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DNA - A Prosecutor’s Practice Notebook Inventory

Attacks on Education, Expertise, and Bias

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After obtaining concessions, the prosecutor's focus should be on attacking the defense expert's areas of education, expertise, and bias. The prosecutor should obtain the defense expert's resume or curriculum vitae pre-trial and review this with the state's DNA expert witness.  Explore with the state's expert any areas of academic specialization and any publications the defense witness may have authored. Many defense experts are schooled in non-human genetics and have little or no forensic background.

In what areas does the defense expert profess to be an expert? If the expert is not an expert in human genetics or forensic DNA analysis, the prosecutor should ask the court to bar the proffered testimony.

The defense expert's prior experience will be a fertile ground for cross-examination. Depending on the lab the expert comes from, they may have little or no hands-on forensic DNA experience. If so, the prosecutor should compare and contrast the state's DNA analyst's credentials with those of the defense's expert and cross-examine the defense expert on various DNA typing procedures to highlight their lack of experience.


The following list contains the types of questions the prosecutor should ask during cross-examination:

  • Does the expert have any forensic experience? Academicians performing analysis on pristine laboratory samples are quite different from the scientist examining crime scene samples.
  • Does the expert have any prior human DNA experience? Most defense experts specialize in plant or animal genetics.
  • Has the expert ever used a commercial DNA forensic typing kit?
  • Did the expert ever perform an extraction, actually extracting DNA from a sample for analysis? Or has the expert ever performed a differential extraction, actually separating male DNA from female DNA in a sample?
  • Has the expert ever performed a quantitation distinguishing human from non-human DNA and deriving the amount present for typing?
  • Has the expert ever performed DNA amplification, thereby exponentially increasing the amount of DNA for analysis? Did the expert ever use a thermal cycler for that purpose?
  • Has the expert ever used a genetic analyzer, or performed the capillary electrophoresis process?
  • Did the expert ever utilize commercial DNA software to interpret typing results?
  • Does the expert even have a laboratory of his or her own?
  • Is the laboratory, if any, used by the expert accredited?
  • Is the expert subject to regular proficiency typing?

"Regular proficiency tests, both within a laboratory and by external examiners, are one of the best ways of ensuring high standards [NRC II at page 88].”

  • Is the expert’s work subject to a technical and/or administrative review as to the results and conclusions?

For example: ID vs. consent; placing the victim at a certain location; impeaching a witness or witness corroboration

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