DNA commission of the International Society of Forensic Genetics: Recommendations on the evaluation of STR typing results that may include drop-out and/or drop-in using probabilistic methods

Abstract

DNA profiling of biological material from scenes of crimes is often complicated because the amount of DNA is limited and the quality of the DNA may be compromised. Furthermore, the sensitivity of STR typing kits has been continuously improved to detect low level DNA traces. This may lead to (1) partial DNA profiles and (2) detection of additional alleles. There are two key phenomena to consider: allelic or locus ‘drop-out’, i.e. ‘missing’ alleles at one or more genetic loci, while ‘drop-in’ may explain alleles in the DNA profile that are additional to the assumed main contributor(s). The drop-in phenomenon is restricted to 1 or 2 alleles per profile. If multiple alleles are observed at more than two loci then these are considered as alleles from an extra contributor and analysis can proceed as a mixture of two or more contributors. Here, we give recommendations on how to estimate probabilities considering drop-out, Pr(D), and drop-in, Pr(C). For reasons of clarity, we have deliberately restricted the current recommendations considering drop-out and/or drop-in at only one locus. Furthermore, we offer recommendations on how to use Pr(D) and Pr(C) with the likelihood ratio principles that are generally recommended by the International Society of Forensic Genetics (ISFG) as measure of the weight of the evidence in forensic genetics. Examples of calculations are included. An Excel spreadsheet is provided so that scientists and laboratories may explore the models and input their own data.

Development of an Italian RM Y-STR haplotype database: Results of the 2013 GEFI collaborative exercise

“…Generally speaking, the future application of these markers in forensic casework poses no different problems than those raised by the analysis of autosomal STR profiles in stains, namely the possibility of allele drop-out and/or drop-in, and the occurrence of mixtures [19,20]. However, such issues have not been fully explored yet in haploid markers [21].…”

Evaluation of mixed-source, low-template DNA profiles in forensic science

“…If Q ≡ AB, where "≡" denotes "has genotype," but the CSP shows only A and low epg peak heights suggest that dropout is possible, then the possibility that B has dropped out must be considered. Under a standard model (8,10), the LR can be written as…”

“…In previous formulations (5)(6)(7)(8)(9)(10), the likelihood at a locus in a profiling run is the product over all allelic positions in the epg of one of four possible terms, according to whether or not the corresponding allele is represented in the crime scene profile (CSP) and whether or not it is included in the profiles of any of the hypothesized contributors (Materials and Methods). I introduce here a fifth possibility corresponding to an absence of information about whether the allele is present, irrespective of whether or not it is included in the profile of a hypothesized contributor.…”

The future of forensic DNA analysis

“…genotyping using what has been termed 'semicontinuous' (where only allele information is used) or 'fully continuous' (where alleles and peak height information are used) [57][58][59][60][61][62][63].…”