Any truth is better than indefinite doubt.
(Sherlock Holmes, The Yellow Face)
For every complex problem, there is a simple solution... and it is wrong.
(Henry Louis Mencken, Mencken's Law)
The points that have been addressed in Chapters 19-21 thus far involve obtaining a 'clean' full DNA profile from a single source. However, crime scene evidence can produce mixed DNA profiles from more than one individual (see Chapter 7). These mixtures can be challenging to interpret and depending on the alleles present in the profile, the individual components may never be unambiguously separated. In addition, partial profiles where entire loci have dropped out sometimes occur due to the presence of degraded DNA or polymerase chain reaction (PCR) inhibitors (see Chapter 7).
As pointed out in Chapter 7, many forensic cases involve multiple pieces of DNA evidence and not all of these will be mixtures. Thus, if additional samples can be tested that are easier to interpret they should be sought after versus complicated mixtures. Fortunately, mixtures do not represent a majority of cases especially if a good differential extraction is performed in a sexual assault case and the sperm fraction can be fully separated from the victim's DNA (see Chapter 3). As an example, over a four-year period, one forensic laboratory worked 1547 criminal cases that involved a total of 2424 samples, yet only 163 showed a mixed profile or 6.7% (Torres et al. 2003).
Mixtures are DNA samples containing two or more contributors. As described in Chapter 7, the presence of a mixture may be ascertained when three or more alleles are observed at multiple loci or notable differences in allele intensities are detected in a short tandem repeat (STR) profile. Sometimes it is relatively straightforward to determine a contributor profile, such as when DNA analysis from an intimate swab reveals a mixture consistent with the victim and perpetrator.
As stated in earlier chapters, there is no one universal formula that fits all situations when interpreting DNA evidence. This reality certainly applies for mixture interpretation and reporting. Various laboratories have adopted different approaches to these challenging situations. Ladd et al. (2001) discuss four primary approaches to statistical evaluation of DNA mixtures: (1) issuing a qualitative statement involving no calculations and simply stating that the suspect is included or excluded from being a possible contributor to the mixture observed; (2) performing match probability estimations after deducing the possible genotypes of the contributors; (3) using exclusion probabilities; and (4) performing likelihood ratio calculations. These four approaches will be discussed below in the context of example data (Figure 22.1).
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