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LECTURE-5
PART- I CYTOGENETICS
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INVERSIONS
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Chromosomal mutationsInversions
Inversions
Definition
Types of deletions
OriginDetection
- Cytological detection
- Genetic detection
Inheritance of aberration
Consequences
Evolutionary significance
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What is an inversion?
If two breaks occur in one
chromosome, sometimes the
region between the breaksrotates 180obefore rejoining
with the two end fragments.
Such an event creates achromosomal mutation called
an inversion
Chromosomal mutationsInversions
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Paracentric Inversions Pericentric Inversions
Two typesofinversion:
Chromosomal mutations- Inversions
Inversion spans centromereInversion does not span centromere
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Inversions with the centromere remaining outside of the inverted
segment are called paracentric inversions or homobrachial
inversions.
Chromosomal mutationsInversions
Inversions that involve the centromere are called pericentric
inversionsor heterobrachial inversions.
Pericentric inversion usually result in a change in chromosomeshape owing to the centromere changing its position.
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Chromosomal aberrationsInversions
Inversions are balanced rearrangements.
They do not change the overall amount of the genetic
material so they are generally viable.
They show no particular abnormalities at the
phenotypic level.
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Chromosomal aberrations- Inversions
Diagnosis
Cytological detection
Karyotype (mitotic metaphase)
No change in chromosome size
Inversion in banding pattern Changes in chromosome arm ratios for pericentric
inversions (changes in shape)
Meiotic chromosomes Inversion loops formed during meiotic pairing of
homologues if inversion is large.
Note: if inversions spans a small region, loops are not formed
but instead form inversion bumps.
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Chromosomal aberrationsInversionsPericentric
In a pericentric inversion, because the centromeres are
contained within the inverted region, the chromosomes that have
crossed over disjoin in the normal fashion during cell division.
However, the crossover produces recombinant chromatids that
contain a duplication and a deficiency for different parts of the
chromosome.
If a nucleus carrying a crossover chromosome is fertilized, the
zygote dies because of its genetic imbalance.
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Chromosomal aberrations
InversionsPericentric
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Chromosomal aberrationsParacentric
Crossing-over within the inversion loop of a heterozygous paracentric
inversion connects homologous centromeres in a dicentric bridge
while also producing an acentric fragment one without a centromere.
In Anaphase I, centromeres remain linked by the bridge.
Centromeres oriented so non-crossover chromatids lie farthest apart.
Forms two chromosomes with terminal deletions.
Acentric fragment is lost.
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Chromosomal aberrations
InversionsParacentric
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Chromosomal aberrations- Inversions: Inheritance
Inheritance of Pericentric and Paracentric inversions.
The consequences of inheritance are different for pericentric and paracentric
inversions.
A single crossover in a pericentric inversion
Gives one normal chromosome, one inversion chromosome and two
recombinant chromosomes that have duplications and deficiencies.
Since the recombinant chromosomes result in lethal gametes, no
crossover products are recovered.
The genetic consequence of this is that pericentric inversions are
apparent crossover suppressors.
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Chromosomal aberrations- Inversions
A single crossover in a paracentric inversion
Gives one normal chromosome, one inversion chromosome and two
recombinant chromosomes that have duplications and deficiencies.
The recombinant chromosomes are different: they are one acentricfragment and one dicentric chromosome with duplications and
deficiencies.
The recombinant chromosomes result in lethal gametes.
Similar to pericentric inversions, paracentric inversions are also apparent
crossover suppressors.
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Chromosomal aberrationsInversions
Diagnosis
All the products of a two strand double crossover in a paracentric
inversion are viable.
This is the one class of recombinant chromosomes that can be
recovered.
They can be used to assess the level of two strand double crossovers
in a paracentric inversion compared to normal chromosomes to
determine if inversions prevent chromosome pairing.
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Chromosomal aberrations- Inversions
Diagnosis
All the gametes
of a two-stranddouble crossover
in a paracentric
inversion are
balanced.
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Chromosomal aberrations- Inversions
Diagnosis
Genetic detection
1. Position Effects
Genes that are located
near heterochromatic
regions normally
expressed less.
Genes that were once
expressed routinely now
show a reduction in
expression.
Wild-type flies with a normal whitegene
have red eyes. If the whitegene is
inactivated by mutation, the eyes become
white (hence the name of the gene).
In flies with a chromosomal inversion that
moves the whitegene near a
heterochromatic region, the eyes are
mottled with red and white patches.
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Chromosomal aberrationsInversions
Genetic detection
2. Crossover Suppression.
Reduction of crossing over within an inversion loop in inversion heterozygotes due to
physical constraints during synapsis.
Crossing over within an inversion loop, when it does occur, leads to defective (deleted
and duplicated) crossover chromosomes and mortality of zygotes carrying them.
Inversions do not suppress crossing over, but their cross over products are usuallyeliminated before incorporation into functional nuclei.
Inversions are therefore used to maintain a sequence of genes on a particular
chromosome, and prevent them from crossing over. Thus recombinant type gametes
are rarely produced.
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Chromosomal aberrations- Inversions
Genetic detection
3.Viability of inversion homozygotes.
Since there are no net loss or gain in genetic material, both inversion
homozygotes and heterozygotes are viable in that they are capable of
producing gametes.
4. Genetic Co-adaptation.
An inversion has a certain segment of its genetic loci reversed to the
normal order of genes. Owing to the prevention of recombination within
that region, each gene represents a sort of super gene.
Natural selection accumulates beneficially interacting alleles within each
inversion. These are said to show genetic co-adaptation.
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Evolutionary Significance
Chromosomal aberrations- Inversions
The main importance of inversions might lie in their ability to produce
genetic isolation between populations and species owing to the evolution
of e.g pericentric homozygotes being distinct from the remainder of the
species. Over evolutionary time they diverge into distinct populations.
Speciation is associated with inversions. E.g. The deer mouse genus
Peromyscus = 2n = 48 for all species. But, P. collatus is composed of
entirely metacentric chromosomes andP. boylei is composed of
acrocentrics.
Inversions can create postzygotic barriers when they reduce the fecundity
of heterokaryotypes (chromosomal heterozygotes).