Cytogentic studies of a patient with mosaicism of isochromosome 13q and a dicentric (Y; 13) translocation showing differential centromeric activity

A case is presened in which both an isochromosome and a dicentric translocation with differential centromere activity are found in one individual. Three karyotypes are present: 46, XY, ‐13, isochromosome 13q is found in 23% of cells in blood and 5% in skin. The dicentric (Y;13), + i(13q)/45,X, ‐13, + psu dic(13)t(13;Y)/45,X, ‐13, + psu dic(Y)t(Y;13). The isochromosome in all of the remaining cells displays differential centromeric activity; the ratio of cells with the acitve 13 centromere to the acstive Y centromere is about 3.5:1. The formation of the isochromosome 13q was a de novo gametic event. The translocation producing the dicentric occurred after fertilization with the breakpoints at band Yq12 and the juxta‐centromeric region of the isochromosome 13. The finding of differential centromeric activity in this chromosome indicates that centromere inactivation is not always permanent in a dicentric translocation.