Dizygotic twins with Down's syndrome.

father and G trisomy in the two mongol children, but revealed 5 cells with 47 chromosomes among 70 cells counted from the mother. In 4 of these cells the extra chromosome was from the G group, and in the remaining cell it was a C group chromosome. On these results maternal mosaicism with karyotype 46,XX/47, XX,G+ was diagnosed. In 1971 the couple elected to have a further child despite the risks involved. A male mongol was born in December 1971. His karyotype has been confirmed as 47,XY,G+.