Bali , 182 have been identified, including 88 Australians. DNA profiling identified 115 people, while fingerprints, dental records and medical records were also used to identify victims.
Australian forensic experts went to Thailand to assist with the identification of bodies following the 2004 tsunami.
For more information on disaster victim identification in Bali, click here.