Conclusions On Interpreting Mixtures

Mixed sample stains are present in many forensic investigations and STR typing procedures have been demonstrated to be an effective means of differentiating components of a mixed sample. However, a case may contain multiple stains and not all of these will be mixtures. In fact, the proportions of a mixture can vary across the forensic stain itself. Thus, if additional samples can be tested that are easier to interpret they should be sought after versus complicated mixtures (Gill et al. 1998b). As recommended by Peter Gill of the Forensic Science Service, the best advice is 'Don't do mixture interpretation unless you have to'.

As an example of the number of mixture samples encountered in typical casework, Torres et al. (2003) reviewed all of the mixture STR profiles seen in their laboratory over a four-year time period. From 1547 criminal cases worked which involved a total of 2424 samples, only 163 showed a mixed profile or 6.7%.

Some forensic DNA laboratories may decide not to go through the trouble of fully deciphering the genotype possibilities and assigning them to the major and minor contributors. An easier approach is to simply include or exclude a suspect's DNA profile from the crime scene mixture profile. If all of the alleles from a suspect's DNA profile are represented in the crime scene mixture, then the suspect cannot be excluded as contributing to the crime scene stain. Likewise, the alleles in a victim's DNA profile could be subtracted out of the mixture profile to simplify the alleles that need to be present in the perpetrator's DNA profile. Approaches to attaching a statistical value to mixture results are presented in Chapter 22.

For many years, DNA extracted from the cell line K562 was supplied as a control sample from the Promega Corporation with their GenePrint® STR typing kits. However, as can be seen from the peak profiles below, some of the STR loci exhibit imbalanced heterozygous alleles and/or multiple peaks that would make the sample appear to come from more than one source of DNA. In this particular case, these extra peaks or peak imbalances are the result of an abnormal number of chromosomes present in the sample rather than a problem with the DNA typing system. K562 cells are derived from a female human subject with a diagnosis of chronic myelogeneous leukemia. Because mutant cells are present with chromosomes that possess somatic mutations that affect the number of repeats in various STR markers, the K562 cell line results do not possess the normal balance of chromosomal material seen in healthy individuals. Thus, balanced heterozygous allele peaks are not always seen. Shown below are six STR markers amplified from K562 genomic DNA that possess a significant variation in the balance of the STR allele peak heights.


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