 The mar del (10) and bisatellited chromosomes were present in every cell examined, but the ring chromosome was only present in 4–8% of the cells.Some have attributed this statistic to the possible mitotic instability of ring chromosomes.Centromeres are conventionally defined by dark-staining heterochromatin which consists of the primary constriction site—this convention exists because heterochromatin is usually found flanking the centromere.