Editing Robertsonian translocation (section)

==Consequences==

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 in balanced form results in no excess or deficit of genetic material and causes no health difficulties. In unbalanced forms, Robertsonian translocations cause chromosomal deletions or addition and result in syndromes of multiple malformations, including trisomy 13 ([[Patau syndrome]]) and trisomy 21 ([[Down syndrome]]). The most frequent forms of Robertsonian translocations are between chromosomes 13 and 14, 14 and 21, and 14 and 15.<ref name=":1" />

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-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 |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=978-1-118-52212-7|language=en}}</ref>