Possible hereditary Y-chromosome instability.

A 9 year old boy, born to a phenotypically normal, non-consanguineous couple was referred for clinical examination due to radio-ulnar synostosis. We made cytogenetic analyses to investigate the genotype-phenotype correlation. Chromosomal studies on the boy and his father revealed a very small Y chromosome in both cases, probably due to loss of the heterochromatic long arm segment. Repeated cytogenetic analysis of the boy was made, using QFQ, CBG, DA/DAPI and fluorescence in situ hybridization (FISH) methods with DYZ1 and DYZ3 probes. The results showed a likely mosaicism of cell lines with either a small Y or a normal Y found in the boy. The small Y appeared to be composed of double centromeric regions, without the heterochromatic segment of the long arm of the Y chromosome. The father refused re-analysis. These findings indicate an inherent instability of the Y chromosome resulting in a familial small Y. The radio-ulnar synostosis may be associated with excessive Y short arms.