Supernumerary chromosome inherited from a maternal balanced translocation leading to pure trisomy 9p.

We describe a child with a supernumerary chromosome defined as der(9)t(9;22) (q12;p11), resulting in trisomy 9p and trisomy 22p. The mother carried the balanced translocation. In G- and C-banding the derivative chromosome 9 appeared to be dicentric and to contain 22q material. Using in situ hybridization we defined the exact breakpoints of the translocation and ruled out the possibility of a centric fission in the mother's chromosomes.