The characterisation of artefacts found in DNA profiles

Abstract

DNA profiling using short tandem repeat loci is commonplace. The Institute of Environmental Science and Research Limited (ESR) provides forensic services in New Zealand. The DNA profiling system used by ESR is AmpF/STR® SGM PlusTM. Artefacts such as stutter are often observed on SGM PlusTM DNA profiles. Under certain circumstances these artefacts can hinder profile interpretation. This is especially true if the DNA in the sample is degraded or low copy, or if the sample contains different concentrations of DNA from more than one individual. In these situations it may not he possible to distinguish alleles from artefacts. Guidelines, based on data collected elsewhere, are in place to aid interpretation of a DNA profile that contains artefacts. However, it was not known if the prevalence and magnitude of the artefacts produced by the instruments used at ESR were similar to observations recorded in other studies. This research collected and analysed data relating to stutter, n-peak, pull-up and heterozygote imbalance from 990 DNA profiles. Results from the pull-up and n-peak studies suggest that the thresholds currently in place for identifying these artefacts are too high. Furthermore, an unusual stutter characteristic was identified at specific alleles of the D8 locus. However, the results of this study generally concurred with results achieved elsewhere indicating that most of the guidelines currently in place are indeed appropriate.