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 ¬ 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 Fl 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

Polyploidy plays an important role in plant breeding and horticulture. 0 Polyploidy has more vigorous effect than the diploids and results in
the production of large sized flowers and fruits. Hence, it has economical significance.
It plays significant role in the evolution of new species.
Polyploidy results in the changes in the season of flowering and fruiting.
Polyploids are vigorous invaders of new habitats.
It leads to the formation of new varieties which show high resistance to disease and increase in yield.
Tetraploid cabbages and tomatoes contain more ascorbic acid whereas tetraploid corn contains more vitamin A.
Both euploidy and aneuploidy in man cause congenital diseases.
Polyploidy varieties like apple, pear, grape and watermelons are cultivated because of their large size.