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 lead to the trisomy 13 (Patau) syndrome. And the Robertsonian translocations between 14 and 21 and between 21 and 22 can and do result in (trisomy 21 (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.
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(rob) translocation involving two acrocentric chromosomes (13, 14, 15, 21, and 22), which fuse at the centromere region and lose their heterochromatic short arms; since these short arms contain no essential genetic material, the rearrangement is considered to be a functionally balanced one. A carrier of a robertsonian translocation involving chromosomes 14 and 21 has a virtually complete chromosomal complement but only 45 chromosomes (including the translocation chromosome), is phenotypically normal, but risks producing offspring with trisomy 21 (translocation Down syndrome). See illustration at aberration.
Medical dictionary.
2011.