Biology · Ch 4 — Principles of Inheritance and Variation
Mutation
Mutation
A mutation is any change in the DNA sequence of an organism. This alteration can affect both the genotype (the genetic makeup) and the phenotype (the observable traits). While recombination shuffles existing genes to create variation, mutation is the original source of new variation — it actually changes the DNA itself.
Types of Mutations Based on Scale
Mutations can be broadly classified into two categories based on how much of the DNA is affected.
1. Chromosomal Aberrations (Large-scale changes)
Each chromatid is a single, continuous DNA helix that runs from one end to the other in a highly supercoiled form. If a segment of this DNA is lost or gained, the structure of the entire chromosome is altered. Since genes are located on chromosomes, these structural changes lead to abnormalities or aberrations in the organism.
- Deletion: Loss of a segment of DNA from a chromosome.
- Insertion/Duplication: Gain of an extra segment of DNA.
These chromosomal aberrations are commonly observed in cancer cells.
2. Point Mutations (Small-scale changes)
A mutation can also arise from a change in just a single base pair of DNA. This is called a point mutation.
The classic textbook example of a point mutation is sickle cell anemia. In this disorder, a single base-pair change in the gene for haemoglobin results in the substitution of one amino acid (valine) for another (glutamic acid), altering the shape of the red blood cells.
3. Frame-shift Mutations
When base pairs are either deleted or inserted into the DNA sequence, the reading frame of the genetic code is shifted. This is known as a frame-shift mutation. Because the genetic code is read in triplets (codons), adding or removing a base changes every codon from that point onward, often producing a completely non-functional protein.