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Describe the structural changes in chromosomal mutation.

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Chromosomal mutation or Chromosomal aberration or Numerical changes in chromosomes:

Any change in gene arrangement or insertion of a gene or deletion of a gene then these changes are known as Chromosomal Mutation. 

They are of two types:

  1. Change in the number of chromosomes
  2. Change in the structure 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).

2. Change in the structure of chromosomes:

  • These mutations result in structural changes in the chromosome. Hence, the original structure of the gene is changed.
  • They involve changes either in the total number of genes or gene loci in a chromosome or their rearrangement.

They are of 4 types –
(a) Deletion:

  • It is the simplest kind of chromosomal aberration.
  • The deletion is loss of chromosomal segment.
  • Terminal deletion refers to the loss of a segment from one or the other end of the chromosome.
  • Intercalary of Interstitial deletion involves an intercalary segment of the chromosome with the reunion of the terminal segment.

(b) Translocation:

  • It is a type of chromosomal aberration.
  • A chromosomal segment goes to another chromosome (Unilateral) or normally, exchange of chromosomal segment (genes) between two non-homologous chromosomes (Bilateral).
  • Sometimes there is a change in the expression of the gene due to Position effect.

(c) Inversion:

  • Some times the number of genes in a chromosome does not change but the sequence of genes is altered by the rotation of the chromosomal segment within the chromosomes by 180°. Due to these change, genotypes remains unchanged but phenotypic characters change.

(d) Duplication:

  • Sometimes, some genes are present more than once in a chromosome i.e. one, segment repeats twice in a chromosome. It is called duplication.

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