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DNA - A Prosecutor’s Practice Notebook Inventory

Questioning the DNA Analyst

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National Institute of Justice (NIJ) (see reuse policy).

The prosecutor should have the DNA analyst list the many uses of DNA outside the forensic community (medical, missing persons identification, etc). The DNA analyst can also discuss how DNA has assisted in the exoneration of individuals accused of a crime and may be able to discuss the rate of exclusion of suspects at the DNA analyst's laboratory. Where appropriate, it may be stressed that the process of testing done for the exoneration is the same as that used in the case at hand (STR typing) and the same instruments, kits, techniques etc. were used, thus stressing the reliability not  just of DNA typing but the exact typing used in the prosecutor's case.

The DNA analyst should be questioned about the DNA laboratory facility, including whether the lab meets national testing standards, as required by the American Society of Crime Lab Directors – Laboratory Accreditation Board (ASCLD-LAB), and/or participates in the Federal Bureau of Investigation's Combined DNA Indexing System (CODIS). The DNA analyst should describe the quality assurance standards employed in the testing lab itself. Prosecutors should be aware that there are laboratories that do adhere to national standards of quality assurance but have not been certified by ASCLD-LAB.

Photo of two chemists in a lab examining vials of data samples
National Institute of Justice (NIJ) (see reuse policy).

After educating the jury on the reliability of DNA and testing procedures, the prosecutor should question the DNA analyst on the testing done in this specific case. The DNA analyst should describe the chain of custody whereby the DNA samples came under the DNA analyst's exclusive control during the DNA testing procedures. Additionally, the DNA analyst should describe the necessity for obtaining a control sample taken from the victim and/or suspect and/or scene (including any non-human DNA) for comparison purposes.

The expert should explain that DNA was extracted from both the evidence and control samples using sophisticated scientific testing equipment for comparison purposes, and the comparison yielded a:

  • Match or
  • Exclusion  or
  • Inconclusive results

Examples of how DNA is used outside the forensic community include:

  • paternity testing
  • medical applications in genetic research, diagnostics, and tissue typing
  • identification of the dead
  • identification of victims of mass destruction
  • identification of missing children
  • wildlife management and protecting endangered species

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