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D1.3.1—Gene mutations

Gene mutations change DNA base sequences through substitution, insertion, deletion or duplication, creating altered alleles and potential protein effects in organisms.

Syllabus
First assessment 2025
Objective
D1.3.1
Level
HL

Exam analysis

Chance of appearing3%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper2
Typical marks1

Common command terms

  • Define
  • Identify
  • Distinguish
  • Compare

Scoring notes

Common mistake
Naming a disease such as sickle-cell anemia instead of naming a mutation type.

Recent exam appearances

November 2025Paper1A ["HL"] · TZ111[ 1 ]D1.3.1—Gene mutations
May 2025Paper2 ["HL"] · TZ27(b)[ 1 ]D1.3.1—Gene mutations
May 2017Paper2 ["HL"] · TZ25(a)(ii)[ 1 ]D1.3.1—Gene mutations
Practice this objective

Coverage 2017–2025 · Updated 16 Jul 2026

A Gene Mutation Changes a DNA Sequence

A gene mutation is a structural change in the base sequence of DNA within a gene.

Mutation Sequence change
Substitution One base is replaced by another
Insertion One or more bases are added
Deletion One or more bases are removed
Duplication A DNA section is copied, producing an extra copy

Changing 5′-ACT-3′ to 5′-AGT-3′ is a substitution; changing it to 5′-ACCT-3′ is an insertion.

A mutation is the DNA sequence change itself. Its effect on a codon, protein or phenotype is a possible consequence, not part of the definition.

Gene mutations

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Define / Identify / Distinguish.

Command terms

Define / Identify / Distinguish / Compare

What earns marks

Build the answer around this relationship: Gene mutations are changes in DNA nucleotide sequence.

Watch for

Naming a disease such as sickle-cell anemia instead of naming a mutation type.

Representative question

Question 1

[Maximum number: 2]

Mutations may increase variation within a species. Compare and contrast substitution and insertion mutations.

Core Mutation Effects

Gene mutations are changes in the base sequence of DNA; main types are substitution, insertion, deletion, and duplication. Base substitutions can create SNPs and change codons; degeneracy can make substitutions silent, missense, or nonsense. Insertions or deletions not in multiples of three cause frameshifts that alter downstream codons and often disrupt protein function. Mutations can arise from replication errors, repair errors, or chromosome damage; mutagens include chemicals, ionizing radiation, and ultraviolet radiation. Mutations occur randomly with respect to organism need or advantage; mutation rate varies with DNA sequence, gene expression, repair, and mutagen exposure. Germ-line mutations can be inherited by offspring; somatic mutations affect only descendant body cells and can contribute to cancer. Mutation is the original source of new alleles and genetic variation; many are neutral or harmful, but variation supplies material for natural selection.

Concept essentials

  • Gene mutations are changes in DNA nucleotide sequence.
  • Substitution, insertion, deletion and duplication are common gene mutation types.
  • Mutation can produce new alleles in a population.
  • Insertion and deletion add or remove bases rather than replacing one base.