Editing Robertsonian translocation (section)

== Mechanism ==
[[File:Centromere_Placement.svg|thumb|320px|Chromosome arms can have different length ratios. Robertsonian translocation occurs in [[Centromere#Acrocentric|'''acrocentric''']] chromosome pairs (number <code style="border:none;padding:0">II</code> in the image), where the short arms are fairly short but not very short.

'''A''': [[P arm|Short arm (p arm)]]<br />
'''B''': [[Centromere]]<br />
'''C''': [[Q arm|Long arm (q arm)]]<br />
'''D''': [[Sister chromatids]]
]]

All chromosomes in animals have a long arm (known as ''q'') and a short arm (known as ''p''), separated by a region called the [[centromere]]. Robertsonian translocations can only occur between chromosomes which have the centromere very close to one end. This means these chromosomes have a long arm which is particularly long, and a short arm which is particularly short. These are known as [[Centromere#Acrocentric|acrocentric]] chromosomes. Humans have five of these acrocentric chromosomes: [[Chromosome 13 (human)|13]], [[Chromosome 14 (human)|14]], [[Chromosome 15 (human)|15]], [[Chromosome 21 (human)|21]] and [[Chromosome 22 (human)|22]]. When these chromosomes break at their [[centromere]]s, the two resulting long arms may fuse. The result is a single, large chromosome with a [[Centromere#Metacentric|metacentric]] centromere. This form of rearrangement is a Robertsonian translocation.{{citation needed|date=May 2022}}

This type of translocation may involve [[Homologous chromosome|homologous]] (paired) or non-homologous chromosomes. Owing to the acrocentric nature of the chromosomes involved, the long arms of these chromosomes contain the majority of genetic material contained on the original chromosomes. 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 division]]s. This type of translocation is [[Cytology|cytologically]] visible, and can reduce chromosome number (in humans, from 23 to 22). However, the smaller chromosome carries so few essential genes that its loss is usually clinically insignificant.<ref name=":0">Robertson WRB. ''Chromosome studies. I. Taxonomic relationships shown in the chromosomes of ''Tettigidae'' and ''Acrididae''. V-shaped chromosomes and their significance in ''Acrididae, Locustidae ''and'' Gryllidae: chromosome and variation.'' J Morph 1916;27:179-331.''</ref><ref>{{Cite book|title = Genetics From Genes to Genomes, 4e|last1 = Hartwell|first1 = Leland|publisher = McGraw-Hill|year = 2011|isbn = 978-0-07-352526-6|location = New York|pages = 443, 454|last2 = Hood|first2 = Leroy|last3 = Goldberg|first3 = Michael|last4 = Reynolds|first4 = Ann|last5 = Silver|first5 = Lee}}</ref>