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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# Types of mutations

A mutation is a change in the DNA sequence; depending on where and how it happens it may do nothing, change one amino acid or ruin the whole protein.

Updated 2026-11-01

## Key facts

- Silent: changes the codon but not the amino acid (code degeneracy).
- Missense: changes one amino acid. E.g. HbS, Glu6Val in β-globin.
- Nonsense: creates a premature stop codon → truncated protein or degraded mRNA.
- Frameshift: insertion or deletion not a multiple of 3 → everything after it changes. E.g. Duchenne.
- Others: splice-site (β-thalassemia), trinucleotide repeat expansion (Huntington, fragile X), large deletions and rearrangements.

## In detail

- Transition: purine for purine or pyrimidine for pyrimidine (A↔G, C↔T). Transversion: purine for pyrimidine.
- In-frame deletions (multiple of 3) remove amino acids without shifting the frame: they often give partly working proteins (Becker dystrophy, cystic fibrosis ΔF508).
- Loss of function: usually recessive (one healthy copy is enough). Gain of function or dominant-negative effect: dominant.
- Germline (heritable, in every cell) versus somatic (in one tissue only; the basis of most cancers). Mosaicism: a mix of cell lines.
- Mutagens: UV light (pyrimidine dimers), ionising radiation (breaks), alkylating agents, base analogues, intercalators (frameshifts).
- Polymorphism: a common variant (> 1% of the population), usually without disease effect.

**Also called**
Genetic variants

## Clinical notes

- Sickle cell disease is the classic missense example; β⁰-thalassemia comes from nonsense or splice mutations.
- Whether a deletion keeps the reading frame largely explains why Becker dystrophy is milder than Duchenne.
- Repeat-expansion diseases show anticipation: the disease starts earlier and is more severe in each generation.

## Related conditions

- [Sickle cell disease](https://durgical.com/en/conditions/sickle-cell-disease/)
- [Thalassemias (α and β)](https://durgical.com/en/conditions/thalassemias/)
- [Duchenne muscular dystrophy](https://durgical.com/en/conditions/duchenne-muscular-dystrophy/)
- [Huntington disease](https://durgical.com/en/conditions/huntington-disease/)
- [Fragile X syndrome](https://durgical.com/en/conditions/fragile-x-syndrome/)

## Related

- [DNA repair](https://durgical.com/en/biochemistry/topics/dna-repair/)
- [Translation and the genetic code](https://durgical.com/en/biochemistry/topics/translation-and-the-genetic-code/)
- [RNA processing (cap, poly-A tail and splicing)](https://durgical.com/en/biochemistry/topics/rna-processing/)
- [Trinucleotide repeat expansion disorders](https://durgical.com/en/biochemistry/topics/trinucleotide-repeat-expansion-disorders/)
- [Hemoglobin variants (HbA, HbA2, HbF, HbS, HbC)](https://durgical.com/en/biochemistry/topics/hemoglobin-variants/)

## Test yourself

Think of the answer, then tap the question to see it.

## Why is a 3-nucleotide deletion usually milder than a 2-nucleotide one?

Deleting 3 removes one amino acid and keeps the reading frame; deleting 2 shifts the frame and changes the rest of the protein.

## What kind of mutation produces haemoglobin S?

A missense mutation: codon 6 of β-globin changes from glutamic acid to valine.

## What is genetic anticipation?

Earlier onset or greater severity in successive generations, typical of trinucleotide repeat expansions.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [What is a gene variant and how do variants occur?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/mutationsanddisorders/genemutation/) (MedlinePlus, US National Library of Medicine, 2026); [What kinds of gene variants are possible?: MedlinePlus Genetics](https://medlineplus.gov/genetics/understanding/mutationsanddisorders/possiblemutations/) (MedlinePlus, US National Library of Medicine, 2026); [Frameshift Mutation](https://www.genome.gov/genetics-glossary/Frameshift-Mutation) (National Human Genome Research Institute (NHGRI), 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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