Match Or Not A Match That Is The Question

Generally, the process of comparing two or more samples is limited to one of three possible outcomes that are submitted in a case report:

1. Match - Peaks between the compared STR profiles have the same genotypes and no unexplainable differences exist between the samples. Statistical evaluation of the significance of the match is usually cited in the match report (see Chapter 21). Alternatives for presentation of a match range from statements of identity, to computations of the likelihood ratio for the hypothesis that the defendant is the source, to descriptions of random-match probabilities in various populations, to a simple qualitative report of a match with no statistics behind its significance (see NRC II, p. 192).

2. Exclusion (Non-match) - The genotype comparison shows profile differences that can only be explained by the two samples originating from different sources.

3. Inconclusive - The data does not support a conclusion as to whether the profiles match. This finding might be reported if two analysts remain in disagreement after review and discussion of the data and it is felt that insufficient information exists to support any conclusion.

If a match is observed between a suspect and crime-scene evidence, then three possibilities exist: (1) the suspect deposited the sample, (2) the suspect did not provide the sample but has the profile by chance, and (3) the suspect did not provide the sample and the matching result is a false positive due to laboratory error. The first explanation is the basis behind the use of DNA testing in the criminaljus-tice system. The second possibility depends on population genetic principles that are covered in Chapters 19-21 from which the probability of a random match is determined. The third explanation of why a match might occur concerns the possibility of laboratory mistakes. Chapter 16 discusses laboratory validation and proficiency tests that are in place to prevent or reduce the possibility of error in performing DNA testing. Generally speaking a great deal of effort goes into insuring reliable forensic DNA testing although laboratory errors have been reported.

In forensic DNA typing, if any one STR locus fails to match when comparing the genotypes between two or more samples, then the profiles between the questioned and reference sample will be declared a non-match, regardless of how many other loci match.

Paternity testing is an exception to this because of the possibility of mutational events (see Chapters 6 and 23). When analyzing and reporting the results of parentage cases, an allowance for one or even two possible mutations is often made. In other words, if 13 loci are used and the questioned parentage is included for all but one locus, the data from the non-inclusive allele will be attributed to a possible mutation.

In the end, interpretation of results in forensic casework is a matter of professional judgment and expertise. Interpretation of results within the context of a case is the responsibility of the case analyst with supervisors or technical leaders conducting a follow-up verification of the analyst's interpretation of the data as part of the technical review process. When coming to a final conclusion regarding a match or exclusion between two or more DNA profiles, laboratory interpretation guidelines should be adhered to by both the case analyst and the supervisor. However, as experience using various analytical procedures grows, interpretation guidelines may evolve and improve. These guidelines should always be based on the use of proper controls and validated methods as described in the next chapter.


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