This document summarizes structural and numerical variations in chromosomes and their implications. Structural variations include deletions, duplications, translocations, and inversions which alter chromosome structure. Numerical variations include aneuploidy, involving extra or missing chromosomes, and euploidy, involving extra sets of chromosomes. Aneuploidy includes hypoploidy, hyperploidy, trisomy and tetrasomy. Euploidy includes monoploidy and polyploidy such as autotetraploid and allopolyploid. Chromosomal variations have implications for plant breeding including determining gene locations and producing new varieties with desirable traits like larger fruits or flowers.
Botany krishna series 2nd semester Only Mcq type questions
chromosomalabnormalitiesppt.pdf
1. Topic: Structural and numerical
variations in chromosomes and
their implication
G.Bhagirath
faculty in Botany
2. TABLE OF CONTENT
Structure of chromosomes
1. Structural Changes (Variations in Chromosome structure):
Deletions
Duplications
Translocations
Inversions
2. Numerical Changes (Variations in Chromosome Number):
a. Aneuploidy:
Hypoploidy
Hyperploidy
b. Euploidy:
Monoploidy
Polyploidy
Autopolyploid and allopolyploid
Implication of chromosomal aberrations in plant breeding and crop improvement
3. What are chromosomes?
Chromosomes are the structures that hold
genes. These are made up of a long DNA
molecule with part or all of the genetic
material of an organism.
Most eukaryotic chromosomes include
packaging proteins called histones bind to
and condense the DNA molecule to
maintain its integrity.
Many chromosomes have two segments,
called arms, separated by a pinched
region known as the centromere. The
shorter arm is called the p arm. The
longer arm is called the q arm.
4. 1. Structural Change (Variations in Chromosome structure)
Any change which alter the basic chromosome structure is known as
structural change.
Chromosomes are the vehicle of hereditary material or genes. Any
alteration, addition or deletion of chromosomal part leads to alteration
of number, position or sequence of genes in the chromosome.
These changes are categorized into four classes-
Deletions
Duplications
Translocations
Inversions
5. Deletions
A portion of the chromosome is missing or deleted. The chromosome becomes shorter
due to loss of one or more genes.
7. Translocations
A portion of one chromosome is transferred to
another chromosome. There are three main
types of translocation.
Simple translocation- In simple translocation,
segment from one chromosome will break and
attached to another chromosome.
Reciprocal translocation- In a reciprocal
translocation, segments from two different
chromosomes have been exchanged.
8. Robertsonian translocation-
In a Robertsonian
translocation, an entire
chromosome arm has attached
to another at the centromere.
Loss of chromosomal segment.
9. Inversions
A portion of the chromosome has broken off, turned upside down, and reattached. As a result, the genetic
material is inverted.
An inversion is produced when there are two breaks in a chromosome and the intercalary segment reunites
in reverse order i.e., the segment rotate by 180°.
Inversion is of two types-
Pericentric inversion- If the inverted segment includes the centromere, the inversion is called pericentric
inversion;
Paracentric inversion- If it does not include centromere the inversion is called as paracentric inversion.
10. 2. Numerical Changes (Variations in Chromosome Number)
The organisms are usually diploid (2n), i.e., they possess two sets of
chromosomes.
A deviation from the diploid state represents a numerical chromosomal
aberration which often referred as heteroploidy.
Individual possessing the variant chromosome numbers are known as
heteroploids
Heteroploidy can be mainly of two types
(1) Aneuploidy
(2) Euploidy
11. (1) Aneuploidy
It involves addition or deletion of one or few chromosomes to the usual diploid set
of chromosomes.
Aneuploid changes in chromosome number do not involve the whole genome.
13. (ii) Nullisomy
These arise by the loss of a particular pair of chromosomes i.e.,
2n-2.
14. Hyperploidy (i) Trisomy
These arise by addition of an extra chromosome to the normal diploid set with
the genetic formula, 2n + 1.
15. (ii) Tetrasomy
These arise by the addition of an extra pair of chromosome to the diploid set with a chromosomal
formula 2n + 2.
By this a particular chromosome is represented in four doses instead of normal two.
16. (2) Euploidy
Normally organism possesses two sets of chromosomes i.e., they are diploid
(2n).
At times there is addition or loss of complete one set (n) or more than one set
of chromosomes is observed. It is called as euploidy.
Euploidy is of following types:
Monoploidy: Presence of a single copy of a single genome is known as
monoploidy, denoted by x.
Polyploidy: Organisms having more than two normal sets of chromosomes
(2n) are called polyploids.
Organisms with three sets of chromosomes (2n + n) = 3n, are triploids.
Those with four sets of chromosomes (2n + 2n) = 4n, are tetraploids and those
with five sets (2n + 3n) = 5n, are pentaploids and so on.
17.
18. There are two major kinds of polyploids according to the
origin of chromosomes-
Autopolyploid and allopolyploid
Autopolyploids are those polyploids which have same basic set of chromosomes
multiplied.
For instance, if a diploid species has two similar sets of chromosomes (AA), an
autotriploid will have three similar sets (AAA) and an autotetraploid will have four
such sets (AAAA).
Allopolyploids are those polyploids which contains two or more distinct (different)
genomes.
If we double the chromosome number in a F1 hybrid (AB) which is derived from two
distinctly different species, the resulting polyploidy will be allotetraploid (AABB).
common wheat is an allohexaploid having genome configuration AABBDD (three
distinct species).
19. Implication of chromosomal aberrations in plant breeding and
crop improvement
Aneuploids have been used to determine the phenotypic effects of loss or gain of
different chromosomes.
Aneuploids are also used to produce alien addition and alien substitution lines which
are useful in gene transfer from one species to another.
Aneuploid analysis permits the identification of location of a gene onto a specific
chromosome
Aneuploids are useful in the identification of chromosomes involved in translocation.
Chromosome doubling of haploids produces homozygous disomic plants (2n) (double
haploids) in just two years as compared to 6-7 generations required for their production
through selfing.
20. Triploid watermelons are produced by crossing tetraploid (4x female) and
diploid (2x male) lines. These watermelons produce only rudimentary seeds
which are not objectionable when chewed.
Triploid sugarbeet produce larger roots and more sugar per unit area than
diploids.
Autotetraploid varieties of some forage crops like Clove variety Tora, and
Berseem variety Pusa Giant have been released for commercial cultivation.
Many ornamentals are autotetraploids, they generally have larger flower and
longer flower duration than do doploids.
