Robertsonian translocation (ROB) is a rare form of chromosomal rearrangement where the participating chromosomes break at their centromeres and the long arms fuse to form a single, large chromosome with a single centromere. In humans, Robertsonian translocations generally occurs in the five acrocentric chromosome pairs, namely 13, 14, 15, 21 and 22. Other Robertsonian translocations can occur, but do not lead to a viable fetus. They are named after the American biologist William Rees Brebner Robertson Ph.D. (1881–1941), who first described a Robertsonian translocation in grasshoppers in 1916
A Robertsonian translocation is a type of translocation involving two homologous (paired) or non-homologous chromosomes (i.e. two different chromosomes, not belonging to a homologous pair). A feature of chromosomes that are commonly found to undergo such translocations is that they possess an acrocentric centromere, partitioning the chromosome into a large arm containing the vast majority of its genes, and a short arm with a much smaller proportion of genetic content.
The short arms also join to form a smaller reciprocal product, which typically contains only nonessential genes also present elsewhere in the genome, and is usually lost within a few cell divisions. This type of translocation is cytologically visible, and can reduce chromosome number (from 23 to 22 pairs, in humans) if the smaller chromosome that results from a translocation is lost in the process of future cellular divisions. However, the smaller chromosome lost may carry so few genes (which are, in any case, also present elsewhere in the genome) that it can be lost without an ill effect to the individual