The big idea: Most mutations change just a few DNA bases inside one gene. A chromosome mutation is bigger: a whole chromosome is gained or lost.
The usual cause is non-disjunction — when chromosomes fail to separate correctly during meiosis (the division that makes gametes).
When this happens, a gamete ends up with the wrong number of chromosomes. If that gamete is fertilised, the new individual has an extra (or missing) whole chromosome — for example three copies of chromosome 21, which causes Down syndrome.
- Chromosome mutation
- A change to whole chromosomes — usually a change in the NUMBER of chromosomes in a cell, rather than a change to the DNA bases of a single gene.
- Non-disjunction
- The failure of chromosomes (in meiosis I) or sister chromatids (in meiosis II) to separate properly during meiosis, so they end up in the same gamete.
- Aneuploidy
- Having an abnormal number of chromosomes — one too many or one too few — rather than a complete extra or missing set.
- Trisomy
- Having three copies of a particular chromosome instead of the normal two (for example trisomy 21).
- Down syndrome
- The condition caused by having three copies of chromosome 21 (trisomy 21).
The word tells you what happens: Disjunction means 'separating'. Non-disjunction therefore means 'not separating'.
The chromosomes that should move to opposite ends of the cell instead go to the same end — so one gamete gets too many and the other gets too few.
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To see how non-disjunction causes Down syndrome, first recall what normal meiosis should do: separate chromosomes so each gamete carries exactly one copy of every chromosome.
Normal meiosis: homologous chromosomes separate in meiosis I and sister chromatids separate in meiosis II, so every gamete ends up with the correct, halved chromosome number. Non-disjunction is the failure of one of these separation steps.
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Step 1 — separation fails in meiosis: In meiosis I, homologous chromosomes are meant to be pulled to opposite poles; in meiosis II, sister chromatids are.
In non-disjunction, chromosome 21 (or its chromatids) fails to separate, so both copies go into the same cell.
The result is a gamete with an extra chromosome 21 (n + 1), while another gamete is left missing that chromosome (n − 1).
Step 2 — fertilisation adds the normal copy: The abnormal gamete already carries two copies of chromosome 21.
At fertilisation it joins a normal gamete, which brings one more copy.
The zygote therefore has three copies of chromosome 21 — trisomy 21. Because every cell of the body is made from this zygote by mitosis, all the cells carry the extra chromosome, giving Down syndrome.
| When it happens | What fails to separate | Result in the gamete |
|---|---|---|
| Meiosis I | A pair of homologous chromosomes does not separate | A gamete with an extra whole chromosome (n + 1) and another with one missing (n − 1) |
| Meiosis II | Sister chromatids do not separate | Again, one gamete gains an extra copy (n + 1) and another lacks one (n − 1) |
| After fertilisation | An n + 1 gamete joins a normal n gamete | A zygote with three copies of one chromosome (trisomy, 2n + 1) |
Normal meiosis
- Chromosomes separate correctly
- Each gamete gets one copy of each chromosome
- Fertilisation restores the normal number (2n)
- The offspring has the correct chromosome number
Non-disjunction
- Chromosomes fail to separate
- One gamete gets an extra copy (n + 1), another is short one (n − 1)
- Fertilisation adds a normal copy → three copies (2n + 1)
- The offspring has an extra whole chromosome (e.g. trisomy 21)
Why it is a WHOLE chromosome: Non-disjunction is not a change to the DNA bases — it does not rewrite a gene.
It moves an entire chromosome into the wrong gamete, so the error is on the scale of whole chromosomes and can even be seen on a karyogram as an extra band.
| Feature | Gene (point) mutation | Chromosome mutation |
|---|---|---|
| Scale of change | A change to a few DNA bases within one gene | A change to whole chromosomes (a whole chromosome added or lost) |
| What is altered | The base sequence of a single gene | The number (or structure) of chromosomes in the cell |
| Typical cause | An error in DNA replication or a mutagen | Non-disjunction during meiosis |
| Visible on a karyogram? | No — far too small to see | Yes — an extra or missing whole chromosome can be seen |
| Example | Sickle-cell anaemia (one base substitution) | Down syndrome (an extra chromosome 21 = trisomy 21) |
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How this is tested: The headline task on Paper 2 is to Explain how meiotic non-disjunction can result in Down syndrome — usually worth 3 marks. You must link non-disjunction → a gamete with an extra chromosome 21 → fertilisation → trisomy 21.
On Paper 1 you may have to identify a chromosome-number error from a karyogram (an extra or missing whole chromosome = aneuploidy from non-disjunction).
A common data twist gives a graph of Down-syndrome incidence against maternal age and asks you to describe the trend — incidence stays low then rises steeply with age.
IB-style question — explain how non-disjunction causes Down syndrome
Explain how meiotic non-disjunction can result in Down syndrome (trisomy 21). [3]
How to score all three marks
- Name the failure. During meiosis, non-disjunction occurs — chromosome 21 (the homologues or the sister chromatids) fails to separate and both copies pass into the same gamete.
- Describe the abnormal gamete. This produces a gamete with an extra chromosome 21 — it carries two copies instead of one (n + 1).
- Add fertilisation. When this gamete is fertilised by a normal gamete (which brings one copy), the zygote has three copies of chromosome 21 (trisomy 21), so the individual has Down syndrome. (Mark 1: non-disjunction / chromosomes fail to separate. Mark 2: gamete has an extra chromosome 21. Mark 3: fertilisation → three copies / trisomy 21.)
Final answer
Non-disjunction in meiosis means chromosome 21 fails to separate, so one gamete carries two copies of chromosome 21; at fertilisation a normal gamete adds a third copy, giving trisomy 21 (Down syndrome).
Why this scores full marks: It tells the whole causal chain in order: non-disjunction → extra-chromosome gamete → fertilisation → three copies.
A 3-mark Explain needs the mechanism, not just the word 'trisomy' — each link in the chain is a separate scoring point.
If separation goes correctly, each gamete carries one copy of every chromosome. Non-disjunction means one chromosome (or chromatid) does not separate, so one gamete gets an extra copy and another gets none.
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