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We all possess specific DNA sequences, which act as genetic fingerprints. DNA is found in every cell of a human - blood, hair, skin, etc- often left at the scene of the crime. They are also left by the victim on the body of the perpetrator. No one's DNA is identical to anyone else's (excluding the case of identical siblings), so if a sample matches the sample taken from the scene of the crime, there is a high probability that they originate from the same person.

A powerful tool for crime solving, DNA evidence helps to determine the guilt/innocence of the accused by identifying, for example, the identity of the individual who deposited semen in a rape case, and the identity of the individual who left blood on the victim, alleged perpetrator or crime scene in a homicide case. DNA testing is useful as a technique in crime solving as everyone's DNA is different (except for identical twins) and DNA can be extracted from any body fluid (blood, saliva, sweat, nasal mucus etc) or from fragments of a body (hair roots, torn skin or flesh).

Process of DNA testing

Some basic steps are performed during DNA testing. These include:

  • the isolation of the DNA from an evidence sample which contains unknown DNA, and at a later stage, the isolation of DNA from a sample from a known individual
  • the processing of the DNA - Under this step, the DNA is extracted from its biological source material. After isolating the DNA from its cells, specific regions (loci) are copied with a technique known as the polymerase chain reaction (PCR). PCR produces millions of copies for each DNA segment of interest and therefore permits very small amounts of DNA to be examined. Multiple STR loci can be examined at the same time (The variability in STR regions (loci) is used to distinguish one DNA profile from another.
  • the determination of the DNA test results, from specific regions of the DNA and
  • the comparison and interpretation of the results from the unknown and known samples to determine whether the known individual is included or excluded as a possible source of the DNA.

See Evidence, Causes of Wrongful Convictions