Robertsonian translocation: A common and significant type of chromosome rearrangement that is formed by fusion of the whole long arms of two acrocentric chromosomes (chromosomes with the centromere near the very end). One in about 900 babies is born with a Robertsonian translocation making it the most common kind of chromosome rearrangement known in people. All five of the acrocentric chromosomes in people -- chromosome numbers 13, 14, 15, 21 and 22 -- have been found to engage in Robertsonian translocations. However, the formation of Robertsonian translocations was discovered by Hecht and coworkers to be highly nonrandom. Far and away the most frequent forms of Robertsonian translocations are between chromosomes 13 and 14, between 13 and 21, and between 21 and 22.
In balanced form, a Robertsonian translocation takes the place of two acrocentric chromosomes and results in no problems for the person carrying it. But in unbalanced form, Robertsonian translocations produce chromosome imbalance and cause syndrome of multiple malformations and mental retardation. Robertsonian translocations between chromosomes 13 and 14 (when transmitted in unbalanced for may lead to Trisomy 13) lead to the trisomy 13 (Patau) syndrome. And the Robertsonian translocations between 14 and 21 and between 21 and 22 (may result in Trisomy 21) in Down syndrome.
Robertsonian translocations are named for the America insect geneticist W.R.B. Robertson who first described this form of translocation (in grasshoppers) in 1916 and are also known as whole-arm or centric-fusion translocations or rearrangements.