Highlights
- •We built a computational tool to infer the number of contributors to a DNA sample.
- •The tool (NOCIt) uses peak heights and accounts for dropout and stutter.
- •NOCIt was tested on 278 samples containing between 1 and 5 contributors.
- •NOCIt correctly identified the number of contributors in 83% of the samples.
- •Using the quantitative information in the signal enhances mixture interpretation.
Abstract
Repetitive sequences in the human genome called short tandem repeats (STRs) are used
in human identification for forensic purposes. Interpretation of DNA profiles generated
using STRs is often problematic because of uncertainty in the number of contributors
to the sample. Existing methods to identify the number of contributors work on the
number of peaks observed and/or allele frequencies. We have developed a computational
method called NOCIt that calculates the a posteriori probability (APP) on the number of contributors. NOCIt works on single source calibration data consisting of known genotypes to compute
the APP for an unknown sample. The method takes into account signal peak heights,
population allele frequencies, allele dropout and stutter—a commonly occurring PCR
artifact. We tested the performance of NOCIt using 278 experimental and 40 simulated DNA mixtures consisting of one to five contributors
with total DNA mass from 0.016 to 0.25 ng. NOCIt correctly identified the number of contributors in 83% of the experimental samples
and in 85% of the simulated mixtures, while the accuracy of the best pre-existing
method to determine the number of contributors was 72% for the experimental samples
and 73% for the simulated mixtures. Moreover, NOCIt calculated the APP for the true number of contributors to be at least 1% in 95% of
the experimental samples and in all the simulated mixtures.
Keywords
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Article info
Publication history
Published online: November 15, 2014
Accepted:
November 9,
2014
Received in revised form:
September 24,
2014
Received:
April 29,
2014
Identification
Copyright
© 2014 Elsevier Ireland Ltd. Published by Elsevier Inc. All rights reserved.
