Abstract:
Low levels of male DNA are commonly encountered in sexual assault cases and are usually present in high backgrounds of female DNA. Y STR analysis gives analysts the ability to detect those low levels of male DNA without interference from the large quantities of female DNA. The way that single source and mixed Y STR profiles behave at low levels is of great interest to the forensic community in order to aid in the understanding and interpretation of these commonly encountered profiles. The main objectives of this study were to firstly further investigate the behaviour of low level single source male samples below optimal amounts of DNA, such as 0.5ng to 0.005ng. Secondly the behaviour of the components in two contributor mixtures at a range of low level starting templates of total DNA from 0.5ng to 0.01ng at various mixture ratios was investigated. Single source male profiles were used to further investigate locus drop out, stutter and overstutter, allelic drop in and sample variability. Mixed profiles were used to investigate locus drop out, sample variability, mixtures appearing as single source, the argument of composite versus consensus profiling and the variability of mixture proportion across loci. It is hoped that the research presented will be used to aid in the interpretation of single source and two contributor mixed male profiles at low template levels. Interpretation of mixed profiles commonly begins by enumerating all possible genotype combinations that collectively explain the mixture. In order to do this the variability in mixture proportion across loci needs to be understood. It was found that average peak height has a strong influence over the consistency of the Promega® PowerPlex® Y profile. A heuristic rule has been suggested to aid in the interpretation of DNA profiles that consist of two male contributors. Aspects surrounding the behaviour of single source male profiles at low DNA template levels have also been identified and these can subsequently be used to aid in the interpretation of these types of profiles.