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DNA - A Prosecutor’s Practice Notebook Inventory

DNA Results (nDNA only)

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Thumbnail Photo of a DNA profile using a graphical software program.
National Institute of Justice (NIJ) (see reuse policy).

When the DNA profile in a known reference sample is the same as that found in a crime scene sample, the profiles are said to match, meaning that the donor of the known reference sample is a potential contributor of the DNA in the crime scene sample. When the DNA profiles are different, the known donor is excluded as a contributor to the evidence sample. When comparing DNA profiles, the results at all tested loci are considered.

DNA profiling is carried out typically at 13 specific genetic loci.  A 14th loci, (amelogenin) is also tested and identifies the gender of the sample contributor.  Profiling attempts can yield a complete profile, a partial profile, or no profile, depending on sample quantity and the extent of sample degradation. Moreover, every laboratory will have an established analytical threshold for accepting a result at a locus as conclusive. DNA profile data that fall beneath the threshold at any locus dictate that the result will be inconclusive and, while used for profile comparison(s), these data cannot be used when calculating the statistical weight that will be assigned in the case of a profile match.

Thumbnail Photo of a DNA profile match using a graphical software program.
National Institute of Justice (NIJ) (see reuse policy).

When the statistical weight of a profile match is calculated, only those loci that demonstrated conclusive profiling results will be incorporated into the calculation. That calculation will be carried out, regardless of how few loci yielded conclusive results. Again, it is important to remember that the resulting alleles developed at all loci are used in the profile comparison, even if some are ultimately not used for statistical calculations.

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