In some cases it may be possible to confidently pull apart the alleles from individual contributors to a mixture. In cases of sexual assault, the victim's DNA profile is typically compared to the mixed profile and this comparison may be helpful in identifying the STR alleles present in the perpetrator's DNA profile.
The DNA Advisory Board recommendations on statistics issued in February 2000 state that 'when intensity differences are sufficient to identify the major contributor in the mixed profile, it can be treated statistically as a single source sample' (DAB 2000; see Appendix V). In such a situation, after deciphering the individual components for the major and minor contributors, the statistical treatment of their profiles could be conducted as described in Chapter 21 for single-source samples.
Interpretation of genotypes present in a mixture is much more complicated when the contributions of the donors is approximately equal and thus a major contributor cannot be definitively determined or when true alleles for a contributor are masked by stutter products (see Chapter 6) or other alleles in the mixture (DAB 2000). It is not always possible to unambiguously determine all of the alleles present in a mixture especially with a partial profile from a degraded DNA sample. Likewise, it is not always possible to infer the complete genotypes of all contributors with a high degree of confidence because the mixture combination may be too complex to easily decipher. Perhaps in the future computer programs will exist that will be able to pull apart mixture components with a high degree of reliability (see Perlin and Szabady 2001, Wang et al. 2002). Until that time, conservative approaches such as the calculation of exclusion probabilities and likelihood ratios will likely be used.
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