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We describe a man with pericentric inversion 9 and constitutive heterochromatin, and a high disomy rate in his sperm cells (with all probes analyzed). The disomy rate was estimated with the following probes: 8, 9, 18, X, and Y, and was significantly higher than that in control sperm cells, while chromosome 9 showed the highest disomy frequency. The probes of X and Y together showed the same disomy frequency as X and Y alone, which indicates the same nondisjunction rate in the first meiotic division. We suggest that the interchromosomal effect found in this man differed from other findings in sperm cells of men carrying an inversion in terms of the difference in the length of the heterochromatin between the two chromosomes 9. Also, it is well known that the effect of inversion 9 with increased heterochromatin is highly variable and may even vary in members of the same family.

Down syndrome (DS) is a multifactorial disorder with a high predisposition to leukemia and other malignancies. A change in the replication pattern from synchronous in normal genes to asynchronous in DS amniocytes has previously been reported. The objective of this study was to evaluate additional molecular cytogenetic factors which could re-emphasize the high correlation between DS cells and genetic instability. We found a higher rate of random aneuploidy in chromosomes 9 and 18 and a higher rate of asynchronous replication in the subtelomeric region or DS leukocytes than in cells from normal newborns. In addition, the telomere capture phenomenon was observed in the DS leukocytes but not in normal controls. The molecular cytogenetic factors observed in the DS individuals are known to correlate with genomic instability and with predisposition to cancer.

We assessed possible inter- and intraindividual variations in the frequency of disomy in sperm cells from three men with abnormal sperm analysis parameters. Mono- and dual-color fluorescence in situ hybridization (FISH) was applied to sperm cells from different samples of the men. Four men with a normal sperm profile were used as controls. The samples were taken separately over a period of 6 months. FISH probes used for the disomy rate analysis were clones from the satellite region of chromosomes 8, 18, X, and Y. The study group showed a significantly higher disomy rate compared with the control group, whereas there was no significant difference in the disomy rate between three different samples from the same individuals. These results suggest that the sampling time has no importance in assessing the rate of nondisjunction in sperm cells.