Application of subpopulation theory to evaluation of DNA evidence

The strength of any evidence can be assessed using a likelihood ratio (from Bayes' point of view). This is the ratio of the probabilities that the evidence would have been obtained given that the suspect is guilty and innocent, respectively. This, in turn, depends upon the probability that a match will be produced if the suspect is innocent. An essential population genetics parameter is the 'coancestry coefficient', or θ, or F(ST), which is the correlation between two genes sampled from distinct individuals within a subpopulation. In this paper, θ coefficients for the southern Polish population were calculated for three loci of forensic interest: TH01, TPOX and CSF1PO. Three small southern Polish subpopulations of different ethnic origin were analysed. The results suggest that values of θ appropriate to forensic applications are quite small in the southern Polish population (they vary in the range of 0.002 to 0.013), and the value of θ=0.03 suggested by the National Research Council is too conservative for the defendant.