Highly polymorphic short tandem repeat analyses clarify complex molecular test results

Judicious application of highly polymorphic short tandem repeat (STR) analy ses and modification of assay conditions readily distinguished nonparentage from true parentage, with occasional failure to transmit one parentally de rived allele. These categories were resolved with a reliability of >99.9%, the standard applied to most DNA evidence presented in a U.S. court of law. While completing a single prenatal diagnosis submitted at 19 weeks gestati on, the authors found that one polymorphic grandpaternal allele did not amp lify, a duplicate control propositus' DNA sample had been switched by an ou tside laboratory, and recombination occurred in maternal meiosis within the mutant gene region. In two parentage cases with one available parent, a Y- linked STR or an autosomal STR was modified when transmitted to the offspri ng. In contrast, some apparently inconsistent results between parental DNA and off-spring were resolved by purifying or diluting the original extracte d DNA samples. Thus, the source of each complex molecular test result was c haracterized unambiguously by testing a sufficient number of highly polymor phic STR loci and by purifying or diluting troublesome DNA samples to dimin ish polymerase chain reaction amplification artifact.