Dissociation as probable origin of mosaic 45,XY,t(15;21)/46,XY,i(21q).

A patient is described with some features of Down's syndrome and a 45,XY, t(15;21)(15qter leads to 15p13::21p11 leads to 21qter)/46,XY,i(21)(qter leads to cen leads to qter) karyotype. Two mechanisms are proposed for the origin of the mosaicism, one assuming the dissociation of a translocation (15;21) chromosome already present in the zygote, and the other involving a chromatid translocation in a 46,XY zygote. The possible independent origin of the two cell lines is also considered.