Replication and inactivation of a dicentric X formed by telomeric fusion.

A woman complaining of secondary amenorrhea and infertility was found to have, in addition to a normal X chromosome, a long compound chromosome that consisted of two X chromosomes attached by telomeric fusion of the short arms without any apparent loss of genetic material, 46,X,dic(X)(pter), hereafter referred to as t(X;X). The t(X;X) chromosome was late replicating; the inactivation pattern was analyzed. It formed Barr bodies that were larger than normal, of which the majority were bipartite. Analysis of the X-chromatin masses supports our hypotheses that there is a region next to the centromere on the short arm of the inactive X chromosome that remains active and that there is an inactivation center located on Xq, a short distance from the centromere, around which the X chromatin condenses.