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Chapter: 11 th 12th std standard Bio Botany plant tree Biology Higher secondary school College Notes

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Numerical and Structural chromosomal aberrations

In an organism, any visible abnormality in chromosome number or structure from the diploid set is known as chromosomal aberration. The chromosomal aberrations based on the structure of the chromosome are of four types - deletion, duplication, inversion and transversion.

Chromosomal aberrations

 

In an organism, any visible abnormality in chromosome number or structure from the diploid set is known as chromosomal aberration. The chromosomal aberrations based on the structure of the chromosome are of four types - deletion, duplication, inversion and transversion.

 

Structural chromosomal aberrations

 

Deletion

 

The loss of a segment of the genetic material in a chromosome is called deletion. It may be terminal or intercalary. When the deletion occurs near the end of the chromosome, then it is called terminal deletion. Eg. Drosophila and Maize. When the deletion occurs in the middle of the chromosome then, it is called intercalary deletion. Most of the deletions lead to death of an organism.

 

Duplication

 

When a segment of a chromosome is present more than once in a chromosome then, it is called duplication. For example, the order of genes in a chromosome is a, b, c, d, e, f, g and h. Due to aberration, the genes 'g' and 'h' are duplicated and the sequence of genes becomes a, b, c, d, e, f, g, h, g and h. In Drosophila, corn and peas a number of duplications are reported. Some duplications are useful in the evolution of the organism.

 

Inversion

 

It is another chromosomal abnormality in which, the order of genes in a chromosomal segment is reversed by an angle of 180'. For example, the order of genes in a chromosome is a, b, c, d, e, f, g and h. Due to aberration, the sequence of genes becomes, a, b, c, d, g, f, e and h. There are two types of inversion - pericentric and paracentric inversion.

 

In pericentric inversion, the inverted segment of the chromosome contains centromere. Sometimes, it is responsible for evolution of the organism. For example the 17th human chromosome is acrocentric, while in Chimpanzee the corresponding chromosome is metacentric. In paracentric inversion, the inverted segment of the chromosome has no centromere.

Translocation

 

It is a kind of a chromosomal abnormality in which the interchange of the chromosomal segments occurs. When translocation occurs between two non-homologous chromosomes, then it is called reciprocal translocation or illegitimate crossingover. It is of two kinds - heterozygous translocation and homozygous translocation.

 

In heterozygous translocation, one member of each pair of chromosomes is normal and the other member is with interchanged segment. But in homozygous translocation, both the members of paired chromosomes have translocated segments.

 

They play an important role in species differentiation. Translocations causes hereditary disorders.

 

Numerical chromosomal aberrations

 

Each species of an organism has a specific number of chromosomes in its somatic cells. These chromosomes are found in pairs. At the time of formation of gametes the chromosome number is reduced. Hence, the gemetes carry haploid set of chromosomes. Alterations in the number of chromosomes from the diploid set is called numerical chromosomal aberration. It is also known as ploidy. There are two types of ploidy x euploidy and aneuploidy.

 

Euploidy

 

Euploidy is the variation in the chromosome number that occurs due to increase or decrease of full set of chromosomes. Monoploidy, diploidy and polyploidy are the types in euploidy.

 

Diploidy

 

In most of the plants and animals, the somatic cells contain two sets of chromosome. Diploidy is formed by the union of two gametes during fertilization.

 

Polyploidy

 

Addition of one or more sets of chromosomes to the diploid set results in polyploidy. It is commonly noticed in plants and rare in animals. They are of two kinds - autopolyploidy and allopolyploidy.

Autopolyploidy

 

Addition of one or more haploid set of its own genome in an organism results in autopolyploidy. Watermelon, grapes and banana are autotriploids, whereas apple is an autotetraploid.

 

Allopolyploidy

 

Increase in one or more haploid set of chromosomes from two different species result in allopolyploidy. Triticale is the first man made cereal. It is obtained by crossing a wheat Triticum durum (2n = 4x = 28) and a rye

 

Secale cereale (2n = 2x = 14). The F1 hybrid (2n = 3x = 21) is sterile. Then the chromosome number is doubled using colchicine and it becomes an hexaploid.


Aneuploidy

 

Variation that involves one or two chromosomes within the diploid set of an organism results in aneuploidy. It is of two types - hypoploidy and hyperploidy.

 

Hypoploidy

 

Decrease in one or two chromosomes from the diploid set is described as hypoploidy. There are two types of hypoploidy - monosomy and nullisomy. Monosomy is due to loss of a chromosome from the diploid set i.e. 2n - 1. Nullisomy is the condition in which a pair of homologous chromosomes is lost from the diploid set i.e. 2n - 2.

 

Hyperploidy

 

Addition of one or two chromosomes to the diploid set of chromosome results in hyperploidy. There are two types of hyperploidy - trisomy and tetrasomy. Trisomy results due to the addition of one chromosome to diploid set of chromosomes. It is represented by 2n + 1. Trisomics are observed in Datura stramonium. Tetrasomy results due to the addition of two chromosomes to diploid set of chromosome. It is represented by 2n+2.

 

Significance of ploidy

 

1.        Polyploidy plays an important role in plant breeding and horticulture.

 

2.        Polyploidy has more vigorous effect than the diploids and results in the production of large sized flowers and fruits. Hence, it has economical significance.

 

3.        It plays significant role in the evolution of new species.

 

4.        Polyploidy results in the changes in the season of flowering and fruiting.

 

5.        Polyploids are vigorous invaders of new habitats.

 

6.        It leads to the formation of new varieties which show high resistance to disease and increase in yield.

 

7.        Tetraploid cabbages and tomatoes contain more ascorbic acid whereas tetraploid corn contains more vitamin A.

 

8.        Both euploidy and aneuploidy in man cause congenital diseases.

 

9.        Polyploidy varieties like apple, pear, grape and watermelons are cultivated because of their large size.

 

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