5. Chromosomal Macromutations- Inversions

8/10/2019 5. Chromosomal Macromutations- Inversions

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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

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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.


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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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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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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Diagnosis

All the gametes

of a two-stranddouble crossover

in a paracentric

inversion are

balanced.
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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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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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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


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).

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5. Chromosomal Macromutations- Inversions

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