Abstract
In some crime cases, the male part of the DNA in a stain can only be analysed using
Y chromosomal markers, e.g. Y-STRs. This may be the case in e.g. rape cases, where
the male components can only be detected as Y-STR profiles, because the fraction of
male DNA is much smaller than that of female DNA, which can mask the male results
when autosomal STRs are investigated. Sometimes, mixtures of Y-STRs are observed,
e.g. in rape cases with multiple offenders. In such cases, Y-STR mixture analysis
is required, e.g. by mixture deconvolution, to deduce the most likely DNA profiles
from the contributors.
We demonstrate how the discrete Laplace method can be used to separate a two person
Y-STR mixture, where the Y-STR profiles of the true contributors are not present in
the reference dataset, which is often the case for Y-STR profiles in real case work.
We also briefly discuss how to calculate the weight of the evidence using the likelihood
ratio principle when a suspect's Y-STR profile fits into a two person mixture. We
used three datasets with between 7 and 21 Y-STR loci: Denmark (n = 181), Somalia (n = 201) and Germany (n = 3443). The Danish dataset with 21 loci was truncated to 15 and 10 loci to examine
the effect of the number of loci. For each of these datasets, an out of sample simulation
study was performed: A total of 550 mixtures were composed by randomly sampling two
haplotypes, h1 and h2, from the dataset.
We then used the discrete Laplace method on the remaining data (excluding h1 and h2) to rank the contributor pairs by the product of the contributors’ estimated haplotype
frequencies. Successful separation of mixtures (defined by the observation that the
true contributor pair was among the 10 most likely contributor pairs) was found in
42–52% of the cases for 21 loci, 69–75% for 15 loci and 92–99% for 10 loci or less
depending on the dataset and how the discrete Laplace model was chosen. Y-STR mixtures
with many loci are difficult to separate, but even haplotypes with 21 Y-STR loci can
be separated.
Keywords
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Article info
Publication history
Published online: September 25, 2014
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© 2014 Elsevier Ireland Ltd. Published by Elsevier Inc. All rights reserved.
