MOLECULAR ANALYSIS TO ASSIGN PARENTAL ORIGIN AND DISTINGUISH DE-NOVO I(21Q) FROM T(21Q21Q) IN 2 DOWN-SYNDROME FETUSES

OBJECTIVE: We sought to determine the origin of two prenatal cases of chromosome 21 rearrangements not amenable to clarification by conventional cytogenetic methodology. METHODS: Hypervariable repeat polymorphisms (chromosome 21) were used to determine the type of structural rearrangement and the parental origin of the rearranged chromosome. The repeats used were highly polymorphic and located very close to the centromere; thus, the likelihood of differences among the parental alleles and overall informativeness were increased. RESULTS: The rea(21q21q) chromosomes were identified as a Robertsonian translocation in one fetus and an isochromosome in the other. The extra chromosome material was found to be maternal in origin in both cases. CONCLUSION: The ability to clarify the origin of abnormal chromosomal rearrangements provides valuable information concerning possible mechanisms of aneuploidy, as well as clinical data that may have an impact in assessing a patient's risk for abnormal offspring.