Sex determination and DNA competition in the analysis of forensic mixed stains by PCR

Sex determination of pure and mixed blood samples and stains was performed by amplification of the X-specific and Y-specific alphoid sequences by PCR (XY-PCR). From mixed blood samples with female DNA present in large excess of male DNA, the Y-specific sequence still amplified efficiently. In the analysis of vaginal secretions in a case of sexual assault, XY-PCR was performed to test the efficiency of the selective lysis procedure in order to investigate whether alleles found with other PCR systems were of male or female origin. Our studies with mixed blood samples revealed pronounced DNA competition in the HLA-DQ alpha and D1S80 PCR systems: the alleles from a minor DNA component could not be detected in the presence of a large excess of DNA from a second person.