Centromeric association of chromosome 16- and 18-derived microchromosomes.

A supernumerary C-band-positive microchromosome was observed in 88% of lymphocyte metaphases from a healthy 24-year-old female. Traditional cytogenetic analyses failed to determine the microchromosome's origin and structure. However, hybridization experiments with a panel of chromosome-specific alpha-satellite probes demonstrated that this microchromosome was derived from chromosome 16 and consisted mainly of transcriptionally inactive alpha-satellite DNA. The microchromosome closely associated with the centromere of most chromosomes. An even more pronounced centromeric association pattern was observed in a further microchromosome that was found to contain chromosome 18-specific alpha-satellite DNA. The latter microchromosome was detected in a female newborn affected with fetal alcohol syndrome. The two microchromosomes described here did not appear to bear major phenotypic risks.