Consensus and pool profiles to assist in the analysis and interpretation of complex low template DNA mixtures

Abstract

Forensic analysis of low template (LT) DNA mixtures is particularly complicated when (1) LT components concur with high template components, (2) more than three contributors are present, or (3) contributors are related. In this study, we generated a set of such complex LT mixtures and examined two methods to assist in DNA profile analysis and interpretation: the "n/2" consensus method (Benschop et al. 2011) and the pool profile approach. N/2 consensus profiles include alleles that are reproducibly amplified in at least half of the replications. Pool profiles are generated by injecting a blend of independently amplified PCR products on a capillary electrophoresis instrument. Both approaches resulted in a similar increase in the percentage of detected alleles compared to individual profiles, and both rarely included drop-in alleles in case mixtures of pristine DNAs were used. Interestingly, the consensus and the pool profiles often showed differences for the actual alleles detected for the LT component(s). We estimated the number of contributors using different methods. Better approximations were obtained with data in the consensus and pool profiles compared to the data of the individual profiles. Consensus profiles contain allele calls only, while pool profiles consist of both allele calls and peak height information, which can be of use in (statistical) profile analysis. All advantages and limitations of the various types of profiles were assessed, and based on the results we infer that both consensus and pool profiles (or a combination thereof) are helpful in the interpretation of complex LT DNA mixtures.

Fig. 1

The various types of profiles used in this study when based on four independent amplifications per DNA extract, namely: individual profiles, real pool profile, virtual pool profile, consensus profile (n = 4 x = 2), and composite profile

Fig. 2

Example of the differences observed in the alleles included in the n = 4 x = 2 consensus and the pool profile (blend of the four amplifications). For this three-person mixture, two differences are obtained for locus FGA: in the consensus allele 21 of minor 2 is missing and in the pool profile allele 22 of minor 2 is absent

Fig. 3

Ratio of observed PH to expected PH for the 30-, 150-, and 300-pg contributors’ alleles in individual profiles and pool profiles (blend of four) obtained from two-person mixtures with ratios 1:1, 1:5, and 1:10. The perfect ratio of observed to expected PH is 1, which is indicated by a horizontal line