Familial supernumerary microchromosome mosaicism: phenotypic effects and an attempt at characterization.

A healthy normally intelligent man and his two daughters who had mild developmental delay and different malformations were determined to have an acrocentric supernumerary microchromosome (MC). The father and both daughters were noted to be mosaic for the MC, perhaps suggesting mitotic instability of the MC cell line with loss over time. The MC was negative staining on G-banding and showed absence of centromeric heterochromatin on C-banding. A centromere was identified with silver staining; no NORs were identified. The MC approximated the size of the short arm of the Y chromosome and showed cytologic behavior similar to that of a Y chromosome. H-Y antigen and Y-DNA gene probe studies failed to confirm a Yp origin.