Short communication| Volume 33, P24-32, March 2018

Estimation of the number of contributors of theoretical mixture profiles based on allele counting: Does increasing the number of loci increase success rate of estimates?

  • Gina M. Dembinski
    Department of Biology, Indiana University-Purdue University Indianapolis, 723 West Michigan Street, Indianapolis, IN, 46202, USA
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  • Carl Sobieralski
    Indiana State Police Laboratory, 550 West 16th Street, Suite C, Indianapolis, IN, 46202, USA
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  • Christine J. Picard
    Corresponding author at: Department of Biology, Indiana University-Purdue University Indianapolis, 723 West Michigan Street, Indianapolis, IN, 46202, USA.
    Department of Biology, Indiana University-Purdue University Indianapolis, 723 West Michigan Street, Indianapolis, IN, 46202, USA

    Forensic and Investigative Sciences, Indiana University Purdue University Indianapolis, 402 N. Blackford Street, Indianapolis, IN, 46202, USA
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Published:November 20, 2017DOI:


      • This study evaluated maximum allele count of mixtures with expanded U.S. core loci.
      • There was no significant improvement as compared to previous U.S. core loci panel.
      • Maximum allele count was accurate for two person male mixtures based on 3 Y-STRs.
      • The maximum allele count method is not reliable beyond three person mixtures.


      DNA mixtures are more frequently encountered in casework due to increased kit sensitivity, protocols with increased cycle number, and requests for low copy number DNA samples to be tested. Generally, the first step in mixture interpretation is determining the number of contributors, with the most common approach of maximum allele count. Although there are previous studies regarding the accuracy of this approach, none have evaluated the accuracy with the newly expanded U.S. core STR loci. In this work, 4,976,355 theoretical mixture combinations were generated with the PowerPlex® Fusion 6C system which includes 23 autosomal STR loci and three Y-STR loci. The number of contributors could be correctly assumed for 100% two-person and 99.99% three-person mixtures, whereas, four-, five-, and six-person mixtures were correctly assumed in 89.7%, 57.3%, and 7.8% of mixtures, respectively. Y-STR analysis showed the 3 Y-STR markers are only accurate for two-person male mixtures (96.7%). This work demonstrates that maximum allele count using the expanded U.S. core loci is not much improved from previous smaller panels, reiterating that this method is not as accurate beyond three contributors.


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