The statistical weight of mixed DNA samples with allelic drop out (Dr. Larry Mueller, University of California, Irvine)

Allelic Dropout: In some instances, an STR test will detect only one of the two alleles from a particular contributor at a particular locus. Generally this occurs when the quantity of DNA is relatively low, either because the sample is limited or because the DNA it contains is degraded, and hence the test is near its threshold of sensitivity. The potential for allelic dropout complicates the process of interpretation because analysts must decide whether a mismatch between two pro.les re¸ects a true genetic diff¨erence or simply the failure of the test to detect all of the alleles in one of the samples.

Figure 6 shows three loci from a case in which a defendant's profile was "matched" to the profile of a saliva sample from a woman's breast. The laboratory reported that the DNA profile of the saliva sample shown in Figure 6 was consistent with the defendant's profile, despite the absence of the defendant's 14 allele at locus D13S317 because the analyst assumed that the 14 allele had "dropped out." However, the occurrence of "allelic dropout" cannot be independently verified-the only evidence that this phenomenon occurred is the "inconsistency" that it purports to explain. Obviously, there is another possible interpretation that is more favorable for this defendant-i.e., that police arrested the wrong man.

Materials

Presentation

P Gill, CH Brenner, JS Buckleton, A Carracedo, M Krawczak, WR Mayr, N Morling, M Prinz, PM Schneider, BS Weir. DNA commission of the International Society of Forensic Genetics: Recommendations on the interpretation of mixtures. Forensic Science International. 2006;160:90-101.