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COMPARISON OF DNA SAMPLES

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  p280-281

2026-08-04

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COMPARISON OF DNA SAMPLES

The goal of all forensic analysis is to associate a piece of evidence with as few people or objects as possible and ideally for there to be only one possible source. For evidence in general, there is no scientific basis for a conclusion of individuality and the very concept may not have a true meaning when referring to comparing evidence to its possible source. DNA typing as practiced today is unique among all physical evidence in that valid, rigorous scientific testing has been done that validates the concepts and testing methodologies for human DNA typing. Like wise, valid scientifically determined statistical frequencies for the alleles at the loci that are used in DNA typing have also been calculated. Once the particular allele(s) are identified at each locus, genotype frequencies and the product rule are used to determine the overall frequency of the DNA profile. If the DNA types of the evidence and of a suspect (or victim or other person of interest), then the odds that this is a coincidence can be computed. The term commonly used to express this comparison is match. The DNA in the biological evidence found at the crime scene is said to have “matched” the DNA from the suspect. But the term “match” may not be understood by a jury in the same way it is by scientists. Scientists define match to mean that there are no significant or unexplainable differences between the known and unknown materials. Thus the term “match” is used to describe the relationship between two fingerprints or shoeprints. In DNA typing, not all of the evidence is examined. The methods used for DNA typing today look at only small, select parts of the DNA. It is possible that, if an exact “match” between two samples for all of the loci tested is achieved, then further examination of additional loci might reveal differences. For this reason, the term “match” should not be used to describe the relationship between two samples of DNA. Scientists instead use the term genetic concordance.

FIGURE 1: Mitochondrial DNA (mtDNA) is circular and contains 16,569 base pairs. The white region at the top of the figure contains the two hypervariable loci, HV1 and HV2. These are used to generate mtDNA profiles.