TETRASOMY 18P DE-NOVO - PARENTAL ORIGIN AND DIFFERENT MECHANISMS OF FORMATION

We have used eight PCR-based DNA polymorphisms to determine the parent al origin and mechanisms of formation in 9 patients with de novo nonmo saic tetrasomy 18p. The 9 patients, 4 girls and 5 boys, had clinical f eatures characteristic of i(18p) syndrome. The supernumerary marker ch romosome was identified by fluorescence in situ hybridization (FISH) a nalysis using centromeric probes and a flow-sorted 18p-specific librar y. The isochromosome was of maternal origin in all 9 cases. The format ion of tetrasomy 18p cannot be explained by a single model. In 6 cases , meiosis II nondisjunction, followed by subsequent postzygotic misdiv sion, and in I case postzygotic nondisjunction and postzygotic misdivi sion were the most likely mechanisms of formation. Alternative mechani sms are suggested in the remaining 2 cases.