Additional Thoughts On Mixtures

Mixtures can be quite complicated to interpret, such as in the case of People of the State of California versus Orenthal James Simpson (Weir and Buckleton 1996). However, that particular case would probably have been simplified if STR testing had been available at the time since STRs have more possible alleles than the dot blot methods used to recover DNA evidence in the O.J. Simpson trial.

Evett and Weir (1998) note that the essence of mixture interpretation is to first identify the alleles in the crime scene evidence sample and alleles carried by the known contributor(s) to the sample, such as the victim. Then any alleles present in the evidence sample that are not provided by the known contribu-tor(s) must be carried by one or more unknown contributors, which may or may not include the suspect.

The DNA Advisory Board recommends that either or both CPE and LR calculations be performed whenever feasible in the event of a mixture (DAB 2000). However, there will be mixture results that due to low-copy number stochastic limits, DNA template degradation or PCR inhibition, no interpretation of the profile can be made. In the end, as pointed out in Chapter 15, the interpretation of results in forensic casework, whether arising from single-source samples or mixtures, is a matter of professional judgment and expertise.

Mixtures will be complicated by the fact that some loci will possess intensity differences that permit contributors to be deciphered while other loci may not be fully interpretable due to overlapping allele combinations (see Figure 7.5). With STRs and peak intensity differences, some loci may be interpretable so that contributors can be statistically treated as single sources, while other loci may be too complex to confidently attribute alleles to their sources. Thus, when performing mixture interpretation do everything possible to first eliminate artifacts such as stutter products from consideration and then interpret remaining alleles to determine how many contributors are present.

Other challenges with interpreting mixtures involve taking into account the ethnicity of contributors (Fung and Hu 2000, 2001, 2002a, 2002b) and assessment of DNA mixtures with the presence of relatives (Hu and Fung 2003a). A computer program has been written to help in evaluating forensic DNA mixtures involving contributors from different ethnic origins (Hu and Fung 2003b). Probabilistic expert systems are also in development for aid in DNA mixture resolution (Mortera et al. 2003).

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