Mutation is a sudden inheritable variation.
Term given by Hugo de Vries while working on the plant Oenothera lamarckiana (Evening Primrose).
1st experimental evidence on mutation was given by Morgan while working on eye color in Drosophila. White eye color is due to a recessive mutation having occurred on the X-chromosome.
Mutations are of 2 types: – Chromosomal mutations and Gene mutations.
It includes chromosomal aberrations and genomic mutations.
Chromosomal aberrations – change in chromosome structure.
They are 4 types: Deficiency, Duplication, Translocation, and Inversion.
It is the loss of certain genes from a haploid genome. It may be terminal or interstitial. E.g.,
Notched wing in Drosophila is due to deletion of a segment in X-chromosome.
Cri-du-chat syndrome in humans is due to deletion of a part of the short arm of chromosome 5.
Certain genes are present twice in a haploid genome.
The duplicated genes are attached to its homologous chromosome or to a non-homologous chromosome. E.g.,
Bar eye Drosophila is due to B gene for the normal eye being present in duplicates in the X-chromosome.
Means position of certain genes has been changed in a haploid genome.
It is of 2 types: simple and reciprocal.
In reciprocal translocation exchange of segments occur between non-homologous chromosomes.
Also referred as illegitimate crossing over.
Complications occur during meiotic pairing.
E.g., Chronic myeloid leukemia is caused due to translocation of a part of chromosome 22 to chromosome 9.
An in-between part of a chromosome is attached in reverse order.
Inversion prevents crossing over.
Genomic mutations – change in chromosome number.
It is of 2 types: Aneuploidy and Euploidy.
Aneuploidy is addition or loss of chromosomes.
It occurs due to non-disjunction of chromosomes.
Monosomic: (2n-1) one chromosome is without its homologue. E.g., Turner’s Syndrome in human female (44 + X).
Nullisomic: (2n-2) complete pair is missing. They do not survive.
Trisomic: (2n +1) one chromosome is in triplicate.
E.g., Down’s Syndrome in man, chromosome 21 in triplicate.
Klinefelter’s Syndrome in human male (44 + XXY).
Patau’s Syndrome in man, chromosome 13 in triplicate.
Euploidy: increase or decrease in the number of genomes.
It is of 2 types haploidy and polyploidy.
Haploidy: only one genome instead of two.
Polyploidy: more than 2 genomes are present. It is very common in plants. It is of 2 types: autopolyploid and allopolyploid.
Autopolyploid: it is increasing in the number of the same genome.
E.g., banana is Autotriploid (AAA)
Allopolyploid: done through hybridization between two species followed by doubling of chromosomes.
E.g., Triticum aestivum (AABBDD),
Triticale produced from wheat and rye.
Changes occurring within a gene.
It includes substitution mutation and frame shift mutation.
Substitution: one nitrogenous base replaces the other.
It is of 2 types:
Transition: purine replaces purine and pyrimidine replace pyrimidine.
Transversion: purine replaces pyrimidine and pyrimidine replace purine.
e.g., the gene for Sickle Cell Anaemia in which 6th amino acid of β-chain of Haemoglobin. Glutamic acid is replaced by valine. As GAG (glutamic acid) is changed into GUG (valine) due to substitution.
Addition/deletion of one or two bases shifts the reading of the genetic code from the point of addition /deletion.
Due to which the codon sequence changes and hence complete sequence of amino acid in a polypeptide chain changes.
E.g., Cystic fibrosis has occurred due to deletion of 3 nucleotides.
Missense mutation: any mutation due to which a single amino acid change occurs.
Nonsense mutation: any mutation due to which a codon coding for an amino acid changes into termination codon.
Silent mutation: any mutation that does not have any effect on the organism.