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 perfo rmed by amplification of the X-specific and Y-specific alphoid sequenc es by PCR (XY-PCR). From mixed blood samples with female DNA present i n large excess of male DNA, the Y-specific sequence still amplified ef ficiently. In the analysis of vaginal secretions in a case of sexual a ssault, XY-PCR was performed to test the efficiency of the selective l ysis procedure in order to investigate whether alleles found with othe r PCR systems were of male or female origin. Our studies with mixed bl ood samples revealed pronounced DNA competition in the HLA-DQ alpha an d 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 per son.