2. Numerical Changes in
Chromosomes: General Overview
Numerical changes in chromosomes
refer to variations in the number of
chromosomes in a cell, deviating
from the normal diploid set. These
changes are categorized into
aneuploidy and euploidy, both of
which play significant roles in
cytogenetics, evolution, and plant
and animal breeding.
3. 1. Aneuploids
Definition: Organisms with chromosome numbers that are not
exact multiples of the haploid set (n). Instead, they have either
one or a few chromosomes more or less than the normal diploid
number.
Types:
• Monosomy (2n-1): Loss of a single chromosome.
• Trisomy (2n+1): Addition of a single chromosome.
• Nullisomy (2n-2): Loss of a pair of homologous chromosomes.
• Tetrasomy (2n+2): Addition of a pair of homologous chromosomes.
Cytogenetics of Aneuploids:
• Aneuploids show abnormal meiotic behavior leading to fertility problems.
• Chromosome pairing and segregation are disturbed.
• Examples: Down syndrome (trisomy 21) in humans; aneuploid crop varieties.
4. 2. Euploids
Definition: Organisms whose chromosome
number is an exact multiple of the basic haploid
number (n).
Types:
• Haploids (n): Single set of chromosomes.
• Diploids (2n): Two sets of chromosomes (normal in most
organisms).
• Polyploids (>2n): More than two complete sets of
chromosomes.
• Triploids (3n)
• Tetraploids (4n)
• Hexaploids (6n), etc.
5. 3. Cytogenetics of Polyploids
A. Autopolyploids:
• Derived from duplication of the
chromosome set of a single species.
• Chromosomes are identical or
homologous.
• Example: Autotetraploid potato (4n=48).
• Features: Larger cell and organ size,
but sometimes reduced fertility due to
abnormal pairing during meiosis.
6. • B. Allopolyploids:
• Formed from the
combination of
chromosome sets from
different species.
• Chromosomes are
homeologous (partially
homologous).
• Example: Wheat (Triticum
aestivum, hexaploid),
Cotton (Gossypium
hirsutum, tetraploid).
• Features: Generally more
stable than autopolyploids;
useful in evolution and
plant breeding.
7. Applications in Botany
and Genetics
Development of improved
crop varieties with higher
yield and disease
resistance.
Study of chromosome
behavior and evolution.
Understanding genetic
disorders caused by
chromosomal imbalances.
8. Aspect Aneuploids Euploids
Definition
Organisms with chromosome
numbers not an exact multiple of
haploid number (n)
Organisms with chromosome
numbers that are exact multiples
of haploid number (n)
Chromosome Change
Addition or loss of one or few
chromosomes
Addition of entire sets of
chromosomes
Types
- Monosomy (2n-1)
- Trisomy (2n+1)
- Nullisomy (2n-2)
- Tetrasomy (2n+2)
- Haploid (n)
- Diploid (2n)
- Polyploid (3n, 4n, 6n, etc.)
Origin
Meiotic nondisjunction or
chromosome mis-segregation
Whole genome duplication
Example (Plant/Animal)
- Down syndrome (trisomy 21 in
humans)
- Datura trisomics
- Wheat (6n)
- Potato (4n)
- Banana (3n)
Cytogenetic Features
Abnormal pairing and segregation
during meiosis, often reduced
fertility
May show normal (in
allopolyploids) or abnormal (in
autopolyploids) chromosome
pairing
Subtypes of Polyploids Not applicable
- Autopolyploids: Chromosome
sets from the same species
- Allopolyploids: Chromosome
sets from different species
Autopolyploids: Homologous
chromosomes, multivalent
formation in meiosis
