Differential accessibility is non-random and reproducible between individuals. A. The light gray and black shading represents the brighter hybridization to either the normal or abnormal homolog, respectively (hatched marks indicate the paternal homolog). Bars depicting higher percentages correspond to the more accessible, brighter homolog in a given cell. This was the abnormal paternal homolog for RGS7 (sample ID: GM10958), abnormal maternal for CACNA1B (GM01921), abnormal paternal for ADORA2B:IVS1, and PMP22:IVS3 (GM06326), and normal maternal homolog for ACR (GM10273). B. Non-random DA was confirmed using cells from individuals in which the parental origin of the specific chromosomal rearrangement was unknown. The light gray and black shading represents the brighter hybridization to either the normal or abnormal homolog, respectively. Bars depicting higher percentages correspond to the more accessible, brighter homolog in a given cell. RGS7 probe had greater probe target accessibility on the normal chromosome 1 (sample ID: L12-1980). CACNA1B had greater accessibility on chromosome 9 with heteromorphic variant (L13-72). ADORA2B:IVS1 and PMP22:IVS3 probes were brighter on the abnormal and normal chromosome 17s, respectively (L12-1980) while ACR showed greater accessibility to the normal chromosome 22 (L11-729). C. Quantification of probe signal fluorescence between homologs are shown by box plots of normalized integrated fluorescence intensity ratios. Single copy probes detecting DA (RGS7, CACNA1B, PMP22:IVS3, ADORA2B:IVS1, ACR) exhibited large differences in hybridization intensities between homologs. This is indicated by the broad inter-quartile range of normalized intensity ratios from 0.55-1 (median intensity ratio, 0.87). By contrast, normalized intensity ratios for single copy FISH probes (CCNB1, Corf66, PMP22: IVS4-Ex 5, ADORA2B:Promoter-Ex1 and 1p36.3 intregenic region) with equal accessibility ranged from 0.07-0.31 (median intensity ratio, 0.14). Intensity differences between homologs were quantified by GVF from 125 metaphase cells for each probe category.