Robertsonian translocation (ROB) is a

chromosomal abnormality A chromosomal abnormality, chromosomal anomaly, chromosomal aberration, chromosomal mutation, or chromosomal disorder, is a missing, extra, or irregular portion of chromosomal DNA. These can occur in the form of numerical abnormalities, where ther ...

wherein a certain type of a

chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...

becomes attached to another. It is the most common form of

chromosomal translocation In genetics, chromosome translocation is a phenomenon that results in unusual rearrangement of chromosomes. This includes balanced and unbalanced translocation, with two main types: reciprocal-, and Robertsonian translocation. Reciprocal transl ...

in humans, affecting 1 out of every 1,000 babies born. It does not usually cause health difficulties, but can in some cases result in

genetic disorders A genetic disorder is a health problem caused by one or more abnormalities in the genome. It can be caused by a mutation in a single gene (monogenic) or multiple genes (polygenic) or by a chromosomal abnormality. Although polygenic disorde ...

such as

Down syndrome Down syndrome or Down's syndrome, also known as trisomy 21, is a genetic disorder caused by the presence of all or part of a third copy of chromosome 21. It is usually associated with physical growth delays, mild to moderate intellectual d ...

and Patau syndrome.Unique: Rare Chromosome Disorder Support Group
/ref> Robertsonian translocations result in a reduction in the number of chromosomes.

Mechanism

Humans have 5 autosomal

chromosomes A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins ar ...

with arms that are particularly discordant in length, known as acrocentric chromosomes. These are 13, 14, 15, 21 and 22. When these chromosomes break at their

centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers ...

s, the two resulting long arms may fuse. The result is a single, large chromosome with a

metacentric Metacentric may refer to: * Metacentric height, the distance between the center of gravity of a ship and its metacenter * Metacentric centromere, the position of a centromere on a chromatid {{disambiguation ...

centromere. This form of rearrangement is a Robertsonian translocation. This type of translocation may involve 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 Cell division is the process by which a parent cell divides into two daughter cells. Cell division usually occurs as part of a larger cell cycle in which the cell grows and replicates its chromosome(s) before dividing. In eukaryotes, there ...

s. This type of translocation is 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.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.''

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 Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. ...

carrier is

phenotypically In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological proper ...

normal because there are two copies of all major chromosome arms and hence two copies of all essential genes. However, the progeny of this carrier may inherit an unbalanced

trisomy A trisomy is a type of polysomy in which there are three instances of a particular chromosome, instead of the normal two. A trisomy is a type of aneuploidy (an abnormal number of chromosomes). Description and causes Most organisms that reprod ...

21, causing

. About one in a thousand newborns have a Robertsonian translocation. The most frequent forms of Robertsonian translocations are between chromosomes 13 and 14, 14 and 21, and 14 and 15.Unique: Rare Chromosome Disorder Support Group
/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 (

). 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 Ribosomal ribonucleic acid (rRNA) is a type of non-coding RNA which is the primary component of ribosomes, essential to all cells. rRNA is a ribozyme which carries out protein synthesis in ribosomes. Ribosomal RNA is transcribed from ribos ...

which is present in multiple copies. 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 A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce ...

), or they may inherit a missing or extra long arm of an acrocentric chromosome (phenotype affected).

Genetic counseling Genetic counseling is the process of investigating individuals and families affected by or at risk of genetic disorders to help them understand and adapt to the medical, psychological and familial implications of genetic contributions to disease; t ...

and genetic testing is offered to families that may be carriers of chromosomal translocations. 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. Outside of humans,

Przewalski's horse Przewalski's horse (, , (Пржевальский ), ) (''Equus ferus przewalskii'' or ''Equus przewalskii''), also called the takhi, Mongolian wild horse or Dzungarian horse, is a rare and endangered horse originally native to the steppes of ...

has 66 chromosomes, while both of domesticated

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 million yea ...

s and the

tarpan The term tarpan (''Equus ferus ferus'') refers to free-ranging horses of the Russian steppe from the 18th to the 20th century. It is generally unknown whether those horses represented genuine wild horses, feral domestic horses or hybrids. The las ...

have 64

s, it is thought that the difference is due to a Robertsonian translocation.

Nomenclature

Human karyotype with bands and sub-bands

The

International System for Human Cytogenomic Nomenclature The International System for Human Cytogenomic Nomenclature (previously International System for Human Cytogenetic Nomenclature), ISCN in short, is an international standard for human chromosome nomenclature, which includes band names, symbols and a ...

(ISCN) is an international standard for human chromosome

nomenclature Nomenclature (, ) is a system of names or terms, or the rules for forming these terms in a particular field of arts or sciences. The principles of naming vary from the relatively informal conventions of everyday speech to the internationally ag ...

, which includes band names, symbols and abbreviated terms used in the description of human chromosome and chromosome abnormalities. Abbreviations include ''rob'' for Robertsonian translocations. For example, rob(21;21)(q10;q10) causes

Name

Robertsonian translocations are named after the American zoologist and

cytogeneticist Cytogenetics is essentially a branch of genetics, but is also a part of cell biology/cytology (a subdivision of human anatomy), that is concerned with how the chromosomes relate to cell behaviour, particularly to their behaviour during mitosis a ...

William Rees Brebner Robertson (1881–1941) who first described a Robertsonian translocation in grasshoppers in 1916. They are also called ''whole-arm translocations'' or ''centric-fusion translocations''.

References

{{DEFAULTSORT:Robertsonian Translocation Cytogenetics Chromosomal abnormalities Chromosomal translocations de:Translokation (Genetik)#Robertson-Translokation (Zentrische Fusion)