Whole-arm translocation: A type of chromosome rearrangement, also called a Robertsonian translocation, in which there is fusion of an entire long arm of one acrocentric chromosome with a similarly intact long arm of another acrocentric chromosome. The short arms of the chromosomes participating in the translocation are usually lost.
Acrocentric chromosomes have their centromere near but not at the very end of the chromosome. In humans, chromosomes 13-15, 21 and 22 are acrocentric.
These translocations are relatively common. In balanced form, they take the place of two acrocentric chromosomes and result in no problems. But in unbalanced form, whole-arm translocations account for some cases of the trisomy 13 (Patau) syndrome and trisomy 21 (Down) syndrome.
This type of chromosome rearrangement is named for W.R.B. Robertson who first described it (in grasshoppers) in 1916.