Origin of a small metacentric chromosome: familial and cytogenic evidence.

A family is described in which the mother has an 18p- chromosome, one normal 18, and a probable i(18p). One of the daughters of this woman inherited the 18p- chromosome, and her phenotype resembles that of other 18p- cases. The other daughter inherited the presumed i(18p) chromosome, and her phenotype resembles that of some cases with extra, small metacentric chromosomes. The clinical, chromosomal, and familial evidence suggest that these abnormal chromosomes originated in the occurrence of one transverse break of the centromere and subsequent misdivision of a chromosome 18 in an earlier generation of this family. According to this interpretation, the mother is trisomic for 18p, one daughter is monosomic and the other daughter is tetrasomic for this chromosomal region.