Properties and significance of a small marker chromosome in amniotic fluid cells.

We report the presence of a small supernumerary metacentric chromosome in an amniotic fluid cell culture. The extra chromosome was inherited from the mosaic mother, and was characterized by the presence of four satellite regions, two on each side of the centromere. The pregnancy proceeded to term, and our findings were confirmed in the clinically normal offspring. The problems posed by the presence of the marker chromosome, in terms of both cytogenetic diagnosis and genetic counseling, are discussed in the light of similar cases from the literature.