Mixture Interpretation

Mixtures of DNA from two or more individuals are common in some forensic cases and must be dealt with in the interpretation of the DNA profiles. In evaluating the evidence, an analyst must decide whether the source of the DNA in the questioned sample is from a single individual or more than one person. This may be accomplished by examination of the number of alleles detected at each locus as well as peak height ratios and/or band intensities on a gel (see Chapters 7 and 22). Occasionally extra peaks occur in the data that should not be confused with true alleles.


Electropherograms may contain extra peaks besides the primary target alleles of interest. These peaks can arise from a number of sources related to the biology of STRs and the technology of detecting fluorescently labeled amplification products. It is important to recognize these peaks and not make a false exclusion because of the presence of supposedly spurious peaks in one of the samples.

A laboratory needs to establish criteria to identify a true allele because a DNA typing analyst must decide which peaks contribute to a donor(s) profile(s) and which are due to an artifact. The following material is intended as a helpful guide to some of the commonly seen artifacts and should not be considered a comprehensive list for troubleshooting purposes.

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