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D1.3.8 (HL)—Gene knockout

Gene knockout makes a target gene non-functional so researchers can infer gene function from the resulting phenotype in model organisms.

Syllabus
First assessment 2025
Objective
D1.3.8
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2025
Most common paperPaper1A
Typical marks1

Recent exam appearances

November 2025Paper1A ["HL"] · TZ311[ 1 ]D1.3.8 (HL)—Gene knockout
May 2025Paper1A ["HL"] · TZ211[ 1 ]D1.3.8 (HL)—Gene knockout
Practice this objective

Coverage 2025–2025 · Updated 16 Jul 2026

Gene Knockout Tests What a Gene Does

HL only

Gene knockout investigates gene function by changing a chosen gene so that it becomes inoperative.

Researchers compare the knockout phenotype with an appropriate control. A consistent difference suggests that the disabled gene contributes to the affected process, while rescue or other controls strengthen the inference.

Libraries of knockout organisms are available for some research species, allowing scientists to study many genes systematically. Common model groups include mice, fruit flies, zebrafish and Arabidopsis plants.

If p53-knockout mice develop tumours and are then used to test anti-cancer treatments, the knockout model connects loss of the tumour-suppressor gene with the cancer phenotype.

Students do not need the technical steps used to create a knockout. A missing phenotype also does not prove that the gene has no function, because another gene may compensate.

Gene knockout

HL only

Assessment in practice

1 marks
How it is assessed

This objective is assessed through multiple choice.

What earns marks

Build the answer around this relationship: Gene knockout deliberately makes a specific gene inoperative.

Representative question

Question 1

[Maximum number: 1]

What is gene knockout used for?

A

Increasing protein production by editing a gene

B

Investigating the function of a gene by replacing it to make it inoperative

C

Identifying the presence of a gene by editing it to produce a different protein

D

Editing a gene to initiate cell death

HL Gene Editing Evidence

HL only

Gene knockout makes a specific gene non-functional to investigate phenotype; model organisms such as mice, Drosophila, zebrafish, and Arabidopsis support KO libraries. Guide RNA directs Cas9 to a complementary DNA target sequence; Cas9 cutting enables deletion, replacement, insertion, or gene disruption. Conserved sequences remain similar across species or long evolutionary times; conservation suggests essential function, lower mutation rate, or strong purifying selection.

Concept essentials

  • Gene knockout deliberately makes a specific gene inoperative.
  • The phenotype of a knockout organism helps reveal gene function.
  • Knockout models can be used to study disease processes.
  • A p53 knockout mouse can support cancer research involving tumour growth or treatment testing.