Forensic scientists use DNA profiling to link people to crime scenes, to rule suspects out and to identify bodies.Collecting and comparing samples
Cells containing DNA are left behind as blood, semen, saliva, hair roots or skin cells. Only a tiny amount is needed because PCR can amplify it. A DNA profile is produced from the crime scene sample and compared with profiles from suspects, which are usually taken from cheek cells using a mouth swab.If the bands of the crime scene profile and a suspect's profile are in exactly the same positions at every locus, the profiles match. This is strong evidence that the DNA came from that person. If even one band is different, the suspect can be excluded: the DNA did not come from them.Worked example: a crime scene sample has bands at 120, 185 and 240 base pairs. Suspect A has bands at 120, 185 and 240 base pairs. Suspect B has bands at 120, 160 and 240 base pairs. Suspect A's profile matches; suspect B is excluded because of the band at 160 base pairs.Profiles from crime scenes can also be compared with a national database of profiles to find a suspect.Limitations
• A match shows that the person's DNA was present, not that they committed the crime. They may have been there at another time, or their DNA may have been transferred indirectly.
• Samples can be contaminated with DNA from other people, including investigators.
• Old or degraded samples may give an incomplete profile.
• Evidence is presented as the probability that a random, unrelated person would match, and relatives are more likely to share bands.
Cells containing DNA are left behind as blood, semen, saliva, hair roots or skin cells. Only a tiny amount is needed because PCR can amplify it. A DNA profile is produced from the crime scene sample and compared with profiles from suspects, which are usually taken from cheek cells using a mouth swab.If the bands of the crime scene profile and a suspect's profile are in exactly the same positions at every locus, the profiles match. This is strong evidence that the DNA came from that person. If even one band is different, the suspect can be excluded: the DNA did not come from them.Worked example: a crime scene sample has bands at 120, 185 and 240 base pairs. Suspect A has bands at 120, 185 and 240 base pairs. Suspect B has bands at 120, 160 and 240 base pairs. Suspect A's profile matches; suspect B is excluded because of the band at 160 base pairs.Profiles from crime scenes can also be compared with a national database of profiles to find a suspect.Limitations
• A match shows that the person's DNA was present, not that they committed the crime. They may have been there at another time, or their DNA may have been transferred indirectly.
• Samples can be contaminated with DNA from other people, including investigators.
• Old or degraded samples may give an incomplete profile.
• Evidence is presented as the probability that a random, unrelated person would match, and relatives are more likely to share bands.