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D2.2.10 (HL)—Monozygotic twin studies

Monozygotic twin studies separate shared genome effects from environmental and epigenetic differences that accumulate over time in individuals and tissues.

Syllabus
First assessment 2025
Objective
D2.2.10
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2021
Most common paperPaper3
Typical marks1–3

Common command terms

  • Identify
  • Compare
  • Analyse

Scoring notes

Common mistake
Using only numerical values without comparing identical and non-identical twins.

Recent exam appearances

November 2021Paper1 ["HL"] · TZ027[ 1 ]D2.2.10 (HL)—Monozygotic twin studies
November 2013Paper3 ["HL"] · TZ04(c)[ 3 ]D2.2.10 (HL)—Monozygotic twin studies
November 2013Paper3 ["HL"] · TZ04(b)[ 3 ]D2.2.10 (HL)—Monozygotic twin studies
November 2013Paper3 ["HL"] · TZ04(a)[ 1 ]D2.2.10 (HL)—Monozygotic twin studies
Practice this objective

Coverage 2013–2021 · Updated 16 Jul 2026

Monozygotic Twins Separate Genetic and Environmental Effects

HL only

Studies of monozygotic twins compare genetically similar individuals to estimate how environment and epigenetic differences contribute to traits.

If twins differ despite near-identical DNA, differing environments, developmental history or epigenetic states are possible explanations. Concordance and study design determine the strength of inference.

Evaluate a twin result by checking: shared genes; shared environment; age and exposure; trait concordance; alternative causes.

Twins may both inherit risk alleles but develop different symptoms after different exposures, suggesting environment modifies expression or phenotype.

Twin differences do not prove a purely environmental cause; measurement error and non-shared biology also matter.

Monozygotic twin studies

HL only

Assessment in practice

1–3 marks
How it is assessed

This objective is assessed through data analysis, multiple choice, commonly using Identify / Compare / Analyse.

Command terms

Identify / Compare / Analyse

What earns marks

Build the answer around this relationship: Monozygotic twins share essentially the same genome.

Watch for

Using only numerical values without comparing identical and non-identical twins.

Representative question

Question 1

[Maximum number: 3]

Analyse the data to find whether it supports the hypothesis that genetic factors cause some people to have a much higher chance of cocaine dependence than others.

HL Gene Expression Control

HL only
  • Transcription factors, promoters, enhancers, activators and repressors control RNA polymerase activity.
  • mRNA lifetime limits translation; poly-A shortening and nucleases help remove transcripts.
  • DNA methylation and histone modification alter chromatin access without changing base sequence, creating epigenetic patterns during differentiation.
  • The genome is all genetic information; the transcriptome and proteome vary with cell type, time and environment.
  • Some epigenetic marks persist through cell division or inheritance, although most are reset during gamete formation; genomic imprinting is an exception.
  • Hormones regulate eukaryotic transcription through receptors and transcription factors; lac and trp operons illustrate bacterial control.
  • Twin studies and environmental exposures help separate genetic, epigenetic and environmental effects on phenotype.

Concept essentials

  • Monozygotic twins share essentially the same genome.
  • Differences between identical twins can involve epigenetic or environmental factors.
  • Greater similarity in identical than non-identical twins can support genetic influence.
  • Adult identical twins may differ in methylation pattern.