AN UNUSUAL DICENTRIC Y-CHROMOSOME WITH A FUNCTIONAL CENTROMERE WITH NO DETECTABLE ALPHA-SATELLITE

We describe an unusual marker chromosome Y. This marker is present in 5% of the lymphocytes of a dysgenetic woman showing a mosaic karyotype 45,X/46,XY/47,XY+mar. Q-banding revealed that the marker was morpholo gically identical to the Y chromosome of the patient but presented the primary constriction in the heterochromatic region. C-banding confirm ed that the heterochromatic region was C-positive; furthermore, it sho wed two spots in the euchromatic region in a position corresponding to that of the centromere in the normal Y. Fluorescence in situ hybridiz ation with the centromere-specific probe pDP 97 and the pancentromeric alpha-satellite probe alpha 27 alpha 30 failed to detect any signal a t the primary constriction site. To improve the characterization of th e marker chromosome, hybridization was performed using pDP 105, a prob e located on the short arm of the Y chromosome, together with chromoso me-Y-specific paint-hybridizing to the single sequence spanning the Y short arm. In both cases, positive signals telomeric to the inactive c entromere were observed. Possible mechanisms resulting in the formatio n of the marker chromosome are discussed.