Differential extraction of DNA types in semen mixtures from multiple male contributors

Abstract

The analysis of samples obtained from sexual assault cases usually involve the separation of mixtures comprising cells from male and female contributors in order to produce the single profile of suspect and victim. However, specific research on the separation of mixed-male samples without the interference from female cells is lacking. This study is necessary since certain cases of sexual assault involve male victim (sodomy cases) or multiple male suspects (gang-rape cases) and their effects to the pattern of separation should not be neglected. Differential extraction (DE) is the most commonly used method to separate the mixed sample as, theoretically, it is able to produce complete separation of mixtures from male and female individual. However, incomplete separation following DE is possible since the separation is affected by various factors. In this research, the pattern of sperm cells separation in male mixtures contributed by multiple numbers of sperm contributors with different proportion of major and minor contributors was studied. This research utilized semen specimens obtained from volunteering donors which were used to prepare four designed mixtures from multiple numbers of contributors in varying proportion of major and minor contributors in order to determine the effects of both factors to the pattern of cells separation by DE method. It was found that the sperm cells from mixtures contributed by as low as two individuals were successfully separated into both sperm cells (SC) and non-sperm cells (NSC) fraction. This verified that the male DNA profiles obtained from the NSC fraction may originate from the sperm cells and not the epithelial cells of male contributors. Mixture samples contributed by maximum of four males with various proportions of major and minor contributors revealed that the sperm cells from all contributors were separated into both SC and NSC fraction. However the ability to identify the major and minor contributors in both fractions were found to be dependent on different major and minor proportion of the original sample.