What is chromosome aberration?
The change in number of chromosome or change in structure of chromosome is nothing but the chromosomal aberration. This chromosomal aberration may occurs during cell division.
What are causes of chromosomal aberration?
In humans, their are 23 pairs of chromosome are present; means total 46 chromosomes.
Chromosomal aberration may occurs due to one or more reasons as following,
- Errors during (meiosis)
- Errors during (mitosis)
- Exposure to substances that cause birth defects (teratogens)
What are 2 types of chromosomal aberration?
Their are mainly two types of chromosomal aberration namely as follows,
- Numerical chromosomal aberration
- Structural chromosomal aberration
Numerical Chromosomal Aberration
The change in the number of chromosomes is called chromosomal numerical aberration. The change may result in either an increase or decrease in the number of chromosomes, that condition is called ploidy.
The ploidy can occur either in the complete set of chromosomes or in the individual chromosomes.
Further numerical aberration is divided into two types named as Aneuploidy and Euploidy.
Aneuploidy is a condition in which there is the addition or deletion of one or more chromosomes in a diploid set of chromosomes.
It causes due to non-disjunction of chromosomes e.g. in humans, 46 chromosome is a diploid condition. If there is 47 or 45 instead of 46, this will be the Aneuploidy.
It was first discovered by Bridges in Drosophila in 1916. Aneuploidy may be hypoploidy (loss of one chromosome from a genome) or hyperdiploid (one or more chromosomes extra in a set of chromosomes).
Their are Three types of Hypoploidy namely,
In this condition, there is one chromosome less in one homologous pair i.e. 2n – 1 condition. The monosomic parent produces two types of gametes like (n) and (n – 1) during gametogenesis. Normally (n – 1) types of gametes die. But if they survive and take part in fertilization, the resulting offspring will have genetic imbalance which leads to reduced fertility of high mortality. e.g. 44A + XO i.e. Turner’s syndrome.
In this condition, a pair of chromosomes is less in the genome i.e. 2n – 2. This condition arises due to the union of two monosomic gametes or from the non-disjunction of chromosomes. Generally, nullisomic conditions do not survive. But their polyploid forms survive which are weak and sterile.
It is a type of hyperdiploid in which there is an increase in one chromosome inset of chromosome i.e. 2n + 1.
Examples of Trisomy,
- Trisomy of 21st chromosome – Down’s syndrome
- Trisomy of 13th chromosome – Patau’s syndrome
- Trisomy of 18th chromosome – Edward’s syndrome
- Trisomy of sex chromosomes – XXY (Klinefelter’s syndrome), XXX (Super female)
The term euploidy is derived from the Greek word (EU = even or true; ploidy = set).
Euploidy is a condition in which an organism possesses one or more full sets of chromosomes.
Euploidy is classified into haploidy, diploidy, triploidy, and polyploidy.
A condition in which an organism contains only one set (n) of chromosomes in its somatic cell is called haploidy or monoploidy.
Haploidy is denoted by ‘n’. It is rare in animals and found in insects like honey bees and wasps and also in rotifers.
The haploidy may be normal or abnormal in a particular species.
E.g. males in honey bees and wasps are haploid with normal features, while in amphibians if it occurs then causes abnormality.
It can be produced in plants but these plants remain small with reducing leaves, weak and sterile e.g. Fungi, Sorghum Datura, etc.
Triploidy is a condition in which an organism contains Three sets (3n) of chromosomes in the nucleus of the body cell.
Triploidy originates from the union of a haploid (n) and diploid (2n) gametes of different strains of the same species.
Such organisms are always sterile. It is studied in some animals like reptiles, man, some cells of birds, and other invertebrates.
However in plants triploidy has economic value and it can be propagated by asexual methods like grafting, budding, etc. It gives fruits of superior quality.
Polyploidy is a condition in which an organism contains more than usual two sets (2n) of chromosomes. Such an organism is called polyploidy.
They may be triploid, tetraploid, pentaploid, and so on. Polyploidy is rare in animals and it results in the sterility of an individual.
Structural Chromosomal Aberration
Their are main four kinds of chromosomal aberrations are as follows,
Deletion or Deficiency
- When there is a loss of some part of a chromosome, it is called deletion or deficiency.
- In this case a chromosome breaks at two places, the broken part gets separated while the two ends of the chromosome join together and give rise to a mutated chromosome.
- There are two types of deletion,
- Intercalary or interstitial deletion – When there is breakage of the middle part of a chromosome with a particular gene, this type of deletion is called an intercalary or interstitial deletion.
- Terminal deletion – When the deletion is at the terminal end of the chromosome, it is called a terminal deletion. The loss of part of chromosome with genes affects the organism adversely. If deletion takes place in homozygous chromosomes, some characters are completely lost resulting in the death of an organism.
Examples of Deletion,
In a human being deletion in the 5th pair of homologous chromosomes results in a condition called cri – du – chat syndrome. A person is physically retarded and produces a sound like the cry of a cat, hence the name cat’s cry or cri – du – chat.
- Some time deleted portion of the chromosome attaches to another chromosome at the centromere resulting in the duplication of a part of the chromosome.
- This condition is called duplication or addition. This extra part of the chromosome behaves like an independent chromosome.
- A gamete with such a duplicated chromosome receives extra genes in a duplicated form which results in the formation of new species.
- Hence it is important in evolution.
- In this process chromosome breaks at two points and the central piece get detached. The broken piece then re-attaches at its original position with the two ends reversed.
- Fusion of the gamete carrying inversion with normal gamete does not show a visible phenotypic change in offspring.
- But during a subsequent generation, it leads to cytological abnormalities. Hence inversion is important in evolution. Inversion is of two types,
- Pericentric inversion – When the centromere is in the region of inversion, it is called pericentric inversion. Such inversion results in the dicentric chromosome.
- Paracentric inversion – It is an inversion in which the centromere is not involved.
- These are changes in the arrangement of genes in the chromosome.
- There is no change in the quality or quantity of genes, only the rearrangement of genes occurs.
- There is no change in phenotypic characters of an individual due to translocation, only change occurs in the position of genes.
There is shifting of a part of one chromosome to another nonhomologous chromosome.
Translocation is of two types,
- Homozygous Translocation – Exchange of linkage group of the genes with homologous counterpart is called homozygous translocation.
- Heterozygous Translocation – It is a change of linkage groups due to the exchange of genes between different chromosomes. A segment of a chromosome is shifted to a non-homologous chromosome.
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FAQs – Chromosomal Aberration
Q. What is chromosome aberration?
Ans: The change in number of chromosome or change in structure of chromosome is known as chromosome aberration.
Q. What are the 4 types of chromosomal aberrations?
Ans: Deletion, Duplication, Inversion, Translocation are main four types of structural chromosome aberration.
Q. Which is the most common chromosomal aberration?
Ans: The most common type of chromosomal aberration is aneuploidy. This occurs due to change in number of chromosome.