7 chromosomal aberrations

Mujahid Hussain (M.Phil. Botany)
Department of Botany
University of Sargodha, Sargodha,
Punjab, Pakistan
Chromosomal Aberrations
10/12/2016mujahid.hussain7877@gmail.com1

Chromosomal Aberrations
“Variation in chromosomal structure or number is called
Chromosomal Aberration”
They can refer to changes in the number of sets of
chromosomes (ploidy), changes in the number of individual
chromosomes (somy), or changes in appearance of individual
chromosomes through mutation-induced rearrangements. They
can be associated with genetic diseases or with species
differences.
FISH and Banding techniques are used to detect
chromosomal aberrations.

Structural Aberrations
The chromosomal aberrations in which alternation of the
structure of chromosome(sequence of genes or kind of genes
in chromosome or no. of genes)occur—Structural Aberration.
Types of Structural Aberrations:
 Changes in the numbers of genes
 Deletion/Deficiency (Terminal, Intercalary)
 Duplication (Intrachromosomal, Interchromosomal)
 Changes in the location of genes
 Inversions (Paracentric, Pericentric)
 Translocations (Intrachromosomal, Interchromosomal)

Translocation Deletion
Insertion
Inversion Isochromosome
Ring
chromosome
Derivative
chromosome
Chromosomal Structure Abnormalities

Deletion
“Loss of a (generally small) segment of chromosome”
 Spontaneously or may be induced (radiation, UV,
chemicals, viruses).
 Detection is based on unpaired region of normal
chromosome that produces a loop during pachytene stage.
 Firstly observed by Bridges in 1917 in Drosophila.
Terminal deletion:
“Loss of either terminal segment of a chromosome”
 In 1938, Muller postulated that loss of telomere makes
chromosome unstable so it is not commonly observed.
 E.g. maize.

Intercalary Deletion:
“Loss of segment in between centromere and telomere”
 It is commonly observed in drosophila.
Effects of deletion:
 Crossing over not occur.
 Harmful effect on diploid organisms.
 Morphological effects.
A B C D E F G A B D E F G
C 10/12/2016mujahid.hussain7877@gmail.com6

Duplication
“Occurrence of a segment twice in the same chromosomes”
 Given by Bridges in 1919.
 Occurs due to unequal crossing over or crossing over in
inversion or translocated heterozygotes.
 It is detected by presence of loop in duplicated region
during pachytene stage.
Intrachromosomal Duplication:
 Tandem – in this case sequence of genes in the duplicated
segment is similar to the sequence of genes in the original
segment of a chromosome.

 Reverse tandem – the sequence of genes in the duplicated
segment is reverse to the sequence of genes in the original
segment of a chromosome.
Interchromosomal Duplication:
 Displaced : Duplicating segment is incorporated away from
corresponding segment on the same chromosome.
 Translocated: Duplicated chromosomal segment is
incorporated on different chromosome.
A B C D E F G A B C D E F F G

Effects of duplication:
 Origin of new genes mainly due to duplication results in
evolution.
 Activity of certain enzymes may be doubled.
Ex: chromosome 6 of barley increased the activity of α-
amylase.

Inversion
“A chromosomal segment is oriented in a reverse position
(180o reversal )”
 Studied by Sturtevant & Punnet in 1921 in Drosophila.
 Detected by presence of inversion loop during pachytene
 Absence of crossing over.
Paracentric Inversion:
“Inverted segment does not include centromere and
confined to one arm”
Pericentric Inversion:
“Included the centromere”

Effects of Inversion:
 Partial male sterility.
 Formation of Recessive mutation.
 Move active genes to sites generally inactive; loose gene
function and vice versa .
A B C D E F G H I J K
A B C H G F E D I J K
180O

Translocation
“In translocation, change in position of the segment occurs in
such a way 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.
 Occurs spontaneously or may be induced by mutagens.
Types:
 Based on involvement of chromosomes:
Intrachromosomal Translocation:
 Intraradial: Shift occurs in same arm.
 Extraradial: Shift occurs in different arm.

Interchromosomal Translocation:
 Fraternal: Shift occurs to homologous chromosome.
 External: Shift occurs to non homologous chromosome.
Reciprocal Translocation:
“Exchange between segments of non homologous
chromosomes or regions of same chromosome”.
Non reciprocal Translocation:
“Movement of a chromosome
segment to non homologous
chromosome or region of same chr.
without reciprocal change”.

 Based on no. of breaks involved:
Simple Translocation:
“It involves one break. Terminal segment of chromosome
integrated at the one end of non homologous chromosome”
Shift Translocation:
“It requires three breaks. Intercalary segment of a
chromosome is integrated within a non homologous
chromosomes”
Ring Chromosome:
“Break occurs in each arm & the 2 sticky ends join while
distal fragments are lost”
Robertsonian Translocation:
“Breakage of 2 acrocentric chr. near centromeres & fusion of

long arms. Short arms are lost”
Effects of Translocation:
 Alters the chromosome
morphology.
 Damage to DNA may result in
formation of recessive lethals.
 Lead to impaired fertility.
Uses of Structural Aberration:
 Study of chromosome pairing and its behaviour during cell
division
 For locating genes on particular chromosome
 Used in plant breeding by increasing the dosage of certain
desirable genes for increasing the activity
 Important role in evolution
 Desirable characters can be detected using inversion 10/12/2016mujahid.hussain7877@gmail.com15

Numerical Aberrations
“Change in the number of chromosomes is called as
numerical aberration or numerical abnormality”.
Numerical Aberration has two types:
 Aneuploidy (Hyperploidy, Hypoploidy)
 Euploidy (Monoploidy, Diploidy, Polyploidy)

Aneuploidy
“Change in number of individual chromosomes, but not in
complete set” e.g., 2n ± 1
Mainly arises due to non disjunction.
– tolerated in plants
– usually lethal in animals
– most well known examples in human genetic diseases.
Types of Aneuploidy:
 Hperploidy
“having chromosomes more than disomic condition (2n)”
 Trisomy(2n+1) :
“Addition of one chromosome to one pair in diploid set”

It has two types :
 Simple trisomics – increase in chromosome number in
one pair only (2n+1)
 Double trisomics – addition of one chromosomes in two
different pairs (2n+1+1)
 Tetrasomy(2n+2):
“Addition of two chromosomes to one pair or two
different pairs”
 Simple tetrasomics – addition of two chromosomes to one
pair(2n+2)
 Double tetrasomics – two chromosomes are added each to
two different pairs(2n+2+2)

 Hypoploidy:
“Having chromosomes less than disomic condition(2n)”
– Mostly occurs in polyploids e.g. wheat, tobacco etc.
 Monosomy (2n-1):
“lacking one chromosome from a diploid set”
– Used to map genes in plants.
 Double Monosomics – lack of one chromosomes each
from two different pairs (2n-1-1).
 Nullisomy (2n-2):
“lacking one pair of chromosomes from a diploid set”

Type No. of chromosomes Example
Normal diploid 2n AABBCC
Monosomic 2n-1 AABBC
Nullisomic 2n-2 AABB
Polysomic Extra chromosomes
a) Trisomic 2n+1 AABBCCC
b) Double
trisomic
2n+1+1 AABBBCCC
c) Tetra somic 2n+2 AABBCCCC
d) Pentasomic 2n+3 AABBCCCCC

Euploidy
“A condition in which one or more full sets of chromosomes
are present in an organism”
Types:
 Monoploidy(n):
“Single basic set of chromosomes”
e.g. in wheat: 2n₌6x₌42 where x₌7, neurospora etc.
 Polyploidy:
“More than two multiples of haploid chromosomes sets”
Triploidy (3n) and Tetraploidy (4n).
 Autopolyploidy, Allopolyploidy

 Autopolyploids:
“Polyploids which originates by multiplication of the
chromosomes of a single species”
a. Autotriploids(3n)
Formation of autopolyploids occurs when:
• Diploid gamete is fertilized by a haploid sperm
• An ovum is fertilized by two sperms
• Fertilization occurs between diploids(2n) and tetraploids(4n)
– e.g. seedless banana, apple, sugarbeet, watermelon etc.
b. Autotetraploids(4n)
A diploid cell is treated with heat, cold or colchicine to
tranform it into autotetraploid.
– e.g. rye, grapes, alfalfa etc.

 Allopolyploids:
“Polyploids which originates by combining complete
chromosomal sets from two or more species”
 Mostly Autotetraploids
 Formation occurs when two different spp. are combined.
Resulting individual is sterile but is treated with colchicine to
form a tetraploid–a new species.
a. Natural Alloploids – wheat, cotton, tobacco, mustard, oats,
brassica etc.
As wheat (Triticum aestivum: 2n ₌42) is actually formed
from three different species:
Triticum aegilopoides (2n₌14), Aegilops speltoides
(2n₌14), Aegilops squarrosa (2n₌14).

 Artificial Alloploids – Domesticated coffee, cotton,
raphanobrassica etc.
As raphanobrassica (2n ₌36) is formed from Raphanus
sativus (2n ₌18) and Brassica oleraceae (2n₌18).
Impotance of Polyploidy:
 plays an imp. role in evolution as Alloploidy gives rise to
new species.
 Autopolyploids are more adaptive to changing
environment as compared to their respective diploids.

 Plants commonly exhibit polyploidy
 30-35% of ferns and flowering plants are polyploids
 Many of the fruits, grains and ornamental plants are polyploids
 Polyploid strains often
display desirable
agricultural
characteristics
 wheat
 cotton
 strawberries
 bananas
 large blossom flowers

7 chromosomal aberrations

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