Structural Chromosomal Aberrations and Their Impact on Genetic Information
undefinedDR. Neha Antal
Assistant Professor
Department of Zoology
Govt. P.G. College for Women, Gandhinagar, Jammu
Structural Chromosomal
Aberrations
undefined•
Chromosomal
Chromosomal
aberrations
aberrations
are
are
substantial
substantial
changes
changes
in
in
chromosome
chromosome
structure and
structure and
number
number
–
These
These
typically
typically
affect
affect
more
more
than
than
one
one
gene
gene
–
They
They
are
are
also
also
called
called
chromosomal
chromosomal
mutations
mutations
•
Chromosomal
Chromosomal
aberrations or mutation
aberrations or mutation
is of two
is of two
types
types
•
Structural chromosomal mutation:
Structural chromosomal mutation:
Changes
Changes
inn the gene number or gene arrangement
inn the gene number or gene arrangement
•
Numerical chromosomal mutation:
Numerical chromosomal mutation:
Change
Change
in the number of chromosome
in the number of chromosome
Alterations
Alterations
in
in
Chromosome
Chromosome
Structure
Structure
There
There
are
are
two
two
primary
primary
ways
ways
in
in
which
which
the
the
structure of
structure of
chromosomes
chromosomes
can
can
be
be
altered
altered
The
The
total
total
amount
amount
of
of
genetic
genetic
information
information
in
in
the
the
chromosome
chromosome
can
can
change
change
–
Decrease:
Decrease:
Deficiencies/Deletions
Deficiencies/Deletions
–
Increase:
Increase:
Duplications
Duplications
&
&
Insertions
Insertions
The
The
genetic
genetic
material
material
may
may
remain
remain
the
the
same,
same,
but
but
is
is
rearranged
rearranged
–
Inversions
Inversions
–
Translocations
Translocations
Alterations
Alterations
in
in
Chromosome
Chromosome
Structure
Structure
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DELETION
P
P
art
art
of
of
a
a
chromosome
chromosome
is
is
missing
missing
•
Deletions
Deletions
start
start
with
with
chromosom
chromosom
al
al
breaks
breaks
induced
induced
by:
by:
Heat
Heat
or
or
radiation
radiation
(especially ionizing).
(especially ionizing).
Viruses.
Viruses.
Chemicals.
Chemicals.
Transposable
Transposable
elements.
elements.
Errors
Errors
in
in
recombination.
recombination.
•
Deletions
Deletions
do
do
not
not
revert,
revert,
because the
because the
DNA
DNA
is
is
gone
gone
(degraded)
(degraded)
•
Types:
Types:
•
Terminal - involve one break
Terminal - involve one break
•
Intercalary- involve two breaks
Intercalary- involve two breaks
DELETION
The
The
effect
effect
of
of
a
a
deletion
deletion
depends
depends
on
on
what was
what was
deleted.
deleted.
A
A
deletion
deletion
in
in
one
one
allele
allele
of
of
a
a
homozygous
homozygous
wild- type
wild- type
organism
organism
may
may
give
give
a
a
normal
normal
phenotype
phenotype
While
While
the
the
same
same
deletion
deletion
in
in
the
the
wild-type
wild-type
allele
allele
of
of
a heterozygote
a heterozygote
would
would
produce
produce
a
a
mutant
mutant
phenotype.
phenotype.
Deletion
Deletion
of
of
the
the
centromere
centromere
results
results
in
in
an acentric
an acentric
chromosome
chromosome
that
that
is
is
lost,
lost,
usually
usually
with serious
with serious
or
or
lethal
lethal
consequences.
consequences.
Human
Human
disorders
disorders
caused
caused
by
by
large
large
chromosomal
chromosomal
deletions are
deletions are
generally
generally
seen
seen
in
in
heterozygotes,
heterozygotes,
since
since
homozygotes usually
homozygotes usually
die.
die.
The
The
number
number
of
of
gene
gene
copies
copies
is
is
important.
important.
Syndromes
Syndromes
result
result
from
from
the
the
loss
loss
of
of
several
several
to
to
many
many
genes.
genes.
Examples
Examples
of
of
human
human
disorders
disorders
caused
caused
by
by
large
large
chromosomal
chromosomal
deletions:
deletions:
Cri-du-chat
Cri-du-chat
(“cry
(“cry
of
of
the
the
cat”)
cat”)
syndrome,
syndrome,
resulting
resulting
from
from
deletion
deletion
of
of
part
part
of
of
the
the
short
short
arm
arm
of chromosome
of chromosome
5
5
Cat like cry,
Cat like cry,
malformation of larynx, moon face,
malformation of larynx, moon face,
micrognathia, microcephally, low set ears,
micrognathia, microcephally, low set ears,
mental
mental
retardation.
retardation.
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Tandem
Tandem
duplications
duplications
are
are
adjacent
adjacent
to
to
each
each
other.
other.
Reverse
Reverse
tandem
tandem
duplications
duplications
result
result
in
in
genes
genes
arranged
arranged
in
in
the
the
opposite
opposite
order
order
of
of
the
the
original.
original.
Tandem
Tandem
duplication
duplication
at
at
the
the
end
end
of
of
a
a
chromosome
chromosome
is
is
a
a
terminal
terminal
tandem
tandem
duplication
duplication
D
D
u
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t
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n
Displaced
Displaced
duplication
duplication
duplicated
region is not situated
adjacent to the
original
.
.
Transposed duplication
Transposed duplication
duplicated
duplicated
part arrange to a non homologous
part arrange to a non homologous
chromosome
chromosome
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An example is the
An example is the
Drosophila
Drosophila
eye shape
eye shape
allele,
allele,
Bar
Bar
, that reduces the number of eye
, that reduces the number of eye
facets, giving the eye a slit-like rather than oval
facets, giving the eye a slit-like rather than oval
appearance.
appearance.
Cytological examination of chromosomes
Cytological examination of chromosomes
showed that the
showed that the
Bar
Bar
allele results from
allele results from
duplication of a small segment (16A) of the X
duplication of a small segment (16A) of the X
chromosome.
chromosome.
In
In
prophase
prophase
I,
I,
the
the
duplication
duplication
loop
loop
can assume
can assume
different
different
configurations
configurations
that maximize
that maximize
the
the
pairing
pairing
of
of
related
related
regions
regions
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Due to change in the position of a gene or
Due to change in the position of a gene or
a gene
a gene
group in relation to other genes in a
group in relation to other genes in a
chromosome, phenotype of organism is
chromosome, phenotype of organism is
altered, this effect is called position effect.
altered, this effect is called position effect.
It
It
also represents a source of genetic
also represents a source of genetic
variation.
variation.
I
I
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n
v
v
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n
Rotation of a part of a chromosome or a set of
Rotation of a part of a chromosome or a set of
genes by 180º on its own axis.
genes by 180º on its own axis.
I
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v
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s
i
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n
•
The
The
meiotic
meiotic
consequence
consequence
depends
depends
on
on
whether
whether
the
the
inversion
inversion
occurs
occurs
in
in
a
a
homozygote
homozygote
or
or
a
a
heterozygote.
heterozygote.
•
A
A
homozygote
homozygote
will
will
have
have
normal
normal
meiosis.
meiosis.
•
The
The
effect
effect
in
in
a
a
heterozygote
heterozygote
depends
depends
on
on
whether
whether
crossing-over
crossing-over
occurs.
occurs.
•
If
If
there
there
is
is
no
no
crossing-over,
crossing-over,
no
no
meiotic
meiotic
problems
problems
occur.
occur.
•
If
If
crossing-over
crossing-over
occurs
occurs
in
in
the
the
inversion,
inversion,
unequal
unequal
crossover
crossover
may
may
produce
produce
serious
serious
genetic
genetic
consequences.
consequences.
I
I
n
n
v
v
e
e
r
r
s
s
i
i
o
o
n
n
•
Heterozygous
Heterozygous
Pericentric
Pericentric
Inversion
Inversion
:
:
One
One
normal
normal
gamete
gamete
One
One
gamete
gamete
with
with
inversion
inversion
One
One
gamete
gamete
with
with
a
a
duplication
duplication
and
and
deletion.
deletion.
One
One
gamete
gamete
with
with
reciprocal
reciprocal
duplication
duplication
and
and
deletion.
deletion.
•
Heterozygous
Heterozygous
Paracentric
Paracentric
Inversion:
Inversion:
One
One
normal
normal
gamete
gamete
One
One
gamete
gamete
with
with
inversion
inversion
Two
Two
deletion
deletion
products
products
Some
Some
material
material
lost.
lost.
C
r
o
s
s
i
n
g
o
v
e
r
i
n
P
e
r
i
c
e
n
t
r
i
c
I
n
v
e
r
s
i
o
n
C
r
o
s
s
i
n
g
o
v
e
r
i
n
P
a
r
a
c
e
n
t
r
i
c
I
n
v
e
r
s
i
o
n
•
In
In
an
an
inversion,
inversion,
the
the
total
total
amount
amount
of
of
genetic
genetic
information
information
stays the
stays the
same
same
Therefore,
Therefore,
the
the
great
great
majority
majority
of
of
inversions
inversions
have
have
no
no
phenotypic consequences
phenotypic consequences
•
In
In
rare
rare
cases,
cases,
inversions
inversions
can
can
alter
alter
the
the
phenotype
phenotype
of
of
an
an
individual
individual
Break
Break
point
point
effect
effect
–
The
The
breaks
breaks
leading
leading
to
to
the
the
inversion
inversion
occur
occur
in
in
a
a
vital
vital
gene
gene
Position
Position
effect
effect
–
A
A
gene
gene
is
is
repositioned
repositioned
in
in
a
a
way
way
that
that
alters
alters
its
its
gene
gene
expression
expression
•
About
About
2%
2%
of
of
the
the
human
human
population
population
carries
carries
inversions
inversions
that are
that are
detectable
detectable
with
with
a
a
light
light
microscope
microscope
Most
Most
of
of
these
these
individuals
individuals
are
are
phenotypically
phenotypically
normal
normal
However,
However,
a
a
few
few
produce
produce
offspring
offspring
with
with
genetic
genetic
abnormalities
abnormalities
T
T
r
r
a
a
n
n
s
s
l
l
o
o
c
c
a
a
t
t
i
i
o
o
n
n
A
A
chromosomal
chromosomal
translocation
translocation
occurs
occurs
when
when
a segment
a segment
of
of
one
one
chromosome
chromosome
becomes
becomes
attached to
attached to
another.
another.
There
There
are
are
two
two
main
main
types
types
of
of
medically
medically
important translocations:
important translocations:
•
Reciprocal
Reciprocal
(balanced)
(balanced)
Translocations
Translocations
•
Robertsonian
Robertsonian
(unbalanced)
(unbalanced)
Translocations
Translocations
Both
Both
types
types
of
of
translocations
translocations
are
are
capable
capable
of causing
of causing
disease
disease
in
in
humans
humans
.
.
.
.
.
.
R
R
e
e
c
c
i
i
p
p
r
r
o
o
c
c
a
a
l
l
T
T
r
r
a
a
n
n
s
s
l
l
o
o
c
c
a
a
t
t
i
i
o
o
n
n
s
s
•
In
In
reciprocal
reciprocal
translocations
translocations
two
two
non-homologous
non-homologous
chromosomes
chromosomes
exchange
exchange
genetic
genetic
material
material
•
Reciprocal
Reciprocal
translocations
translocations
lead
lead
to
to
a
a
rearrangement
rearrangement
of
of
the
the
genetic
genetic
material
material
not
not
a
a
change
change
in
in
the
the
total
total
amount
amount
Thus,
Thus,
they
they
are
are
also
also
called
called
balanced
balanced
translocations.
translocations.
.
.
Robertsonian
Robertsonian
Translocations
Translocations
•
In
In
Robertsonian
Robertsonian
Translocations
Translocations
the
the
transfer
transfer
of genetic
of genetic
material
material
occurs
occurs
in
in
only
only
one
one
direction
direction
•
Robertsonian
Robertsonian
translocations
translocations
are
are
associated
associated
with
with
phenotypic
phenotypic
abnormalities
abnormalities
or
or
even
even
lethality.
lethality.
•
Example:
Example:
Familial
Familial
Down
Down
Syndrome
Syndrome
In
In
this
this
condition,
condition,
the
the
majority
majority
of
of
chromosome
chromosome
21
21
is
is
attached
attached
to
to
chromosome
chromosome
14.
14.
The
The
individual
individual
would
would
have
have
three
three
copies
copies
of
of
genes
genes
found
found
on
on
a
a
large
large
segment
segment
of
of
chromosome
chromosome
21
21
Therefore,
Therefore,
they
they
exhibit
exhibit
the
the
characteristics
characteristics
of
of
Down
Down
syndrome
syndrome
.
.
•
This
This
translocation
translocation
occurs
occurs
as
as
follows:
follows:
Breaks
Breaks
occur
occur
at
at
the
the
extreme
extreme
ends
ends
of
of
the
the
short
short
arms
arms
of
of
two non-homologous
two non-homologous
acrocentric
acrocentric
chromosomes
chromosomes
The
The
larger
larger
fragments
fragments
fuse
fuse
at
at
their
their
centromeic
centromeic
regions
regions
to form
to form
a
a
single
single
chromosome
chromosome
The
The
small
small
acrocentric
acrocentric
fragments
fragments
are
are
subsequently
subsequently
lost.
lost.
This
This
type
type
of
of
translocation
translocation
is
is
the
the
most
most
common
common
type
type
of chromosomal
of chromosomal
rearrangement
rearrangement
in
in
humans.
humans.
•
Robertsonian
Robertsonian
translocations
translocations
are
are
confined
confined
to
to
chromosomes
chromosomes
13,
13,
14,
14,
15,
15,
21
21
•
The
The
acrocentric
acrocentric
chromosomes
chromosomes
A
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r
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Burkitt’s lymphoma
Chromosomal aberrations involve significant changes in chromosome structure and number, impacting multiple genes. These mutations can be structural or numerical, resulting in alterations such as deletions, duplications, inversions, and translocations. Deletions, for example, involve missing chromosomal segments and can lead to various outcomes depending on the affected genes. Human disorders linked to large chromosomal deletions often manifest in heterozygous individuals. The study of these abnormalities provides insights into genetic variations and their consequences.
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Structural Chromosomal Aberrations DR. Neha Antal Assistant Professor Department of Zoology Govt. P.G. College for Women, Gandhinagar, Jammu
Chromosomal aberrations are substantial changes in chromosome structure and number These typically affect more than one gene They are also called chromosomal mutations Chromosomal aberrations or mutation is of two types Structural chromosomal mutation: Changes inn the gene number or gene arrangement Numerical chromosomal mutation: Change in the number of chromosome
Alterations in Chromosome Structure There are two primary ways in which the structure of chromosomes can be altered The total amount of genetic information in the chromosome can change Decrease: Deficiencies/Deletions Increase: Duplications & Insertions The genetic material may remain the same, but is rearranged Inversions Translocations
Alterations in Chromosome Structure Deletion Loss of a chromosomal segment Duplication Repetition of a chromosomal segment Inversion A change in the direction of genetic material along a single chromosome Translocation A segment of one chromosome becomes attached to a non-homologous chromosome Simple translocations One way transfer Reciprocal translocations Two way transfer
DELETION Part of a chromosome is missing Deletions start with chromosomal breaks induced by: Heat or radiation (especially ionizing). Viruses. Chemicals. Transposable elements. Errors in recombination. Deletions do not revert, because the DNA is gone (degraded) Types: Terminal - involve one break Intercalary- involve two breaks
DELETION The effect of a deletion depends on what was deleted. A deletion in one allele of a homozygous wild- type organism may give a normal phenotype While the same deletion in the wild-type allele of a heterozygote would produce a mutant phenotype. Deletion of the centromere results in an acentric chromosome that is lost, usually with serious or lethal consequences.
Human disorders caused by large chromosomal deletions are generally seen in heterozygotes, since homozygotes usually die. The number of gene copies is important. Syndromes result from the loss of several to many genes. Examples of human disorders caused by large chromosomal deletions: Cri-du-chat ( cry of the cat ) syndrome, resulting from deletion of part of the short arm of chromosome 5 Cat like cry, malformation of larynx, moon face, micrognathia, microcephally, low set ears, mental retardation.
Cri du chat Syndrome Cri du chat Syndrome Variation in phenotype Variation in phenotype associated with region deleted associated with region deleted has been observed has been observed regions with genes involved in regions with genes involved in larynx and nervous system larynx and nervous system development development Researchers have identified Researchers have identified Pseudo-dominance: In the presence of deletion, sometimes a recessive allele of the normal homologous chromosome will behave like a dominant allele i.e. it will phenotypically express itself.
Duplication Duplications result from doubling of chromosomal segments Presence of a part of a chromosome in excess of the normal complement
Duplication Tandem duplications are adjacent to each other. Reverse tandem duplications result in genes arranged in the opposite order of the original. Tandem duplication at the end of a chromosome is a terminal tandem duplication
Duplication Displaced duplication duplicated region is not situated adjacent to the original. Transposed duplication duplicated part arrange to a non homologous chromosome
Duplication An example is the Drosophila eye shape allele, Bar, that reduces the number of eye facets, giving the eye a slit-like rather than oval appearance. Cytological examination of chromosomes showed that the Bar allele results from duplication of a small segment (16A) of the X chromosome.
Duplication loops form when chromosomes pair in duplication heterozygotes In prophase I, the duplication loop can assume different configurations that maximize the pairing of related regions
Position Effect Due to change in the position of a gene or a gene group in relation to other genes in a chromosome, phenotype of organism is altered, this effect is called position effect. It also represents a source of genetic variation.
Inversion Rotation of a part of a chromosome or a set of genes by 180 on its own axis.
Inversion The meiotic consequence depends on whether the inversion occurs in a homozygote or a heterozygote. A homozygote will have normal meiosis. The effect in a heterozygote depends on whether crossing-over occurs. If there is no crossing-over, no meiotic problems occur. If crossing-over occurs in the inversion, unequal crossover may produce serious genetic consequences.
Inversion Heterozygous Pericentric Inversion: One normal gamete One gamete with inversion One gamete with a duplication and deletion. One gamete with reciprocal duplication and deletion. Heterozygous Paracentric Inversion: One normal gamete One gamete with inversion Two deletion products Some material lost.
Crossing over in Pericentric Inversion Crossing over in Pericentric Inversion
Crossing over in Paracentric Inversion Crossing over in Paracentric Inversion
In an inversion, the total amount of genetic information stays the same Therefore, the great majority of inversions have no phenotypic consequences In rare cases, inversions can alter the phenotype of an individual Break point effect The breaks leading to the inversion occur in a vital gene Position effect A gene is repositioned in a way that alters its gene expression About 2% of the human population carries inversions that are detectable with a light microscope Most of these individuals are phenotypically normal However, a few produce offspring with genetic abnormalities
Translocation A chromosomal translocation occurs when a segment of one chromosome becomes attached to another. There are two main types of medically important translocations: Reciprocal (balanced) Translocations Robertsonian (unbalanced) Translocations Both types of translocations are capable of causing disease in humans .
Reciprocal Translocations In reciprocal translocations two non-homologous chromosomes exchange genetic material Reciprocal translocations lead to a rearrangement of the genetic material not a change in the total amount Thus, they are also called balanced translocations. .
Robertsonian Translocations In Robertsonian Translocations the transfer of genetic material occurs in only one direction Robertsonian translocations are associated with phenotypic abnormalities or even lethality. Example: Familial Down Syndrome In this condition, the majority of chromosome 21 is attached to chromosome 14. The individual would have three copies of genes found on a large segment of chromosome 21 Therefore, they exhibit the characteristics of Down syndrome .
This translocation occurs as follows: Breaks occur at the extreme ends of the short arms of two non-homologous acrocentric chromosomes The larger fragments fuse at their centromeic regions to form a single chromosome The small acrocentric fragments are subsequently lost. This type of translocation is the most common type of chromosomal rearrangement in humans. Robertsonian translocations are confined to chromosomes 13, 14, 15, 21 The acrocentric chromosomes .
Burkitts lymphoma An aggressive (fast An aggressive (fast- -growing) type of B type of B- -cell non cell non- -Hodgkin lymphoma that occurs most lymphoma that occurs most often in children and young often in children and young adults. The disease may adults. The disease may affect the jaw, central affect the jaw, central nervous system, bowel, nervous system, bowel, kidneys, ovaries, or other kidneys, ovaries, or other organs. organs. growing) Hodgkin
