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Specificity

Home > Evidence Handling Procedures > Associative Evidence > Value of Associative Evidence > Specificity

Unique DNA sequence. The side-by-side
arrangementof bases along the DNA strand
(e.g., ATTCCGGA).

Image courtesy of the U.S. Department
of Energy Human Genome Program
http://www.ornl.gov/hgmis

The value of associative evidence in regard to specificity or identity can be illustrated with a comparison of blood versus glass evidence. 

Blood can be identified using DNA analysis.  The typical DNA testing procedure results in the identification of the chemical composition at each of thirteen different locations on a DNA molecule.  Every DNA molecule in the body of an individual will have the same composition at these sites.  A reported DNA profile is therefore a specific description of a person’s DNA.

In contrast, there are minute differences in the physical and trace chemical properties of a sheet of glass.  Testing fragments from the glass sheet will reflect the variations in these properties; these variations will be greater in fragments originating in different locations across the same sheet of glass. The differences are small, but they do result in some uncertainty as to the absolute identity of the material tested and therefore the true value of the associative evidence.

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