TETRASOMY 18P DE-NOVO - IDENTIFICATION BY FISH WITH CONVENTIONAL AND MICRODISSECTION PROBES AND ANALYSIS OF PARENTAL ORIGIN AND FORMATION BY SHORT SEQUENCE REPEAT TYPING

We report a de novo supernumerary isochromosome 18p in a child with te trasomy 18p, analyzed by a straightforward combination of cytogenetic and molecular cytogenetic methods. The diagnostic procedure consisted of standard banding techniques and fluorescence in situ hybridization (FISH) with centromere and library DNA probes for chromosome 18, and 1 8p-specific FISH probes prepared by chromosome microdissection and in vitro amplification. The maternal origin as well as the most probable cell stages of formation of the supernumerary isochromosome were deter mined by typing of short sequence repeats (SSRs). The pattern of allel ic distribution suggests a nondisjunction during meiosis followed by a centromeric misdivision in an early postzygotic mitosis as the most p robable mode of isochromosome 18p formation. The combination of the ap plied methods represents a powerful tool to investigate the nature and the origin of de novo marker chromosomes.