DNA is in most cells of the human body. Police can obtain DNA samples from bodily fluids such as saliva left on a bottle, blood on clothes, teeth, the roots of hair or bone, or from surfaces or objects that people have touched (because people shed their skin cells which contain DNA).
Because DNA is the same in all a person’s cells, regardless of the type of sample, scientists will get the same results. Scientists only need a tiny sample of DNA to analyse it and produce a DNA profile (see below).
Under the Criminal Investigations (Bodily Samples) Act 1995, police officers can obtain a DNA sample directly from people if they volunteer or consent to this. Usually, the police officer obtains a DNA sample from a fingerprick blood sample or a saliva swab, also known as a buccal swab.
If someone does not consent, then, in some circumstances, the police officer has the power to obtain a sample without the person’s consent. (Read more about how and when police officers obtain DNA samples).
Forensic scientists from the Institute of Environmental Science and Research (ESR) analyse DNA samples for Police at their laboratory. From their analysis, the scientists generate a DNA profile from each sample.
A DNA profile is a series of numbers produced as a result of forensic analysis of a DNA sample.
Just as everyone’s DNA is different (with the exception of identical twins), each DNA profile is different and will contain a different sequence of numbers.
Another way of thinking about a DNA Profile is that it is a unique numerical identifier – a bit like having your own passport number or IRD number, but encoded in each cell.
To get these numbers the DNA sample is analysed at 21 particular points (loci) on the DNA. (This has increased from six points when DNA analysis first began). This analysis happens at points on the DNA where each person has a different number of ‘repeats’ of the genetic code. These repeats are measured and the numbers recorded in the DNA profile.
Analysis at these points on the chromosome does not reveal sensitive individual genetic information – for instance about someone’s physical appearance, ethnicity or certain inherited medical conditions. However, if there is a defect in a person’s chromosome at any of the points, then scientists could in theory work out whether that person has a rare disease caused by the chromosomal defect– such as Down syndrome or Huntington’s disease. ESR scientists in New Zealand do not do this.
Because a DNA profile is a series of numbers, it can be stored electronically (on a computer) and can be compared with other DNA profiles.