Abstract
The clinical significance of marker chromosome depends on the origin of the marker, size and presence of euchromatin. We report the results of fluorescence “in situ” hybridization (FISH) studies on ten marker chromosomes. Six of these markers were derived from chromosome 15 and ranged in size from a small bi-satellite supernumerary marker to a large isodicentric inverted duplication including the euchromatic region 15q11-q13. This large marker was found in a 25-year-old severely mentally retarded male with oval face, seizures and abnormal speech and gait. Two of the medium size inv dup(15) were detected prenatally. These markers had a small euchromatic region labeled with the whole chromosome 15 probe and no signal for GABR3 and SNRPN of the 15q11-q13 region. Two chromosome 18 markers were analyzed and one of them was an isochromosome 18p labeled with 18p specific probe. The other two markers were derived from chromosome 13. One was a small centromeric supernumerary chromosome, the other was a small metacentric chromosome with two signals for the 13q32-q33 region and for the telomeric probe 13q34→qter and no signal for any alpha satellite centromere. Thus, this isodicentric 13q marker had a functional neo-centromere. The patient was a mosaic with one and two extra markers causing tetrasomy and polysomy 13q32→qter and clinical features of trisomy 13q3.
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