Dicentric human X chromosomes

Cytological findings in 5 individuals with structurally abnormal X chromosomes are presented. Of 4 patients with an i(Xq) chromosome, 2 had an apparently dicentric, and 2 an apparently monocentric isochromosome. The former type of chromosome had twice as much centric heterochromatin (2 distinct C‐bands) as the latter. In the apparently dicentric isochromosomes, it could not be determined whether one or both centromeres were functional. These findings confirm our previously presented hypothesis that transverse breakage of the X chromosome in the centromere region leading to isochromosome formation may occur at more than one site. A 60‐year‐old female, 154 cm in height, with primary amenorrhoea and no other signs of Turner's syndrome, had 46 chromosomes, including a normal X and a giant chromosome composed of two X chromosomes joined end to end by their short arms. By conventional staining this chromosome had 1 centromere, but C‐banding revealed centric heterochromatin both in the centromere and at the site where the centromere of the second component chromosome should have occurred. It is inferred that centric heterochromatin can occur elsewhere than at the centromere and is not necessarily associated with kinetochore function.