Unstable telocentric chromosome produced after centric misdivision of a 21q/21q translocated element.

An unstable telocentric chromosome was found in an individual with Down's syndrome and an unusual chromosomal mosaic, 46,XX, t(21q21q)+,21-/46,XX,21q-/45,XX,21-. As the 21q/21q chromosome was of paternal origin, based on the characteristics of its centromeric heterochromatin and on the characteristics of both 21 chromosomes of the father, it was concluded that its formation involved centric breakage and loss of centromeric material. The cell line with the 21q- chromosome may have originated from the translocation by an asymmetric misdivision of the reduced centromeric material. Of the two telocentrics produced by this fracture, one, possessing the smaller amount of centromeric apparatus, would be immediately lost; the other would be retained, but complete activity of its centromere would not be restored. It would therefore be unstable and might be lost.