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STR Data Analysis and Interpretation for Forensic Analysts

Mixtures with Major/Minor Contributors

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The presence of major and minor contributor(s) is distinguishable when samples display alleles that have distinct contrast in signal intensities. All loci should be used for this evaluation, and laboratories should follow established procedures that are based upon validation studies.

The calculations of peak height/area percentages and percent contribution of donors can be used to support the declaration or a major and/or minor contributor.

In these instances, the DNA profile of the major source may be easily determined at unambiguous loci.

In general, determining the DNA profile of the major contributor is easier than determining that of the minor source.

An electropherogram demonstrating a mixture
An electropherogram demonstrating a mixture
National Institute of Justice (NIJ) (see reuse policy).

The set of peaks on the right (FGA locus) represent a clear major and minor contributor: there are two tall peaks and two short peaks.
The major contributor appears to be a 17, 18. It is unclear, however, whether the major contributor is or is not masking the minor contributor, who could be a 16, 16; a 16, 17; a 16, 18; or a 16, ---  (where --- means the sister allele dropped out due to stochastic effects.

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