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Robertsonian translocations
are a common form of chromosomal
rearrangement that occurs in the
five acrocentric human
chromosome pairs, namely 13,
14, 15, 21, and 22. They are named
for the American insect geneticist
W. R. B. Robertson, who first
described a Robertsonian
translocation in
grasshoppers in
1916. They are also called
whole-arm translocations or
centric-fusion translocations.
They are a type of
chromosomal translocation.
Robertsonian translocation, a
different sort of chromosomal
mutation called centric fusion,
can cause Down Syndrome. It
produces three copies of the long
arm chromosome 21. This form of
Down Syndrome is called familial
Down Syndrome. A Robertsonian
translocation is a type of
nonreciprocal translocation in
which two nonhomologous
acrocentric chromosomes
(chromosomes with centromeres near
their ends) break at their
centromeres, following which the
long arms become attached to a
single centromere. The short arms
also join to form the reciprocal
product, which typically contains
nonessential genes and usually is
lost within a few cell divisions.
In humans, when a Robertsonian
translocation joins the long arm
of chromosome 21 with the long arm
of chromosome 14 (or 15), the
heterozygous carrier is
phenotypically normal because
there are two copies of all major
chromosome arms and hence two
copies of all essentail genes. 'Peter
J. Russel= Essential Genetics
2003'
One in 900 humans is born with
a Robertsonian translocation. The
most frequent forms of
Robertsonian translocations are
between chromosomes 13 and 14, 13
and 21, and 21 and 22, and occur
when the long arms of two
acrocentric chromosomes fuse at
the
centromere and the two short
arms are lost.
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).
A Robertsonian translocation
results when the long arms of two
acrocentric chromosomes fuse at
the centromere and the two short
arms are lost. In this case, the
long arms of chromosomes 13 and 14
have fused, but no genetic
material was lost - this person is
completely normal despite 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.
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 and
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.
Genetic counseling and
genetic testing is offered to
families that may be carriers of
chromosomal translocations.
See also
Reference
- 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.
