Abstract
We discuss the interpretation of DNA profiles obtained from low template DNA samples.
The most important challenge to interpretation in this setting arises when either
or both of “drop-out” and “drop-in” create discordances between the crime scene DNA
profile and the DNA profile expected under the prosecution allegation. Stutter and
unbalanced peak heights are also problematic, in addition to the effects of masking
from the profile of a known contributor. We outline a framework for assessing such
evidence, based on likelihood ratios that involve drop-out and drop-in probabilities,
and apply it to two casework examples. Our framework extends previous work, including
new approaches to modelling homozygote drop-out and uncertainty in allele calls for
stutter, masking and near-threshold peaks. We show that some current approaches to
interpretation, such as ignoring a discrepant locus or reporting a “Random Man Not
Excluded” (RMNE) probability, can be systematically unfair to defendants, sometimes
extremely so. We also show that the LR can depend strongly on the assumed value for the drop-out probability, and there
is typically no approximation that is useful for all values. We illustrate that ignoring
the possibility of drop-in is usually unfair to defendants, and argue that under circumstances
in which the prosecution relies on drop-out, it may be unsatisfactory to ignore any
possibility of drop-in.
Keywords
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Article info
Publication history
Published online: May 04, 2009
Accepted:
March 26,
2009
Received in revised form:
February 27,
2009
Received:
October 20,
2008
Identification
Copyright
© 2009 Elsevier Ireland Ltd. Published by Elsevier Inc. All rights reserved.
