Robertsonian translocation and an extra microchromosome: independent origin identified by in situ hybridization

Robertsonian translocation may result in either a monocentric or a dicentric product involving the long arms of the participating chromosomes. In the former case a microchromosome involving the short arms of both acrocentrics would be expected to arise, but in practice is rarely observed. Where such small bisatellited markers are seen in association with Robertsonian translocations, they are assumed to represent such reciprocal products. We present a case, involving a t(14q21q) and a DA/DAPI positive microchromosome, where using in situ hybridization we show that this assumption is incorrect and that the microchromosome has arisen independently.