Editing Chromosomal translocation (section)

==Robertsonian translocations==
{{main|Robertsonian translocation}}
[[Robertsonian translocation]] is a type of translocation caused by breaks at or near the centromeres of two [[centromere#Acrocentric|acrocentric]] chromosomes. The reciprocal exchange of parts gives rise to one large [[centromere#Metacentric|metacentric]] chromosome and one extremely small chromosome that may be lost from the organism with little effect because it contains few genes. The resulting [[karyotype]] in humans leaves only 45 chromosomes, since two chromosomes have fused together.<ref>{{cite book|last=Hartwell|first=Leland H.|title=Genetics: From Genes to Genomes|year=2011|publisher=McGraw-Hill|location=New York|isbn=978-0-07-352526-6|page=443}}</ref>   This has no direct effect on the phenotype, since the only genes on the short arms of acrocentrics are common to all of them and are present in variable copy number (nucleolar organiser genes).

Robertsonian translocations have been seen involving all combinations of acrocentric chromosomes. The most common translocation in humans involves chromosomes [[chromosome 13 (human)|13]] and [[chromosome 14 (human)|14]] and is seen in about 0.97 / 1000 newborns.<ref>{{cite journal | author = E. Anton |author2=J. Blanco |author3=J. Egozcue |author4=F. Vidal  | date = April 29, 2004 | title = Sperm FISH studies in seven male carriers of Robertsonian translocation t(13;14)(q10;q10) | journal = Human Reproduction | volume = 19 | issue = 6 | pages = 1345–1351 | issn = 1460-2350 | pmid = 15117905 | doi = 10.1093/humrep/deh232| doi-access = free }}</ref> Carriers of Robertsonian translocations are not associated with any phenotypic abnormalities, but there is a risk of unbalanced gametes that lead to miscarriages or abnormal offspring.  For example, carriers of Robertsonian translocations involving [[chromosome 21 (human)|chromosome 21]] have a higher risk of having a child with [[Down syndrome]]. This is known as a 'translocation Downs'. This is due to a mis-segregation ([[nondisjunction]]) during gametogenesis. The mother has a higher (10%) risk of transmission than the father (1%). Robertsonian translocations involving chromosome 14 also carry a slight risk of [[uniparental disomy]] 14 due to [[trisomy rescue]].