Combination of Autosomal and Y-STR Analysis, an Alternative to Differential Extraction: A Case Study

The samples from the sexual assault cases are often found as a imbalanced mixture of epithelial cells of victim and accused sperms, with an excess of victim's material resulting in an unfavorable ratio of male to female. As the male-DNA contribution is much lower than the female epithelial cell population in gynecological sample, analysis and interpretation of the individual contributions, has always been a challenge for the forensic scientists. Various techniques have been tried to separate the male DNA from the female part, including Differential Extraction (DE), but none of those are cost effective and time saving. The Y-STR haplo-typing, has been used for the mixed sample where the autosomal STR failed to produce visible peaks for the male DNA. In those cases combined autosomal and Y-STR analysis provided additional leads for the investigation. Here we report one case, where we conducted both autosomal-STR and Y-STR analysis, in combination, from the gynecological mixture sample, completely avoiding the complex ‘Differential Extraction'. The male to female contribution ratio in a gynecological sample, was found to be 1: 3.39 as deduced by the real-time qPCR using QuantiFilerTM Trio kit. As the mixed autosomal profile included both the suspect and the victim , using the qualitative and semi-quantitative binary model of interpretation, the Y-STR analysis confirmed the offender. This method is applicable when a preliminary suspect list is available to the investigative agency to compare with. Since this method is simple and is not influenced either by the azoospermic and vasectomized male samples or mixed samples other than semen viz. male DNA under fingernails of the victim, male touch DNA on the skin of the victim, and the clothing or belongings of a female victim, we propose this combination method may be highly effective in resolving a broad spectrum of cases in the Forensic laboratory.