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Sensitivity of inferences in forensic genetics to assumptions about founding genes

Bayesian networks, with inferences computed by probability propagation methods, offer an appealing practical modelling framework for structured systems involving discrete variables in numerous domains, including forensic genetics. However, when standard assumptions are violated -- for example when allele frequencies are unknown, there is identity by descent or the population is heterogeneous, dependence is generated among founding genes, that makes exact calculation of conditional probabilities by propagation methods less straightforward. Here we illustrate different methodologies for dealing with these problems by assessing sensitivity to assumptions about founders in forensic genetics problems. These methods include constrained steepest descent, linear fractional programming and representing dependence by structure. We illustrate these methodologies on several real casework forensic genetics examples comprising criminal identification, simple and complex disputed paternity and DNA mixtures.

Selected Grappa codes and Hugin nets that can be used to replicate results in the paper are available at the links below; others may be added to this page in due course. Please use the email link below if you wish to contact the authors.

• Full paper: Annals of Applied Statistics, 3, 731-763 (2009).
  doi: 10.1214/09-AOAS235. ArXiv: http://arxiv.org/abs/0908.2862
• Presentation given at RSS2009, Edinburgh.
• Grappa code:
  • Criminal Identity:
    Data in Table 1, and Baseline, UAF and IBD results in Table 5
  • Simple paternity:
    Data in Table 3, and Baseline, UAF and IBD results in Table 9
  • Disputed sibship:
    Data in Table 4, and Baseline, UAF and IBD results in Table 11
• Hugin OOBNs: crime_IBD | crime_UAF | crime_HET

Software systems

• Grappa
• Hugin