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DNA - A Prosecutor’s Practice Notebook Inventory

Investigating Cases Involving DNA

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Introduction to Investigating Cases Involving DNA

The ability to type various areas of human DNA in order to distinguish among individuals dates back approximately 20+ years. Prior to this ability, forensic analysts were able to perform blood typing, then advanced to new tests that typed blood or semen based on variances in the protein and enzyme types. The fact that a questioned sample contained characteristics identical to a known sample, while helpful, was not determinative when those characteristics were shared by large percentages of the population.  DNA typing increased the ability of analysts to narrow the group of possible contributors to smaller and smaller segments of the population. The goal, of course, has always been to reach the point where testing results become the equivalent of identity.

Forensic DNA typing is now conducted by typing areas of DNA known as Short Tandem Repeats (STRs). This method brought with it the ability to individualize DNA samples if enough STRs sites (known as "loci") are typed. Large numbers of these "STRs" exist throughout the human genome and are useful in forensic testing for a variety of reasons. In the late 1990s, as a result of a push to identify and standardize the number and identity of those loci to be used in typing, a panel of 13 STR loci was chosen. These became known as the "13 CODIS core loci," and they form the basis of our national DNA databasing system.

Since that time, almost all forensic labs in the United States and most foreign countries conduct DNA typing using kits that type at least the 13 CODIS core loci.

Deoxyribonucleic Acid (DNA). Prosecutors must be aware of the complexities surrounding DNA as it relates to all facets of the proceeding. Prosecutors should take a leadership role in training law enforcement and DNA analysts regarding admissibility of evidence, the relevant weight of the evidence, and the importance the chain of custody plays in the case. Prosecutors need to be familiar with the protocols and standard operating procedures of the police agencies in their jurisdiction and the protocols of crime labs involved in processing the DNA for each case. These procedures may determine the admissibility of the evidence and will certainly have an impact on the weight of the evidence. It is important for the prosecutor to be thoroughly familiar with all aspects of a DNA-related case, including those areas ripe for defense attacks.

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