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  Numerical Chromosomal Aberrations

Introduction to Numerical Chromosomal Aberrations

Every species of an organism has a particular number of chromosomes in the somatic cells of it.  These chromosomes are initiated in pairs. At the time of creation of gametes the chromosome number is decreased. Therefore, the gemetes take haploid set of chromosomes. Changes in the number of chromosomes from the diploid set are known as numerical chromosomal aberration. It is as well termed as ploidy.  There are two kinds of ploidy. They are as foolow:

1. Euploidy and

2. Aneuploidy.

Euploidy

Euploidy is the difference in the chromosome number that takes place because of increase or decrease of full set of chromosomes. There are three types of it:

1. Monoploidy,

2. diploidy and

3. polyploidy

Diploidy

In several plants and animals, the somatic cells comprise two sets of chromosome. Diploidy is created through the union of two gametes during fertilization.

Polyploidy

In addition of one or more than one sets of chromosomes to the diploid set results in polyploidy. It is generally noticed in plants and infrequent in animals.  They are of two types. They are as follow:

1. autopolyploidy and

2. allopolyploidy.

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Ploidy - flow chart

Autopolyploidy

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

Allopolyploidy

Raise in one or more haploid set of chromosomes from two dissimilar species result in allopolyploidy. Triticale is the first man prepared cereal. It is acquired through crossing a wheat Triticum durum (2n = 4x = 28) and a rye Secale cereale  (2n = 2x = 14). The F1 hybrid (2n = 3x = 21) is sterile. After that the chromosome number is doubled by using colchicine and it becomes an hexaploid.

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Aneuploidy

Variation that includes one or two chromosomes in the diploid set of an organism results in aneuploidy. It is of two kinds. They are as follow:

1. hypoploidy and

2. hyperploidy.

Hypoploidy

Decrease in one or two chromosomes from the diploid set is explained like a hypoploidy.  There are two kinds of hypoploidy. They are as follow:

1. monosomy and

2. nullisomy.

Monosomy is because of loss of a chromosome from the diploid set that is 2n - 1. Nullisomy is the state where a pair of homologous chromosomes is lost from the diploid set that is 2n - 2.

Hyperploidy

In Addition of one or two chromosomes to the diploid set of chromosome results in hyperploidy.  There are two categories of hyperploidy. That is as follow:

1. Trisomy and

2. Tetrasomy.

Trisomy results because of the adding up of one chromosome to diploid set of chromosomes. It is presented through 2n + 1. Trisomics are monitored in Datura  stramonium. 

Tetrasomy results because of the addition of two chromosomes to diploid set of chromosome. It is presented through 2n+2.

Significance of ploidy

1. Polyploidy plays a significant role in plant breeding and horticulture.

2. Polyploidy comprise more strong influence than the diploids and results in the production of large sized flowers and fruits.  Therefore, it has economical significance.

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

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

5. Polyploidy are strong invaders of new habitats.

6. Polyploidy leads to the formation of new varieties that depict high resistance to disease and increase in yield.

7. Tetraploid cabbages and tomatoes consist of more ascorbic acid whereas tetraploid corn consists of more vitamins A.

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

9. Polyploidy varieties such as apple,  pear,  grape and watermelons are cultivated due to their large size.

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