Mutations and Their Consequences

Point mutations: one base changed

• Silent — the new codon still codes for the same amino acid. The genetic code is redundant, so many substitutions cost nothing.
• Missense — a different amino acid. The effect ranges from none to fatal depending on where it sits: a change deep in the active site or in a folding core matters far more than one on the surface.
• Nonsense — a stop codon appears early, so translation ends and the protein is truncated. Usually severe.

Insertions and deletions

Adding or removing bases in a number not divisible by three shifts the reading frame. Every codon downstream is misread, so almost the whole protein is wrong — a frameshift is generally far more damaging than a substitution. Add or remove exactly three bases and the frame survives; one amino acid is gained or lost and the rest of the protein is untouched.

Where the mutation sits decides who is affected

• In a coding sequence — changes the protein itself.
• In a regulatory sequence (promoter, enhancer, operator) — the protein is normal but is made in the wrong amount, the wrong tissue, or at the wrong time. Many disease alleles and many evolutionary changes are of this kind.
• In a somatic cell — affects only that individual, as in most cancers.
• In a germ-line cell — heritable, and therefore the only kind that matters to evolution.

Mutation is the only source of genuinely new alleles. Meiosis and sexual reproduction shuffle existing variation but create none. Whether a new allele is harmful, neutral or beneficial depends on the environment, not on the mutation itself.