Opinion ID: 1160857
Heading Depth: 2
Heading Rank: 3

Heading: Interpreting Autorads Through Application of Match Criteria

Text: The second step of DNA analysis is to compare the DNA patterns produced by the processing step in order to determine whether the suspect's DNA pattern matches the DNA pattern of bodily material found at the crime scene. [¶] First, the patterns are visually evaluated ... to determine whether there is a likely match. Most exclusions will be obvious, since the patterns will be noticeably different. ( Barney, supra, 8 Cal.App.4th at p. 808, 10 Cal.Rptr.2d 731.) If an exclusion is obvious on any of the autorads, there is a conclusive nonmatch of the samples. Otherwise,  the bands in the patterns are subjected to computer-assisted analysis to determine the length of the represented DNA fragments as measured in base-pair units. The measurements are taken by comparing the bands for the sample fragments with the bands for the size-marker fragments of known base-pair lengths. ( Barney, supra, 8 Cal.App.4th at p. 808, 10 Cal. Rptr.2d 731.) Because of inherent limitations in the DNA processing system, it is not possible to obtain exact base-pair measurements of the sample DNA fragments. For that reason, forensic laboratories have developed DNA match criteria based on the variations they have experienced in repeated measurements of DNA from the same source. Those criteria determine the match windowor range of sizesconstructed around each band for purposes of declaring a match. For example, under the FBI's match criterion of plus or minus 2.5 percent, the window around a band that measures 1,000 base pairs is from 975 to 1,025 base pairs. If the window of either band, or a single band, on one sample fails to overlap the window of the corresponding band on another sample, there is an exclusion of any match between the samples. If the windows of both bands, or of the single bands, of each sample overlap, there is a match at the locus disclosed by that probe. [The OCSD crime laboratory arrived at its own match criterion by recording variations in its repeated measurements of the same thing. The OCSD's match criterion of 3.4 percent (plus or minus 1.7 percent) was narrower than the FBI's match criterion of 5 percent (plus or minus 2.5 percent).] Some conditions adverse to reliability of measurement may call for a determination that a match at that locus is inconclusive. [11] That determination, however, does not invalidate matches at the other loci. There can be a match at multiple loci only if (1) the match criteria are met for all the bands at those loci, and (2) there is no locus at which a match of any band was excluded.