
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 ...
where the entire long arms of two different
chromosomes
A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most importa ...
become fused to each other. 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 three main types: "reciprocal", "nonreciprocal" and "Robertsonian" transloc ...
in humans, affecting 1 out of every 1,000 babies born. It does not usually cause medical problems, though some people may produce
gametes
A gamete ( ) 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. The name gamete was introduced by the Ge ...
with an incorrect number of chromosomes, resulting in a risk of miscarriage. In rare cases this
translocation results in
Down syndrome and
Patau syndrome
Patau syndrome is a syndrome caused by a chromosomal abnormality, in which some or all of the cells of the body contain extra genetic material from chromosome 13. The extra genetic material disrupts normal development, causing multiple and co ...
.
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 ape
The Hominidae (), whose members are known as the great apes or hominids (), are a taxonomic family of primates that includes eight extant species in four genera: '' Pongo'' (the Bornean, Sumatran and Tapanuli orangutan); '' Gorilla'' (the ...
s, is the reason humans have 46 chromosomes while all other primates have 48. Detailed DNA studies of
chimpanzee
The chimpanzee (; ''Pan troglodytes''), also simply known as the chimp, is a species of Hominidae, great ape native to the forests and savannahs of tropical Africa. It has four confirmed subspecies and a fifth proposed one. When its close rel ...
,
orangutan
Orangutans are great apes native to the rainforests of Indonesia and Malaysia. They are now found only in parts of Borneo and Sumatra, but during the Pleistocene they ranged throughout Southeast Asia and South China. Classified in the genus ...
,
gorilla
Gorillas are primarily herbivorous, terrestrial great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or five su ...
and
bonobo
The bonobo (; ''Pan paniscus''), also historically called the pygmy chimpanzee (less often the dwarf chimpanzee or gracile chimpanzee), is an endangered great ape and one of the two species making up the genus ''Pan (genus), Pan'' (the other bei ...
ape
Apes (collectively Hominoidea ) are a superfamily of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and Europe in prehistory, and counting humans are found global ...
s has determined that where
human
Humans (''Homo sapiens'') or modern humans are the most common and widespread species of primate, and the last surviving species of the genus ''Homo''. They are Hominidae, great apes characterized by their Prehistory of nakedness and clothing ...
chromosome 2
Chromosome 2 is one of the twenty-three pairs of chromosomes in humans. People normally have two copies of this chromosome. Chromosome 2 is the second-largest human chromosome, spanning more than 242 million base pairs and representing almost ei ...
is present in our DNA in all four great apes this is split into two separate chromosomes typically numbered 2a and 2b.
Similarly, the fact that horses have 64 chromosomes and donkeys 62, and that they can still have common, albeit usually infertile, offspring, may be due to a Robertsonian evolutionary fusion at some point in the descent of today's donkeys from their common ancestor.
Mechanism

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
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 fiber ...
. 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
acrocentric
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 ...
chromosomes. Humans have five of these acrocentric chromosomes:
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 fiber ...
s, the two resulting long arms may fuse. The result is a single, large chromosome with a
metacentric 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 (biology), 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 eukar ...
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.
Mos ...
carrier is
phenotypically 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
Down syndrome.
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
Patau syndrome is a syndrome caused by a chromosomal abnormality, in which some or all of the cells of the body contain extra genetic material from chromosome 13. The extra genetic material disrupts normal development, causing multiple and co ...
) 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.
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 ribosomal ...
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 ( ) is a Ploidy#Haploid and monoploid, haploid cell that fuses with another haploid cell during fertilization in organisms that Sexual reproduction, reproduce sexually. Gametes are an organism's reproductive cells, also referred to as s ...
), 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. ...
and
genetic testing
Genetic testing, also known as DNA testing, is used to identify changes in DNA sequence or chromosome structure. Genetic testing can also include measuring the results of genetic changes, such as RNA analysis as an output of gene expression, or ...
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 (''Equus ferus przewalskii'' or ''Equus przewalskii''), also called the takhi, Mongolian wild horse or Dzungarian horse, is a rare and endangered wild horse originally native to the steppes of Central Asia. It is named after t ...
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 mi ...
s and the
tarpan
The tarpan (''Equus ferus ferus'') was a free-ranging horse population of the Eurasian steppe from the 18th to the 20th century. What qualifies as a tarpan is subject to debate; it is unclear whether tarpans were genuine wild horses, feral domest ...
have 64
chromosome
A chromosome is a package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
s and
donkey
The donkey or ass is a domesticated equine. It derives from the African wild ass, ''Equus africanus'', and may be classified either as a subspecies thereof, ''Equus africanus asinus'', or as a separate species, ''Equus asinus''. It was domes ...
s have 62; it is thought that the difference is due to a Robertsonian translocation.
Nomenclature

The
International System for Human Cytogenomic Nomenclature (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 theoretical field studying nomenclature is sometimes referred to as ''onymology'' or ''taxonymy'' ). The principl ...
, 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
Down syndrome.
Name
Robertsonian translocations are named after the American
zoologist
Zoology ( , ) is the scientific study of animals. Its studies include the structure, embryology, classification, habits, and distribution of all animals, both living and extinct, and how they interact with their ecosystems. Zoology is one ...
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 an ...
William Rees Brebner Robertson (1881–1941) who first described a Robertsonian translocation in
grasshopper
Grasshoppers are a group of insects belonging to the suborder Caelifera. They are amongst what are possibly the most ancient living groups of chewing herbivorous insects, dating back to the early Triassic around 250 million years ago.
Grassh ...
s 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)