Wel come• Topic :- structural Chromosomes aberrations and

there role in plant breeding

presented by,

shrishaila PG14AGR6066

UAS Raichur

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Variation in chromosome structure or number is called chromosomal aberration

Chromosome aberration

structuralNumerical

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Chromosome Structural aberrations

• The chromosome aberrations in which they alters the structure of chromosome(sequence of genes or kind of genes in chromosome or no. of genes)

• There are mainly four types which areChanges in the numbers of genes

Deletion/DeficiencyDuplication

Changes in the location of genesInversions :- 180o rotationTranslocations:- Exchange

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Origin of structural aberrations

Deletion • spontaneously or may induced• Mis-division of the centromere• radiation, UV, chemicals, viruses may increase

breakage

Duplication• Unequal crossing over• Crossing over in inversion heterozygotes• Crossing over in translocated heterozygotes

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Inversion • Spontaneously and induced

• Naturally in Drosophila

• Produced through breakage and reassociation of chromosome

Translocation

• Spontaneously

• Induced by mutagens

• Growing a plants in ca deficit medium

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Deletion• First chromosome aberration discovered by

the Bridges in 1917 in Drosophila

• Usually detected by based on unpaired region of normal chromosome produces a loop during pachytene stage

Two types which are

Terminal

Intercalary

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Deletions

• Loss of a (generally small) segment of chromosome

A B C D E F G A B D E F G

C

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Terminal deletion

• Loss of segment contain a telomere of affected chromosome

• In 1938 Muller postulated that loss of telomere makes chromosome unstable so it is not commonly observed

Ex:maize

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Intercalary deletion

• Loss of segment in between centromere and telomere

• It is commonly observed

Ex:Drosophila

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Effects of deletion

• Pseudo dominance

• Crossing over not there

• Harmful effect on diploid organisms

• Most are as recessive Lethals

• Morphological effects

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Duplication • Bridges in 1919

• It is detected by presence of loop formation during pachytene stage in duplicated region

or cross pairing between segments of non-homologus chromosomes

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Duplications

• It is type of structural aberration in which resulting the doubling of the segment

A B C D E F G A B C D E F F G

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Mainly four types

Tandem Intrachomosomal

Reverse tandem

Displaced

Translocated Interchromosomal

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Effects of duplication

• According to Ohno(1970) origin of new genes mainly due to duplication resulted the evolution

• Position effect

Ex: Bar eye is result of duplication of 16A region of X chromosome of Drosophila

• Activity of certain enzymes may be doubled

Ex:chromosome 6 of barley increased the activity of α-amylase (hagberg in 1965)

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Inversion

• Sturtevant & punnet in 1921 in Drosophila

Detected by

• presence of partial male sterility

• comparing with the normal strain by using linkage map

• presence of inversion loop during pachytene

• absence of crossing over.

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Inversions

•It is the structural aberration in which the change in the sequence of chromosomal arrangement•180o reversal of chromosome segment

A B C D E F G H I J K

A B C H G F E D I J K

180O

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Types of inversion

• Paracentric:- inverted segment does not

include centromere and confined to one arm

• Pericentric:- included the centromere

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Effects of inversion

• Partial male sterility

• Formation of Recessive mutation

• Position effect

• Effects the activity of the NOR of complement chromosome

• move active genes to sites generally inactive; lose gene function and vice versa

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Translocations • It is a type of structural chromosome in which

change in position of the segment in such that they become integrated into same or homologous or non homologous chromosome

Detected by

• formation of cross shaped configuration at pachytene between two non homologous chromosomes

• Ring of 4 chromosomes at metaphase 1 due to pairing between homologous chromosomes

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Translocations

• Exchange of segments between non-homologous chromosomes

F

A

B

C

D

E

L M NO

P

Q

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Types of translocations

Based on involment of chromosomeIntrachromosomal

Intraradial:-shift occur in same armExtraradial:-shift occur in different arm

Interchromosomalfraternal:-shift occur to homologous

chromosomeExternal:-shift occur to non homologous

chromosome

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Based on number of breaks involved

Simple

• It involves one break

• terminal segment of chromosome integrated at the one end of non homologous chromosome

• Reported by painter and muller in 1929 in Drosophila

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Shift translocation

• It requires three breaks

• Intercalary segment of a chromosome is integrated within a non homologous chromosomes

• Commonly seen in Drosophila ,Neurospora

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Reciprocal translocations

• It requires one break in each of the chromosomes

• Segments will exchanged between non homologous chromosomes

• Designated by prefix ‘T’

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Effects of translocations

• Alters the chromosome morphology

• Position effect

• Damage to DNA may result in formation of recessive lethals

• Lead to impaired fertility

• complications to synopsis and segregation

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Uses of structural aberration

Study of chromosome pairing and its behaviour during cell division

For locating genes on particular chromosome

To resolve special problems such as relation between chaisma and crossing over

Used in plant breeding by increasing the dosage of certain desirable genes for increasing the activity

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Continue...Homozygous duplication in heterozygous

condition will gives the permanent hybrid vigour

Useful in study of position effect

New genes can be produced only through duplications

Paracentric inversion will used to produce acentric fragments intact

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Cont.... Used to study the specific problems in genetics

Ex:-C1B Technique by muller in Drosophila C:-Inversion

1:-1 Recessive lethal B:-Bar eye

Important role in evolution Desirable characters can be detected using inversion Translocation used in determination of unknown locus

of gene and chromosome mapping Testing the association between linkage groups Used to know the point of intiation of chromosome

pairing Production of duplication Evolution of allopolyploidy

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Effects on humansDeletions in Humans

• Cri-du-chat syndrome

– Micro deletion of chromosome 5

• DiGeorge syndrome

– Micro deletion of chromosome 22

• Angelman syndrome

– Micro deletion of chromosome 15

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Translocations

• Several human cancers are associated with reciprocal translocations

– Chronic myelocytic leukemia

– Burkitt’s lymphoma

The Philadelphia Chromosome

Reciprocal translocation between chr. 22 and

chr. 9

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T...................U

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