Molecular and cytogenetic data for patient 00.16. A. Genotyping analyses using STS markers spanning the duplication region reveal evidence for an interchromosomal exchange leading to the formation of the der(15). The shift from biallelic to monoallelic maternal contribution could arise from a second exchange within the duplication or from a crossover on the normal maternal homologue. B. Graph of the Log2T/R ratios for probes across chromosome 15q11-q14. The positions of the BP are shown as vertical lines. The dotted box indicates control probes distal to BP5 and from other autosomes. The positions of the FISH probes are indicated. The dosage data are consistent with tetrasomy of the region between BP2-BP3. C, Metaphase chromosomes hybridized with centromere probe, pcm15, and 770c6 show signals on the normal homologues as well as two centromeric signals on both der(15)s (inset). D and E. Images of idic(15) chromosomes from a single metaphase spread hybridized with pDJ437h9. F and G. Images of idic(15) chromosomes from a single metaphase spread hybridized with pDJ69i9. H. Schematic of the duplication chromosomes for subject 00.16. The der(15) der(15)s are satellited on both p arms, the heteromorphic region encompassing the NF1 pseudogene region is shown in light blue. The region between BP1-BP2 is shown in blue and present on both ends of the chromosome with a single copy of the region between BP2-BP3 shown in yellow. The approximate position of a potential crossover within the der(15) is shown by the dashed line.