Familial supernumerary non-satellited microchromosome.

A small, non-satellited and metacentric supernumerary chromosome was ascertained in an amniotic fluid cell culture. The supernumerary chromosome was present in four of seven family members karyotyped. Of these four carriers, two were phenotypically normal, while the other two, the fetus and its elder sister, suffered from methylmalonic acidemia. The supernumerary chromosome was apparently stable, being transmitted through three generations without causing mosaicism. It had a single C band, seven G bands and was silver staining negative, a hitherto undescribed situation.