Cytogenetic studies in seven individuals with an i(Xq) karyotype

Seven individuals with stigmata of the Turner syndrome, large Barr bodies and a late replicating, structurally abnormal X‐chromosome, have been reinvestigated with different banding techniques to elucidate the mechanisms leading to formation of isochromosomes. Dicentric chromosomes With separated C‐blocks were found in five of these individuals. The two remaining patients had a slightly larger and a nearly double sized block of centromeric heterochromatin. Using the centromeric model proposed by Lima‐de‐Faria in 1956, the size of these blocks can be related to different locations of break points within the centromere or in the short arm. The presence of a 45, X cell line in all cases is probably due to anaphase lag secondary to instability of an abnormal isochromosome centromere. The mechanisms behind centromere inactivation in dicentrics are still obscure and not likely to be disclosed before the ultrastructure of these abnormal centromeres is known in more detail, The necessity of a more specific centromere nomenclature is emphasized. Additional rare or unexpected findings were normal menstruation in one individual, bipartite Barr bodies in two cases and one pericentric inversion of chromosome 9.