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D2.1.10—Down syndrome

Down syndrome usually results when non-disjunction produces a gamete with an extra chromosome 21, causing trisomy after fertilization occurs in reproduction.

Syllabus
First assessment 2025
Objective
D2.1.10
Level
SL

Exam analysis

Chance of appearing11%of analysed past papers
Latest appearanceNovember 2024
Most common paperPaper1
Typical marks1

Common command terms

  • Explain
  • Outline
  • Determine
  • State
  • Identify
  • Describe

Scoring notes

Common mistake
Calling Down syndrome a gene mutation instead of a chromosome-number abnormality.

Recent exam appearances

November 2024Paper1 ["SL"] · TZ214[ 1 ]D2.1.10—Down syndrome
November 2024Paper1 ["SL"] · TZ113[ 1 ]D2.1.10—Down syndrome
May 2024Paper2 ["SL"] · TZ14(c)[ 3 ]D2.1.10—Down syndrome
May 2019Paper2 ["SL"] · TZ16(b)[ 1 ]D2.1.10—Down syndrome
May 2019Paper1 ["SL"] · TZ213[ 1 ]D2.1.10—Down syndrome
Practice this objective

Coverage 2010–2024 · Updated 16 Jul 2026

Nondisjunction Can Produce Trisomy

Nondisjunction is failure of homologous chromosomes or sister chromatids to separate, producing gametes with abnormal chromosome numbers.

If an extra chromosome enters a gamete, fertilization can create a zygote with three copies of one chromosome. The phenotype depends on chromosome and gene dosage.

Trace: division error; abnormal gamete; fertilization; chromosome count; developmental consequence.

A gamete containing two copies of chromosome 21 can combine with a normal gamete to produce trisomy 21.

Nondisjunction is a chromosome-segregation error, not a point mutation in one gene.

Down syndrome

Assessment in practice

1 marks
How it is assessed

This objective is assessed through structured response, commonly using Explain / Outline / Determine.

Command terms

Explain / Outline / Determine / State / Identify / Describe

What earns marks

Build the answer around this relationship: Non-disjunction is failed chromosome separation during meiosis.

Watch for

Calling Down syndrome a gene mutation instead of a chromosome-number abnormality.

Representative question

Question 1

[Maximum number: 4]

Describe how non-disjunction can cause Down syndrome.

Core Cell Division

  • Cell division produces daughter cells for growth, repair or reproduction; cytokinesis divides cytoplasm by a contractile ring in animals or a cell plate in plants.
  • DNA replication creates sister chromatids joined at centromeres before nuclear division.
  • Mitosis preserves chromosome number: chromosomes condense, align, sister chromatids separate and nuclei reform, producing genetically identical nuclei.
  • Meiosis follows one replication with two divisions: homologous chromosomes separate in meiosis I and sister chromatids in meiosis II, producing haploid cells.
  • Crossing over, independent orientation and random fertilization generate allele combinations.
  • Non-disjunction is failed chromosome separation and can produce aneuploid cells, including trisomy 21.
  • Identify stages in micrographs from chromosome condensation, equatorial alignment, separation and nuclear-envelope cues.

Concept essentials

  • Non-disjunction is failed chromosome separation during meiosis.
  • Non-disjunction can produce gametes with extra or missing chromosomes.
  • Down syndrome is usually caused by trisomy 21.
  • A karyogram can identify the chromosome affected by trisomy.