The evaluation of mixed stains from different ethnic origins: general result and common cases.

In some situations, it can be inferred from the crime circumstances that the mixed stain donors are of different ethnic groups. The evaluation of DNA mixtures with contributors coming from more than one ethnic group is considered under the assumption of independence of alleles within and between ethnic groups. A general formula is derived for the assessment of the weight of evidence in mixed stain problems. This formula is equivalent to that of Fukshansky and Bär, but we give a different derivation. For the convenience of practitioners, the explicit expressions of the likelihood ratios for 14 common cases are presented. The effect of different ethnic groups to the assessment of the evidence is shown in the well-known Simpson case.