Medical Genetics Three to four percent of all cases of trisomy 21 are due to Robertsonian Translocation. In this case two breaks occur in separate chromosomes, usuall the 14th and 21st or 21st and 22st chromosomes. There is rearrangement of the genetic material so that some of the 14th chromosome is replaced by extra 21st chromosomeMedical Genetics Three to four percent of all cases of trisomy 21 are due to Robertsonian Translocation. In this case, two breaks occur in separate chromosomes, usually the 14th and 21st or 21st and 22st chromosomes. There is rearrangement of the genetic material so that some of the 14th chromosome is replaced by extra 21st chromosome