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Variation and Mutation

Variation and Mutation:
As we know offspring are not exact copies of their parent. This aspect of differing from parent is called Variation. New combination of genes gives rise to variation. The process responsible for creating variation is called recombination.

Variation occurs due to

  • Any change in the number of chromosomes
  • Any structural change in individual chromosomes
  • Any change in the constitution of genes.

Mutation:
If any sudden change occurs in a hereditary character, then it is called as mutation. The term ‘Mutation” was first used by De Vries. In short, Mutation is a change in DNA material which contains hereditary characters. Mutation may result into an observable character or may be a functional, developmental or behavioral change.

Importance of Mutation:

  • Only process due to that variation in heritable character arises.
  • Evolution of species becomes possible.

Types of Mutation:
  1. Spontaneous Mutation: Mutation occurs spontaneously.
  2. Induced Mutation: Mutation occurs by various chemicals and radiations.
  3. Inherited Mutation: Mutation occurs in reproductive cells.
  4. Somatic Mutation: Mutation occurs in somatic cells and limited to that individual only.

Numerical mutations in chromosomes:
Number of chromosomes is fixed in each species and is denoted as ‘n’. Generally chromosomes occurs in pairs and this paired number is diploid (=2n).
Example: In Human, n is 23 and 2n means 23 pairs.


Polyploidy:
The increase in number of chromosomes in a multiple of the basic number n is called Euploidy or Polyploidy. It is very common in plants. Most of the cultivated crops are developed by polyploidy method because it produces larger leaves and flowers and heavier fruits and seeds.


Types of Polyploidy:
1.  Auto polyploidy: If added sets of chromosomes are of the same species, it is called auto polyploidy. Triploids, Tetraploids and Hexaploids are the example of Auto polyploidy. It occurs by change in chromosome behavior during meiosis.
2.  Allopolyploidy: If the added sets of chromosomes are from a different species, it is called allopolyploidy. More frequently occurs in self compatible species.



Aneuploidy – Abnormal number of chromosome:
There is a change in the number of members in individual homologous pairs of chromosomes that means instead of two members in a homologous pair of chromosomes, there may be only one or nil or three or four members. It is a cytogenetic abnormality. It causes disturbance in the balance of gene on chromosome and becomes incompatible for life.

Types of Aneuploidy:
  1. Monosomy (2n-1) - one chromosome: Only one member in the pair of homologous chromosomes. Turner’s syndrome is the example of Monosomy. It is fatal during prenatal development.
  2. Nullisomy (2n-2): Nil members or totally missing the pair of homologous chromosomes. It is a fetal condition in diploids and grows less vigorously.
  3. Trisomy (2n+1): Three members in a pair of homologous chromosomes. Down syndrome is the example of trisomy.
  4. Tetrasomy (2n+2): Four members in a pair of homologous chromosomes.

Structural abnormalities in chromosomes:
A chromosome contains fixed number of genes located in a fixed position and in a definite sequence. It is necessary to maintain this sequence. Any change in this sequence is called as abnormality. That is of four kinds.

  1. Deletion: Due to any region if any part of chromosome is separated, genes on that part are deleted. Deletion may be terminal or intercalary.
  2. Duplication: A part of chromosome occurs twice or duplicate and occurs during replication of genetic material. Duplication occurs either by maintaining same sequence of genes or by inverted sequence.
  3. Inversion: Sometimes chromosome cuts into two pieces and separated pieces rotates at 180° and rejoin with original one by altering the sequence. This is call inversion.  
  4. Translocation: A part of chromosome becomes separated and regions with another chromosome which is not its homologous chromosome. This is called Translocation.

This is all chromosomal abnormalities which do not produce new genes. Constitution of gene remains same. So, it is not considered as a true mutation.        

Gene Mutation:
A change in the constitution of gene is called gene mutation. During the process of replication of DNA, a change occurs in the sequence of nucleotides in a gene and causes gene mutation. A change in the sequence of nucleotide occurs by adding one or more nucleotide or by losing nucleotide or by replacing normal nucleotide with different one. In all such cashes, genetic codons alters and becomes senseless. Hence specific protein is not formed which alters the action of specific enzyme and produce a mutated organism develops.

Self study:
Example-1: Any change in number of chromosome is called ……………
a)  Variation
b)  Inheritance

c)  Mutation
d)  All of above

Answer: Any change in number of chromosome is called mutation.


Example-2:  Match the following

A B
Polyploidy Process responsible for variation
Aneuploidy Change in the constitution of gene
Gene mutation Increase in number of chromosomes
Recombination Change in the number of members of homologous chromosomes

Answer:

A B
Polyploidy Increase in number of chromosomes
Aneuploidy Change in the number of members of homologous chromosomes
Gene mutation Change in the constitution of gene
Recombination Process responsible for variation


Example-3: Which of the following in not considered as a structural abnormalities in chromosomes?
a)  Monosomy
b)  Deletion
c)  Duplication
d)  Inversion

Answer: Monosomy is not considered as a structural abnormality in chromosomes but is a kind of mutation in which only one member is present in the pair of homologous chromosomes.


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