It has long been speculated, from cytogenetic observation of primary constrictions, that when the two centromeres of a dicentric chromosome are close then they may both be active. That is, orientation of the respective kinetochores on each chromatid to opposite spindle poles is possible so that the dicentric can evade the “fusion-breakage-bridge” cycle of Barbara McClintock and segregate efficiently. Thus, for example, Robertsonian translocation chromosomes usually have two active centromeres. Conversely, it is suggested that widely spaced centromeres of a dicentric may misalign on the spindle, since the chromatin between them can twist; the dicentric is thus forced to somehow undergo inactivation of …