Calculating likelihood ratios for a mixed DNA profile when a contribution from a genetic relative of a suspect is proposed

Roberto Puch-Solis*, Sue Pope, Ian Evett

*Corresponding author for this work

Research output: Contribution to journalArticle

Abstract

This technical note describes a practical method for evaluating evidence in the case of a two person conditioned DNA mixture where the defence proposition is that the unknown contributor is genetically related to the suspect. A conditioned mixture is one where the presence of DNA from one of two individuals is accepted by both prosecution and defence. A typical example would be a vaginal swab in an alleged rape case, where the presence of the complainant's DNA would be expected and samples have been taken from the complainant and a suspect. Much has been written about the interpretation of such mixtures and the calculation of the conditional genotype probabilities that must be carried out. In general, such treatments assume that the unknown contributor, under the defence proposition, is unrelated to the known individuals. In this paper, we consider the case where the defence proposition is that the unknown contributor is genetically related to the suspect. We describe a method, incorporating a flow chart and reference tables that facilitate manual calculations of the likelihood ratio for several postulated genetic relationships.

Original languageEnglish
Pages (from-to)205-209
Number of pages5
JournalScience and Justice
Volume50
Issue number4
DOIs
Publication statusPublished - 1 Dec 2010

Keywords

  • Conditional genotype probability
  • DNA profiling
  • Likelihood ratio
  • Match probability
  • Mixtures
  • Relatives

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