Polyploidy

Polyploidy in Organisms

In an organism all the characters are controlled by genes. All these genes are arranged in one set of chromosomes. If the nucleus contains only one set of chromosomes, it is called haploid state. The total complement of genes present in one set of chromosomes will be called Genome. If the cell contains two sets of chromosomes it is called Diploid. If an organism contains more than two sets of chromosomes it is called Polyploidy. Polyploidy plants may show.

Triploid state (3x)

Tetraploid state (4x)

Pentaploid state (5x)

Hexaploid state (6x)

Polyploidy is very common in plants.

Ex .Wheat variety

Triticum monococcum.2x (14 Chromosomes)

Emmer wheat - 4x (28 Chromosomes)

Triticum vulgare - 6x (42 Chromosomes)

Ex. Cotton plant.

Gosipium herbacium - 2x (26 Chromosomes)

Gosipium hirsutum -4x (52 Chromosomes) .

(Thus ploidy is reported in many cultivated plants).

On the basis of the source of chromosomes two types are recognised

 

a) Autopolyploidy  b) Allopolyploidy.

 

Autopolyploidy In this polyploidy condition, all the sets of chromosomes will come from same species.

The mutant verities of Oenothera are really autopolypboidy plants. In Oenothera 4x condition is seen. It contains 28 chromosomes. Because of this tetraploid condition, it becomes tall and its vegetative parts are thick and big. Tetraploidis shows aggressive characters. They can grow in different environment. They can grow in less favourable areas.

 

Allopolyploidy : It is common than autopolyploidy. In nature allopolyploidy can be seen. In this polyploidy plants, the genomes will come from different species. Such a plant is called allopolyploid.

 

Aneuploldy In polyploidy the set of chromosomes will increase.

But in some individuals one are two chromosomes will increases or decreases in genome. It is called heteroploidy or Aneuploidy. Such condition is obtained from diploid individuals.

a) Monosomic : In the diploid number of chromosomes one chromosome will be less. It is called monosomic. They are common. (2n -1)

ex .In Nicotiana tobacum now and then monosomics are obtained.

Ex. In human beings Turners syndrome is good example. He shows 45 chromosomes.

 

6) Nullisomic : In this diploid individuals one homologous pair of chromosome will be ‘missing. Such individuals are sterile (2x - 2)

c) Trisomic: Now and then one chromosome will be extra (2x -1)

ex. Klinefleter,s syndrome of man (47 chromosomes) Trisomics are unbalanced organisms.

  

c) Tetransoinice In a diploid organism if one homologous chromosomal pair is added, it is called tetrasomic. (2x + 2)

Thus if the number of chromosomes of genome will be changed it is called Aneuploidy. Because of non-disjunction of chromosomes during meiosis aneuploidy is developed.