Unit 3
Genetics
Chapter 3.4
Mutation and PolyploidyClass 12 Biology – Mutation and Polyploidy Notes PDF
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Introduction
Mutations may alter a gene, a chromosome segment or chromosome number. They create new genetic variation; polyploidy is especially important in plant evolution and crop breeding.
1. Mutation
General Characteristics
- May arise spontaneously or be induced by mutagens.
- Can occur in somatic cells or germ-line cells.
- May be beneficial, harmful or neutral.
- Creates new alleles and new genetic variation.
- Heritable effects require transmission through reproductive cells or clonal propagation.
Fig. 1 – Levels of Genetic Mutation
2. Gene Mutation
A gene mutation changes the nucleotide sequence of DNA.
Substitution
One nucleotide/base pair is replaced by another.
Insertion
One or more nucleotides are added to the sequence.
Deletion
One or more nucleotides are removed from the sequence.
Frameshift
An insertion or deletion not in multiples of three may shift the reading frame of a coding sequence.
Fig. 2 – Simple Gene Mutations
3. Structural Chromosomal Mutations
| Type | Change |
|---|---|
| Deletion | Loss of a chromosome segment. |
| Duplication | A chromosome segment is repeated. |
| Inversion | A segment breaks, reverses orientation and rejoins. |
| Translocation | A segment moves to another position, often involving a non-homologous chromosome. |
Fig. 3 – Major Structural Chromosome Changes
4. Mutagens
Physical Mutagens
Examples include ionizing radiation and ultraviolet radiation.
Chemical Mutagens
Various chemicals can alter bases, DNA structure or replication.
Biological Causes
Some mobile genetic elements and biological agents can disrupt DNA sequences.
Spontaneous Mutation
Can arise naturally from replication errors or spontaneous chemical changes in DNA.
5. Polyploidy
Polyploidy is common in plants and can arise through failure of chromosome separation, formation of unreduced gametes or chromosome doubling.
Fig. 4 – Diploid and Polyploid Chromosome Sets
6. Types of Polyploidy
Autopolyploidy
Multiple chromosome sets originate from the same species.
Allopolyploidy
Chromosome sets originate from different species, typically after hybridization followed by chromosome doubling.
Fig. 5 – Formation of an Allopolyploid
7. Aneuploidy vs Polyploidy
| Feature | Aneuploidy | Polyploidy |
|---|---|---|
| Change | Gain or loss of individual chromosome(s) | Gain of complete chromosome set(s) |
| Examples of notation | 2n + 1, 2n – 1 | 3n, 4n, 6n |
| Common cause | Nondisjunction | Unreduced gametes or chromosome doubling |
| Importance in plants | May produce abnormal phenotypes | Major role in speciation and crop improvement |
Fig. 6 – Nondisjunction and Abnormal Chromosome Number
8. Significance of Mutation and Polyploidy
- Mutations generate new alleles and genetic diversity.
- Genetic variation provides raw material for natural selection and evolution.
- Some mutations are useful in breeding; others are harmful or neutral.
- Polyploidy can increase cell and organ size in plants.
- Polyploidy can contribute to reproductive isolation and speciation.
- Allopolyploidy can restore fertility to some interspecific hybrids.
9. Quick Revision & Exam Points
- Define mutation and mutagen.
- Differentiate gene and chromosomal mutations.
- Explain substitution, insertion and deletion.
- Explain deletion, duplication, inversion and translocation.
- Define polyploidy.
- Differentiate autopolyploidy and allopolyploidy.
- Differentiate aneuploidy and polyploidy.
- State the significance of mutation and polyploidy.
- Mutation = heritable change in genetic material.
- Substitution changes one base.
- Insertion/deletion may cause frameshift.
- Deletion removes a chromosome segment.
- Duplication repeats a segment.
- Inversion reverses a segment.
- Translocation moves a segment.
- Polyploidy = more than two complete sets.
- Autopolyploid = same-species sets.
- Allopolyploid = sets from different species.
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