2. INTRODUCTION
Chromosomes are the structures which carry genes in a definite linear
order.
Ordinarly, the chromosomes remain unchanged, but under certain natural
or artificial conditions, changes may occur in the structure or arrangement
of genes or in the number of chromosomes.
These changes are called chromosomal aberrations (or) chromosomal
mutations.
Chromosomal aberrations can be broadly grouped into two categories
1.Variations in chromosome number
2. Variations in chromosome structure.
3. VARIATION IN CHROMOSOME NUMBER
Each species has a characteristic number of chromosomes in the nuclei of
its gametes and somatic cells.
The gametic chromosome number constitutes a basic set of chromosomes
called genome.
A gametic cell contains single genome and is called haploid.
When haploid gametes of both sexes fuse in the process of fertilization, a
diploid zygote with two genomes is formed.
The diploid zygotes undergoes development and forms an adult
individual.
4. However, sometimes irregularities occur in normal diploid (2n)
chromosome number.
Such chromosomal aberrations may include whole genomes or a single
chromosome.
Changes in number of whole chromosomes is called heteroploidy.
Heteroploidy may involve entire sets of chromosomes (euploidy), or loss or
addition of single whole chromosomes (aneuploidy).
5. EUPLOIDY
The term euploidy (Gr. eu even or true; ploid= unit) designates genomes
containing chromosomes that are multiples of Some basic number (x).
The number of chromosomes, in a basic set is called monoploid (x) number.
Individuals with two sets of chromosomes are known as diploids (2x), and with
more than two sets are called the polypoids.
The polyploid types are 3x (triploid), 4x (tetraploid), 5x (Pentaploid), 6x
(hexaploid) and so on.
The haploid (n) refers to strictly to the number of chromosomes in gametes.
In most animals and plants, the haploid number (n) and the monoploid number
(x) are the same. Hence n or x (or 2n or 2x) can be used interchangeably.
However in case of polyploids, the usage of n may create confusion.
e.g. wheat has 42 chromosomes with 6x = 42 and x 7. However, gametes
contain 21 chromosomes, hence 2x = 42;n=21. i.e., 2n = 6x = 42
6. MONOPLOIDY
The monoploid organisms have one set ofchromosomes or one genome (n)
in the nuclei of their body cells.
When monoploidy occur in gametes it is termed as haploidy.
Most micro organisms (e.g. bacteria, fungi, and algae); gametophytic
generation of plants (e.g. bryophytes) and certain male insects are
monoploids.
7. TRIPLOIDY
The triploid organisms have three sets of chromosomes or genomes (3n) in
the nuclei of their body cells.
A triploid may originate by the union of a monoploid gamete (n) with a
diploid gamete (2n).
The extra set of chromosomes of the triploids lack any homologue to pair
with, therefore they are distributed in various combinations to the germ
cells resulting in the genetically unbalanced gametes.
Triploids are usually sterile e.g. some cells of birds, reptiles, man and
other vertebrates.
8. TETRAPLOIDY
The organism with four genomes (4n) in the nuclei of their somatic cells
are called tetraploids.
The tetraploidy arises due to doubling of the diploid chromosome number.
Doubling is accomplished either spontaneously or it can be induced in high
frequency by exposure to chemicals such as colchicine, acenapthene and
veratrine and also by exposure to heat or cold.
e.g. wheat, Chrysanthemum, Solanum, Salamander etc.
9. POLYPLOIDY
An organism with more than two genomes (2x) is called a polyploid. Ploidy
levels higher then tetraploid are not commonly encountered in natural
population, but our most important crops and ornamental flowering plants
are polyploids.
e.g. wheat (hexaploid, 6x), strawbenies (octaploid, 8x), many commercial
fruits (Octaploids, 8x).
About one third of all grasses are polyploids.
Polyploidy is broadly of two types
(1) Autopolyploids and
(2) Allopolyploids.
10. AUTOPOLYPLOIDS
The autopolyploids are those polyploids, which consists of same basic set
of chromosomes multiplied.
For example, if a diploid species has two similar sets of chromosomes or
genomes (AA), an autotriploid will have three similar genomes (AAA) and
an autotetraploid will have four such genomes (AAAA).
Autopolyploids may arise in any one of the following ways:
(i) by the union of diploid gamets produced in the absence of meiosis or
due to abnormal meiosis (autotetraploid),
(ii) by somatic doubling of the chromosomes in a zygote due to abnormal
mitosis (autotetraploid),
(iii) by the union of a haploid gamete with the diploid gamete
(autotriploid),
Autotriploids are generally sterile and cannot produce seeds. Therefore,
they have great commercial value in producing seedless varieties of fruits.
e.g. grapes, banana, sugarbeet, and watermelons.
Many important crop plants are autotetraploids. They include rye (Secale
cereales), corn (Zea mays), red clover (Trifolium pratense) etc.
11. ALLOPOLYPLOIDS
When the polyploidy results due to the doubling of chromosome number in a
F1, hybrid which is derived from two distinctly different species, then it is
called allopolyploidy.
The result Species is called an alloploid.
For example if 'A' represent a set of chromosomes (genome) in species X, and
'B' represent another genome in a species Y.
The hybrids of these species then would have one A genome and another B
genome. The doubling chromosomes in the F1, hybrids will give rise to
allotetraploids with two A and two B genomes.
13. ANEUPLOIDY
Aneuploidy is the addition or loss of one or more chromosomes to the
complete diploid chromosome complement of an organism.
The organisms with such chromosome complement are known as
aneuploids. (aneu uneven; picid= unit).
The aneuploidy may be of the following types.
(i) Monosomic: Diploid organisms which lack one chromosome of a
homologous pair are called monosomics.
Their genomic formula is 2n-1. If one more chromosome of some other
pair is also lost (2n-1- 1), these are known as double monosomics.
The number of possible monosomics will be equal to the number of haploid
number of chromosomes
14. (ii) Nullisomics: The diploid organism which have lost a pair of homologous
chromosomes are called nullisomics, with the genomic formula 2n-2.
A nullisomic diploid often does not survive, however, a nullisomic
polyploid (e.g. haxaploid wheat, 6x - 2) may survive but exhibit reduced
vigour and fertility.
(iii) Trisomics: Individuals having one chromosome extra to the diploid
genome are called trisomics.
One of the pairs of chromosome has an extra chromosome, so that a
trivalent structure may be formed during meiotic prophase.
If two chromosomes of the trivalent go to one pole and the third goes to
the opposite pole, then two types of gametes are formed.
The gametes will be n + 1 and n respectively.
15. (iv) Double trisomics: In a diploid organism, when two different
chromosomes are represented in triplicate, the double trisomic is
resulted. The double trisomics cause great genetic imbalance and has the
genomic formula 2n+1+1.
(v) Tetrasomic: The diploid organisms having two extra chromosomes are
known as tetrasomic. They have the genomic formula (2n+2).
Origin of aneuploids: Aneuploids could arise by nondisjunction occuring
during first meiotic division. Translocations also lead to the formation of
trisomics and monosomics.
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17. VARIATION IN CHROMOSOME STRUCTURE
Chromosomes are the structures with definite organization.
In ordinary conditions, the chromosomes are unchangeable.
But under certain natural or artificial conditions, some structural changes may
occur in them.
They involve changes either in the total number of genes or gene loci in a
chromosome or their arrangement.
Such structural changes are collectively called chromosomal aberrations.
Most chromosomal aberrations are caused due to accidental, natural or induced
breakage of chromosomes.
The induced breakage may be caused by radiation, and chemicals such as Lysergic
acid diethylamide (LSD) etc.
The chromosomal aberrations may remain confined to a single chromosome (intra
chromosal) or may extend to both of the members of a pair or may involve two or
more homologous chromosomes (Interchromosomal aberrations)
18. INTRA CHROMOSOMAL ABERRATIONS
The intra chromosomal aberrations occur in one and the same chromosome
and are of the following kinds.
1. Deletion or Deficiency: The deficiency is the deletion of a the chromosomal
segment resulting in the loss of genes.
If the break occurs near the end of a chromosome, it is called terminal
deletion. It results in the loss of a small piece at the terminal end.
Sometimes two breaks may occur at any two points, releasing an inter-calary
segment. The broken ends of original chromosome are again fused and thus
has an intercalary or interstitial deficiency results.
19.
20. 2. Duplication: When a segment of chromosome is represented two or more
times, it is known as duplication.
The repeated block of genes is called a repeat.
Three types of duplications have been recognised on the basis of attachment
of the chromosome segment.
(i) Tandem duplication: The newly added segment has the same genetic
sequence as is present in the original state in the chromosome. Moreover, the
added segment lies in close association. with the original segment. For
example, if duplication piece (repeat) is ABC, a tandem duplication will be
ABC ABC DEF.
21. (ii) Reverse Tandem duplication: The sequence of genes in the attached
repeat is just the reverse of the original alignment. For example, if the
duplication piece (repeat) is ABC, a reverse tandem duplication will be ABC
CBA DEF.
(iii) Displaced duplication: The chromosomal segment gets attached to some
non-homologous chromosome. For example if the repeat is ABC, a displaced
duplication will be KLM ABC NOP.
22. 3. Inversion: Sometimes the sequence of genes in a chromosome is altered by the
rotation of a chromosome segment by 180°. If a chromosome having gene
alignment a b c d e f g h i, breaks at points b and g and the middle segment c d e
f g undergoes inversion, then the gene sequence in the inverted chromosome will
be a b g f e d c h i.
23. Inversions are of two types
(i) Paracentric inversion: When both the breaks in the chromosome occur on
the same side of the centromere, the inversion is known as paracentric. The
inverted segment of chromosome is without centromere.
(ii) Pericentric inversion: In this type of inversion, the inverted segment
contains the centromere i.e., it involves one break on either side of the
centromere. (Pericentric means surronding the centromere). Pericentric
inversions are less frequent than paracentric type.
24. INTER CHROMOSOMAL ABBERATIONS
Translocation: When a segment of one chromosome (a set of genes) is
transferred to a non-homologous chromosome, it is called translocation.
There is no addition or loss of genes during translocations, only a
rearrangement (i.e., change in the sequence and position of a gene) occurs.
Translocations may be of the following three types.
(i) Simple translocations: They involve a single break in a chromosome. The
broken piece gets attached to one end of a non homologous chromosome. This
type of translocation is very rate in nature.
(ii) Shift translocations: In this type of translocation, the broken segment of
one chromosome gets inserted interstitially in a non homologous chromosome.
(iii) Reciprocal translocations: In this case, a segment from one chromosome
is exchanged with a segment from another non homologous chromosome. As a
result, two translocation chromosomes are simultaneously achieved.
25.
26. Reciprocal translocation is of two types:-
a) Homozygotic translocation: In this case, translocation involves both the
members of each of the homologous chromosomes.
(b) Heterozygotic translocation: Here, translocation involves only one
member of each of the homologous pair.
