Criminal Law Evidence Legal Topics

10 Tips for Attacking DNA Evidence

DNA evidence can help make or break a case. If you find yourself confronting DNA evidence, consider these ten tips when developing your challenge.

  1. Examine case materials. Examine all case materials, including crime reports, property reports, lab reports, tabular data, electropherograms, bench notes, correspondence, billing, etc.
  2. Examine the chain of custody and the analyst’s background. Check into who handled the evidence, and when and how they did so. Also find out about the analyst’s record, résumé, prior testimony, and reputation.
  3. Scrutinize lab procedures. Scrutinize the lab’s standard operating procedure, quality assurance/quality control manual, internal validation, contamination and unexpected results logs, lab audits, accreditation, corrective actions, proficiency tests, etc. Request information on the lab’s procedures through discovery or pursue other methods of obtaining them. Laboratory materials help you to compare the procedure actually undertaken with the standard procedure; if there is any variation in what the lab actually did versus what it was supposed to do, this should trigger additional scrutiny. In any event, such discrepancies are good material for cross-examination.
  4. Analyze laxity of standards used. Even if the lab followed its own standards, those standards may be very lax. Compare the lab’s procedures to industry standards, guidelines established by the Scientific Working Group on DNA Analysis Methods, and publications of the International Organization for Standardization.
  5. Re-run raw data. Hire an expert with proprietary software programs (i.e., GeneScan, GenoTyper, or Identifiler) to scrutinize the test results to see whether any of the parameters have been altered, contrary to the lab’s own protocol, to artificially achieve a desired result. This will also show whether additional tests were attempted and had different results.
  6. Identify artifacts. Identify all possible artifacts, i.e., spurious test results that are caused by the test itself rather than being a true indication of the DNA type, which is the subject of the test.
  7. Look for indications of contamination. Unexpected DNA results might occur if the sample has been contaminated. This may be occur, for example, with a degraded crime scene sample.
  8. Consider plausible alternative interpretations. Consider any and all plausible alternative interpretations of the data. Particularly when two or more persons’ DNA is mixed, it is arguably not a certainty which individual contributed a particular allele to the mixture.
  9. Review the pertinent scientific literature. The literature of forensic DNA testing is voluminous, and no one can read it all before a trial or a Kelly hearing. But, particularly with an expert’s help, you can narrow the scope of research to articles focused on the particular issues presented by a given case, such as mixture interpretation, contamination, or degraded samples.
  10. Kelly preparation and research. Study case law for any unresolved issues regarding technology. Even if the technology has been generally accepted, you may be able to show a that “the consensus” among experts in the relevant scientific community has changed.

For more information on how to introduce and challenge DNA evidence, check out Scientific Evidence and Expert Testimony in California.

© The Regents of the University of California, 2010. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited.

4 replies on “10 Tips for Attacking DNA Evidence”

Interesting point made here. One of the most important points is actually dna contamination. This can happen easily in many instances, from sample collections to lab work. If possible we should always ask for a second test in a different lab firm.

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