Occasionally, spontaneous variations (without any known causal factor) in chromosome number or structure do arise in nature these variations are called chromosomal aberrations.
They may be grouped into two broad classes
1) Structural and
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2) Numerical
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Structured chromosomal aberrations:
In this class, included those chromosomal aberrations which alter the chromosome structure i.e. the number, the sequence or kind of genes present in chromosome (s) and ordinarily do not involve change in chromosome number.
Origin of structural aberrations:
All structural aberrations are produced following chromosome breakage. Chromosome breakage occurs spontaneously in a low frequency (Ca. 1% of the cells studied) in almost all tissue studied. The causes are not definitely known but possible factors have been suggested.
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1) Cosmic radiation
2) Nutritional deficiencies
3) Environmental condition
The frequency of spontaneous chromosome breakage is modified by several factors e.g. age, oxygen availability, temperature and metabolic stage of cell.
Types of chromosomal aberrations:
There are four common types of structural aberrations:
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1) Deletion
2) Duplication
3) Inversion
4) Translocation
1) Deletion:
Loss of a chromosome segment is known as deletion or deficiency.
When the missing segment contains a telomere of the affected chromosome, it is called as terminal deletion.
However, the missing segment does not contain a telomere such deletions are known as interstitial or intercalary deletion.
2) Duplication:
The presence of an additional chromosome segment as compared to that normally present in a nucleus is known as duplication.
I) Reversed Tandem:
Added piece is in the reverse order.
II) Displaced Tandem:
Added piece is displaced and inserted into a displaced chromosome.
Tandem duplication is associated with bar eye phenotype in Drosophila. The normal male has 16 A region one, bar male twice and double bar thrice. The occurrence is due to unequal crossing over.
Duplications are more frequent in nature and are less lethal to individuals than deficiencies.
3. Inversion:
It is observed in Tradescantia and Drosophila occasionally, a portion of a chromosome will break out to produce deletion and the broken portion will become reattached in its original position but with two strands reverted.
Thus inverted chromosome is one in which a sequence of genes in piece of chromosome has become rearranged in reverse order. There are two types
Paracentric:
It is more common form of inversion in which inverted segment does not include centromere and which is confined to a single arm of a chromosome.
In salivary gland chromosome during pachytene, inversion loop is formed and all portions of two chromosomes synapse in a homologous fashion.
In anaphase I Inversion Bridge is observed within acentric fragment. In inversion, crossing over results in formation of acentric and dicentric chromatids which are abnormal.
The former will be lost while later will be broken by cell wall. In Drosophila female, it reduces egg viability.
I. Pericentric:
It includes centromere. The pairing of homologous chromosomes with identical inversion will undergo normal meiotic pairing but when only one chromosome undergoes inversion which having a characteristic appearance.
However, the products of crossing over and configurations at subsequent stages differ. In this case two of the four chromatids will have deficiencies and duplications. But no dicentric bridge or acentric fragment will be observed.
Due to inversion the gametes formed may lack one or more genes due to loss of portion and become nonviable or un-functional and sometimes sterile, thus changes viability of the gametes.
II. Translocation:
It results due to transfer of a segment of a chromosome to a different part of the same chromosome or to a different chromosome which may be between homologous chromosomes or nonhomologous chromosomes. Homozygous translocation behaves as normal chromosomes from which they arise.
Homozygous translocations exhibit normal meiosis but heterozygous translocations shows cross-like configuration of pachytene which often opens out into a ring as chiasmata terminalize.