10Mutation and Polypoidy

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INTRODUCTION
▢ Genetics:
❖ Topic: Mutation and Polyploidy
❖ Definition: Mutation is a sudden change in phenotype due to a change in the genome of a cell or virus.
❖ Discoverer:
◉ Name: Hugo de Vries
◉ Contributions:
  1. Discovered mutation in Oenothera lamarckiana (Evening primrose)
  2. Coined the term 'Mutation'
  3. Defined mutation as 'Sudden phenotypic changes which are heritable'
    4. ■ Note: Later discovered that mutation in Oenothera lamarckiana was due to polyploidy.
❖ Scientific Study: Given by Morgan
❖ Causes:
◉ Radiations:
◈ Gamma-rays: Produce free radicals
◈ X-rays: Cause deletion
◈ UV-rays: Cause dimer formation
◉ Mutation Effects:
  1. 99.9% mutations are harmful (lethal)
  2. Most common mutations: lethal, followed by recessive
Q1.
The most common type of mutation is
Q2.
Which of the following is correct about mutation?
❖ Stages of Occurrence:
  1. Before formation of gamete
  2. In gamete
  3. In zygote
❖ Importance:
  1. Useful in crop variety development
  2. UV-treated Penicillium strains yield high penicillin
  3. Phenotypic changes due to gene mutation are reversible; due to chromosomal changes are irreversible
  4. Allelic variations arise from mutation
  5. Ultimate cause of variation is mutation; major cause is sexual reproduction
  6. Basic cause of evolution
❖ Types:
  1. Natural
  2. Induced (via mutagens)
❖ Mutagens:
◉ Definition: Agents that cause mutations
◉ Types:
◈ Physical:
  1. Temperature
  2. Radiations
    3. ● Ionizing:
    1. X-rays
    2. γ-rays
    ● Non-ionizing:
    1. UV-rays
◈ Chemical:
  1. Mustard gas
  2. Acridine dye
  3. Colchicine
  4. Nitric oxide (NO)
  5. Maleic hydrazide
  6. Hydroxylamine
  7. Ethyl methane sulfonate (EMS)
◉ Notable Facts:
  1. H. J. Muller first induced mutation in Drosophila using X-rays
  2. H. J. Muller is called father of Actinobiology
  3. X-rays, β-rays used for deep-seated nucleus tissues; UV-rays for superficial ones
  4. First chemical mutagen made by C. Auerbach
❖ Mutation Rate: Natural mutation frequency is very low (1×10⁻⁵ to 1×10⁻⁶)
❖ Types of Mutation:
◉ Forward: Wild type changes to new type
◉ Reverse: Mutant type changes back to wild
❖ Study Material:
◉ Best: Haploids
◉ Model Organisms:
  1. Drosophila (2n = 8)
  2. Neurospora (haploid; called Drosophila of plant kingdom, pink mould)
  3. E. coli
  4. Bacteriophage
◉ Discoveries:
  1. Beadle and Tatum discovered 'One gene one enzyme hypothesis' in Neurospora (Nobel Prize 1958)
  2. Yanofsky: 'One gene one polypeptide' theory
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TYPES OF MUTATION
▢ Gene Mutation / Point Mutation:
❖ Frame Shift Mutation:
◉ Description: Occurs due to insertion or deletion at initial/internal position; affects all downstream codons.
◉ Effect: Most dangerous mutation type.
◉ Examples:
  1. Deletion
  2. Addition/Insertion
❖ Substitution Mutation:
◉ Transition: Purine↔Purine or Pyrimidine↔Pyrimidine
◉ Transversion: Purine↔Pyrimidine
◉ Example: Sickle cell anemia caused by transversion
▢ Chromosomal Mutation:
❖ Structural Changes:
◉ Deletion: Loss of chromosome part
◉ Inversion: Reinsertion in reverse order (180° twist)
◉ Duplication: Part joins homologous chromosome
◉ Translocation: Part joins non-homologous chromosome
❖ Numerical Changes:
◉ Aneuploidy:
◈ Definition: Change in individual chromosome number
◈ Types:
■ Hypoaneuploidy:
● Nullisomic (2n–2): Loss of chromosome pair (non-survivable)
● Double monosomic (2n–1–1): Loss of 2 dissimilar chromosomes
● Monosomic (2n–1): Loss of one chromosome; causes Turner's syndrome (44+XO)
Q1.
If 2n = 14, how many monosomics are possible?
■ Hyperaneuploidy:
● Trisomy (2n+1):
  1. Patau syndrome (Trisomy 13)
  2. Edward's syndrome (Trisomy 18)
  3. Down's syndrome (Trisomy 21)
● Sex Chromosomal Trisomies:
  1. Super female (XXX)
  2. Klinefelter's (XXY)
  3. Super male (XYY)
  4. Double trisomy (2n+1+1)
● Tetrasomy (2n+2): e.g., Super female (XXXX)
◉ Euploidy:
◈ Definition: Change in entire haploid set
◈ Types:
■ Homoploidy: n
■ Polyploidy:
● Autopolyploidy:
⟣ Definition: Multiple sets from same genome
⟣ Examples:
➢ Autotriploid (AAA):
  1. Banana
  2. Grapes
  3. Doob grass
➢ Autotetraploid (AAAA):
  1. Barseem
⟣ Note: Autotriploids are always sterile
● Allopolyploidy:
⟣ Definition: Sets from different species
⟣ Examples:
➢ Allotetraploid (AAA'A'): Raphano brassica
➢ Allohexaploid (AAA'A'A"A"): Triticum spelta (Wheat)
● Autoallopolyploidy: e.g., AAAAA'A' / AAABBB
◈ Special Notes:
  1. Triticale is the first man-made cereal crop
  2. Autopolyploids form multivalents during synapsis
  3. Allopolyploids form bivalents