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Explain in detail changes in chromosomal numbers.

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Change in the number of chromosomes.
It is of two types:
(1) Euploidy or Polyploidy:

  • The total set of a number of chromosomes in cell or organism is called monoploidy. The gaining of one or more complete sets of chromosomes is called euploidy.

This can be of two types –
(a) Autopolyploidy:

  • It is a type of euploidy, where the additional set of chromosomes is derived from a parent or identical parental species.
  • For example Triploid, Tetraploid, Pentaploid etc. Colchicine which is isolated from the corn of Colchicum autumnale has an ability to destroy spindle fibres during cell division so that the chromosomes do not move to the respective poles and ultimately chromosome number gets double.

(b) Allopolyploidy:

  • In this type, more than two haploid sets of chromosomes that are dissimilar and derived from different species are added.
  • Example: Raphnobraccia (2n = 36). Russian scientist GD. Carpichako(1927) make a cross between Raphanus sativus (Radish) and Brassica oleracea (Cauliflower);. It is sterile. Triticale is a cross product of Triticum (wheat) and Secale (Rai) is also used as man-made cereal “Triticale” and is commercially exploded all over.

(2) Aneuploidy:

  • Aneuploidy is the addition or loss of one or more chromosomes to the complete diploid chromosome complement of an organism. It is caused by nondisjunction of the chromosome during segregation of chromosomes.

It is of two types –
(a) Hypoploidy:

  • Hypoploidy is the loss of one or more chromosomes from the diploid genome. If there is a loss of one chromosome (2n – 1), it is called monosomy and if the loss of pair of chromosome (2n – 2), it is called Nullisomy.

(b) Hyperploidy:

  • Addition of one or more chromosome it is called Hyperploidy.
  • Excess of one chromosome is called trisomy.
  • Example: Down’s syndrome or 21st trisomy or Mongolism or Mongoloid Idiocy. (2n + 1)
  • Excess of two chromosomes is called as tetrasomy (2n + 2).

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