Definition: A Robertsonian translocation is when part of a chromosome, or packet of genetic information, attaches to another chromosome. Robertsonian translocations occur between acrocentric chromosomes (chromosomes 13, 14, 15, 21, and 22). Each chromosome has one short arm and one long arm. Acrocentric chromosomes have one arm that is very small. Robertsonian translocations occur when the long arms of two acrocentric chromosomes join together and the short arms are lost.