The origin and behavior of two isodicentric bisatellited chromosomes

Karyotyping revealed three cell lines in a boy with mental retardation and few other abnormalities. Thirty cells exhibited a normal karyotype, and 54 had an extra acrocentric chromosome of E group size with satellites on the long and short arms. The remaining 20 cells each had, in addition to the first marker (M1), a second tiny bisatellited chromosome (M2). C-banding demonstrated that both markers were dicentric. G-,C-, and Q-banding and satellite association data were consistent with the markers having originated from chromosome 15 material. We propose that M1 was formed from a meiotic breakage and a chromatid fusion in the proximal long arms of an acrocentric pair. This would have produced a symmetrical isodicentric chromosome, plus one or two acentric fragments. M2 then could have resulted from a dicentric bridge-break-synthesis-reunion phenomenon. This model of abnormal meiotic exchange can be generalized to encompass the formation of other dicentric isochromosome cases of isochromosome X.