Forensic DNA Testing

Analyzing DNA evidence in a criminal case can take weeks or months to complete and can be expensive, especially if multiple pieces of evidence are submitted. During a criminal investigation, the police department or the prosecutor's office often pays for the analysis. In criminal DNA cases, laboratory personnel should work with the police department and prosecutor to decide which evidence may be most probative to the case. Once a decision is made about what evidence should be tested, the laboratory will extract and analyze the DNA.

The laboratory analysis of DNA involves the examination of a DNA strand at 13 specific locations (loci). The DNA profile from biological evidence collected from a crime scene can be compared with a profile from a known source (for example, a semen stain from a crime scene could be compared with a blood or saliva sample from a suspect). The DNA profiles from the biological evidence are compared with the profile from a suspect at the loci tested. If the profiles are the same, a statistic is generated which reflects how often one would expect to find this particular DNA profile in the general population. This helps in the analysis to determine whether the evidence likely came from the suspect or not. In addition, this process can seek the source of DNA evidence found at a crime scene by comparing the profile to convicted offenders in the CODIS database in situations where investigators may not yet have a suspect.

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Understanding DNA Evidence: A Guide for Victim Service Providers
April 2001