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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
HL

Exam analysis

Chance of appearing4%of analysed past papers
Latest appearanceMay 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

May 2024Paper2 ["HL"] · TZ27(b)[ 4 ]D2.1.10—Down syndrome
November 2020Paper1 ["HL"] · TZ013[ 1 ]D2.1.10—Down syndrome
May 2018Paper1 ["HL"] · TZ217[ 1 ]D2.1.10—Down syndrome
May 2013Paper2 ["HL"] · TZ22(b)(ii)[ 1 ]D2.1.10—Down syndrome
May 2013Paper1 ["HL"] · TZ111[ 1 ]D2.1.10—Down syndrome

Coverage 2013–2024 · Updated 16 Jul 2026

Non-Disjunction Sends Both Copies to One Pole

1

A homologous pair can fail to separate in meiosis I, or sister chromatids can fail to separate in meiosis II. This failure is non-disjunction.

2

Both copies move to one pole while the opposite pole receives none. Later division therefore cannot give every gamete the standard chromosome number.

3

The affected meiosis produces gametes with an extra chromosome (n + 1) and/or a missing chromosome (n − 1), depending on when the error occurred.

4

After fusion with a normal haploid gamete, an n + 1 gamete can produce a trisomic zygote; an n − 1 gamete can produce a monosomic zygote.

Non-disjunction can create too many or too few chromosomes. It is not defined only by an extra chromosome.

Trisomy 21 Usually Begins with a Meiotic Separation Error

chromosome 21 non-disjunction during gamete formation → gamete receives two copies of chromosome 21 → fertilization by a normal gamete adds one more copy → zygote has three copies of chromosome 21

The resulting trisomy 21 gives the zygote 47 chromosomes rather than the usual 46 and can cause Down syndrome.

The developmental effects vary among individuals because an extra chromosome changes the dosage of many genes, not just one gene.

Down syndrome is a chromosome-number condition. It is not caused by a point mutation within chromosome 21.

Track What Separates to Explain Reduction, Error and Variation

Event What changes Main consequence
replication before meiosis each chromosome gains a sister chromatid DNA prepared for two divisions
meiosis I homologous chromosomes separate chromosome number is halved
meiosis II sister chromatids separate four haploid products can form
non-disjunction one required separation fails aneuploid gametes may form

Variation is layered: crossing over recombines alleles within chromosomes, random orientation assort homologues independently, and random fertilization combines two independently produced gametes.

When explaining any meiotic outcome, name the stage, the chromosome object that should separate, the chromosome number before and after, and whether the result changes number, allele combination or both.

Down syndrome

[ 4 ]

Describe how non-disjunction can cause Down syndrome.

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.