Abstract
One of the main objectives of the Spanish and Portuguese-Speaking Group of the International
Society for Forensic Genetics (GHEP-ISFG) is to promote and contribute to the development
and dissemination of scientific knowledge in the area of forensic genetics. Due to
this fact, GHEP-ISFG holds different working commissions that are set up to develop
activities in scientific aspects of general interest. One of them, the Mixture Commission
of GHEP-ISFG, has organized annually, since 2009, a collaborative exercise on analysis
and interpretation of autosomal short tandem repeat (STR) mixture profiles. Until
now, three exercises have been organized (GHEP-MIX01, GHEP-MIX02 and GHEP-MIX03),
with 32, 24 and 17 participant laboratories respectively. The exercise aims to give
a general vision by addressing, through the proposal of mock cases, aspects related
to the edition of mixture profiles and the statistical treatment.
The main conclusions obtained from these exercises may be summarized as follows. Firstly,
the data show an increased tendency of the laboratories toward validation of DNA mixture
profiles analysis following international recommendations (ISO/IEC 17025:2005). Secondly,
the majority of discrepancies are mainly encountered in stutters positions (53.4%,
96.0% and 74.9%, respectively for the three editions). On the other hand, the results
submitted reveal the importance of performing duplicate analysis by using different
kits in order to reduce errors as much as possible. Regarding the statistical aspect
(GHEP-MIX02 and 03), all participants employed the likelihood ratio (LR) parameter
to evaluate the statistical compatibility and the formulas employed were quite similar.
When the hypotheses to evaluate the LR value were locked by the coordinators (GHEP-MIX02)
the results revealed a minor number of discrepancies that were mainly due to clerical
reasons. However, the GHEP-MIX03 exercise allowed the participants to freely come
up with their own hypotheses to calculate the LR value. In this situation the laboratories
reported several options to explain the mock cases proposed and therefore significant
differences between the final LR values were obtained. Complete information concerning
the background of the criminal case is a critical aspect in order to select the adequate
hypotheses to calculate the LR value. Although this should be a task for the judicial
court to decide, it is important for the expert to account for the different possibilities
and scenarios, and also offer this expertise to the judge. In addition, continuing
education in the analysis and interpretation of mixture DNA profiles may also be a
priority for the vast majority of forensic laboratories.
Keywords
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Article info
Publication history
Published online: February 10, 2014
Accepted:
January 27,
2014
Received in revised form:
December 30,
2013
Received:
November 16,
2013
Identification
Copyright
© 2014 Elsevier Ireland Ltd. Published by Elsevier Inc. All rights reserved.
