Robertsonian translocation: Difference between revisions

'''Robertsonian translocation''' ('''ROB''') is a [[Chromosome abnormality|chromosomal abnormality]] where the entire long arms of two different [[chromosome|chromosomes]] become fused to each other. It is the most common form of [[chromosomal translocation]] in humans, affecting 1 out of every 1,000 babies born.<ref>E. Therman, B. Susman and C. Denniston. The nonrandom participation of human acrocentric chromosomes in Robertsonian translocations. ''Annals of Human Genetics'' 1989;53:49-65.</ref> It does not usually cause medical problems, howeverthough suchsome individualspeople are almost always infertile because they are unable tomay produce [[Gamete|gametes]] with thean correctincorrect number of chromosomes, resulting in a risk of miscarriage. In rare cases this [[Chromosomal translocation|translocation]] results in [[Down syndrome]] and [[Patau syndrome]].<ref name=":1">{{Cite web |url=https://www.rarechromo.org/media/information/Other/Robertsonian%20Translocations%20FTNW.pdf |title=Unique: Rare Chromosome Disorder Support Group |access-date=2019-02-17 |archive-date=2019-02-18 |archive-url=https://web.archive.org/web/20190218082031/https://www.rarechromo.org/media/information/Other/Robertsonian%20Translocations%20FTNW.pdf |url-status=dead }}</ref> Robertsonian translocations result in a reduction in the number of chromosomes. A Robertsonian evolutionary fusion, which may have occurred in the common ancestor of humans and other [[great apesape]]s, is the reason humans have 46 chromosomes while all other primates have 48. Detailed DNA studies of [[chimpanzee]], [[orangutan]], [[gorilla]] and [[bonobo]] [[ape]]s has determined that where [[human]] [[chromosome 2]] is present in our DNA in all four great apes this is split into two separate chromosomes typically numbered 2a and 2b.<ref name="pmid28333343">{{cite journal |last1=Chiatante |first1=Giorgia |last2=Giannuzzi |first2=Giuliana |last3=Calabrese |first3=Francesco Maria |last4=Eichler |first4=Evan E. |last5=Ventura |first5=Mario |title=Centromere Destiny in Dicentric Chromosomes: New Insights from the Evolution of Human Chromosome 2 Ancestral Centromeric Region |journal=[[Molecular Biology and Evolution]] |date=1 July 2017 |volume=34 |issue=7 |pages=1669–1681 |doi=10.1093/molbev/msx108 |pmid=28333343 |pmc=5722054}}</ref><ref name="pmid25984837">{{cite journal |last1=Pratas |first1=Diogo |last2=Silva |first2=Raquel M. |last3=Pinho |first3=Armando J. |last4=Ferreira |first4=Paulo J.S.G. |title=An alignment-free method to find and visualise rearrangements between pairs of DNA sequences |journal=[[Scientific Reports]] |date=18 May 2015 |volume=5 |issue=1 |pages=1203 |doi=10.1038/srep10203 |pmid=25984837 |pmc=4434998|bibcode=2015NatSR...510203P }}</ref> Similarly, the fact that horses have 64 chromosomes and donkeys 62, and that they can still have common, albeit usually infertile, offspring,<ref>More details under [[Mule]] and [[Hinny]].</ref> may be due to a Robertsonian evolutionary fusion at some point in the descent of today's donkeys from their common ancestor.<ref name="equus" />

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 = 9780073525266978-0-07-352526-6|location = New York|pages = 443, 454|last2 = Hood|first2 = Leroy|last3 = Goldberg|first3 = Michael|last4 = Reynolds|first4 = Ann|last5 = Silver|first5 = Lee}}</ref>

In humans, when a Robertsonian translocation joins the long arm of chromosome 21 with the long arm of chromosomes 14 or 15, the [[heterozygous]] carrier is [[phenotype|phenotypically]] normal because there are two copies of all major chromosome arms and hence two copies of all essential genes.<ref>Peter J. Russel; Essential Genetics 2003</ref> However, the progeny of this carrier may inherit an [[Chromosomal translocation|unbalanced]] [[trisomy]] 21, causing [[Down syndrome]].<ref>{{Cite journal |last=Plaiasu |first=Vasilica |date=September 2017 |title=Down Syndrome - Genetics and Cardiogenetics |journal=Maedica |volume=12 |issue=3 |pages=208–213 |issn=1841-9038 |pmc=5706761 |pmid=29218069}}</ref>

A Robertsonian translocation results when the long arms of two acrocentric chromosomes fuse at the centromere and the two short arms are lost. If, for example, the long arms of chromosomes 13 and 14 fuse, no significant genetic material is lost—and the person is completely normal in spite of the translocation. Common Robertsonian translocations are confined to the acrocentric chromosomes 13, 14, 15, 21 and 22, because the short arms of these chromosomes encode for [[rRNA]] which is present in multiple copies.<ref>{{cite journal | pmid= | volume= |title=Recombination between heterologous human acrocentric chromosomes | year=2022 |vauthors= Guarracino A, Buonaiuto S, Potapova T, Rhie A, Koren S, Rubinstein B, Fischer C, Gerton J, Phillippy A, Colonna V, Garrison E | journal=bioRxiv| doi=10.1101/2022.08.15.504037| s2cid= 251647679| hdl=2117/393184| hdl-access=free}}</ref>

Most people with Robertsonian translocations have only 45 chromosomes in each of their cells, yet all essential genetic material is present, and they appear normal. Their children, however, may either be normal, carry the fusion chromosome (depending which chromosome is represented in the [[gamete]]), or they may inherit a missing or extra long arm of an acrocentric chromosome (phenotype affected). [[Genetic counseling]] and [[genetic testing]] is offered to families that may be carriers of chromosomal translocations.<ref>{{Cite journal |last1=Sheets |first1=Kathryn B. |last2=Crissman |first2=Blythe G. |last3=Feist |first3=Cori D. |last4=Sell |first4=Susan L. |last5=Johnson |first5=Lisa R. |last6=Donahue |first6=Kelly C. |last7=Masser‐FryeMasser-Frye |first7=Diane |last8=Brookshire |first8=Gail S. |last9=Carre |first9=Amanda M. |last10=LaGrave |first10=Danielle |last11=Brasington |first11=Campbell K. |date=October 2011 |title=Practice Guidelines for Communicating a Prenatal or Postnatal Diagnosis of Down Syndrome: Recommendations of the National Society of Genetic Counselors |url=https://onlinelibrary.wiley.com/doi/10.1007/s10897-011-9375-8 |journal=Journal of Genetic Counseling |language=en |volume=20 |issue=5 |pages=432–441 |doi=10.1007/s10897-011-9375-8 |pmid=21618060 |s2cid=19308113 |issn=1059-7700|doi-access=free }}</ref>

Rarely, the same translocation may be present homozygously if heterozygous parents with the same Robertsonian translocation have children. The result may be viable offspring with 44 chromosomes.<ref>{{cite journal| pmid=6510025 | volume=38 | title=Homozygosity for a Robertsonian translocation (13q14q) in three offspring of heterozygous parents | year=1984 |vauthors=Martinez-Castro P, Ramos MC, Rey JA, Benitez J, Sanchez Cascos A | journal=Cytogenet Cell Genet | issue=4 | pages=310–2 | doi=10.1159/000132080}}</ref> Outside of humans, [[Przewalski's horse]] has 66 chromosomes, while both of domesticated [[horse]]s and the [[tarpan]] have 64 [[chromosome]]s and [[donkey]]s have 62; it is thought that the difference is due to a Robertsonian translocation.<ref name="equus">{{Cite book|url=https://books.google.com/books?id=RK8kRaiAvCYC&q=robertsonian+translocation&pg=PA2139|title=Equine Genomics|last=Chowdhary|first=Bhanu P.|date=2013-01-22|publisher=John Wiley & Sons|isbn=9781118522127978-1-118-52212-7|language=en}}</ref>