Forensics is the application of science in a legal setting. An example of modern forensics evidence is the use of DNA profiling. Sources of DNA include blood, hair, semen, saliva, bone and tissue.
Collecting DNA samples
DNA can be isolated from a wide range of evidence left at a crime scene – from skin, hair and semen samples to bacteria in dirt!
Forensic scientists, for example those who work at Environmental Science and Research (ESR), are required to collect biological material from a crime scene. Blood is an excellent source of DNA. It is collected from the white blood cells (mature red blood cells do not contain DNA). DNA can also be obtained from the heads of individual sperm cells, from hair follicles or from any other cellular tissue.
For example, imagine someone has broken into the school office where exam records are kept. Perhaps some hairs dropped off without the person realising, or maybe the person cut their arm on the broken window when they entered, leaving a small trace of blood. It would be up to the forensics team to find these samples and collect them for analysis in the lab.
Determining a DNA profile
Forensic scientists can use DNA profiles to identify criminals or determine parentage. A DNA profile is like a genetic fingerprint.
Every person has a unique DNA profile, making it very useful for identifying people involved in a crime. The only exception to this is identical twins. Find out more in the article DNA profiling.
DNA profiles in solving crimes
The results from DNA profiles may be used in court. For example, the samples collected from a crime scene might match the DNA of a suspect. This could be used as evidence that the suspect had been present at the crime scene but it does not necessarily prove that the suspect committed the crime.
DNA evidence is rarely the sole basis of a prosecution case. It is most useful when placed alongside other evidence, such as fingerprints, footprints, crime scene examination and eyewitness accounts. Other biological evidence may also be collected, for example blood splash patterns (showing the direction of the injury) and microbial information (which may give clues as to the time of death). Find out more in the article Crime scene evidence.
There is a New Zealand databank of DNA profiles. It contains over 70,000 DNA profiles of convicted offenders and some volunteers. This national databank collection can be matched against DNA profiles collected from unsolved crimes. Find out more in the article New Zealand DNA databank and try the activity DNA Q&A.