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Detailed notes on Inheritance for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Mitosis produces identical diploid cells for growth and repair. Meiosis produces haploid gametes with genetic variation. Cambridge tests the purpose and outcome of each type — not the detailed phases.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
Mitosis produces two genetically identical daughter cells with the same chromosome number as the parent cell.
Mitosis: type of cell division producing TWO daughter cells that are genetically identical to each other and to the parent cell. Daughter cells are diploid (same chromosome number as parent).
When does it occur?
What happens:
Cancer and mitosis: Cancer = uncontrolled cell division. Mutated genes (proto-oncogenes → oncogenes) cause cells to divide without normal regulation → tumour formation. Risk factors: smoking (chemical mutagens), UV radiation (DNA damage), viruses (insert oncogenes).
Meiosis produces four haploid cells with genetic variation — the process that generates the diversity needed for evolution.
Meiosis: type of cell division producing FOUR daughter cells with HALF the chromosome number (haploid). Each cell is genetically DIFFERENT from the others and from the parent.
When does it occur? ONLY in reproductive organs (testes and ovaries in animals; anthers and ovaries in plants). Produces gametes (sperm, eggs, pollen).
Two divisions in meiosis:
Sources of genetic variation in meiosis:
Comparison:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Divisions | 1 | 2 |
| Daughter cells | 2 | 4 |
| Chromosome number | Diploid (2n) | Haploid (n) |
| Genetic variation | None (identical) | Yes |
| Occurs in | All body cells | Reproductive organs only |
| Purpose | Growth, repair | Gamete production |
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: 'State TWO differences between mitosis and meiosis' (2 marks — must be clear differences e.g. number of cells produced AND genetic variation). 'Explain why gametes are haploid' (2 marks — so that fertilisation produces the correct diploid number). 'State TWO ways in which meiosis produces genetic variation' (2 marks — independent assortment + crossing over). MCQ tests whether a scenario describes mitosis or meiosis.
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (B21); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on cell division , written exactly the way a tutor would explain them at the board.
Question
State TWO differences between mitosis and meiosis.
Step-by-step solution
Step 1
Number of daughter cells: mitosis produces 2 daughter cells; meiosis produces 4 daughter cells.
Step 2
Genetic content: mitosis produces daughter cells that are genetically identical to the parent cell (and to each other), diploid; meiosis produces daughter cells that are genetically different, haploid.
Step 3
Additional differences: mitosis = one division; meiosis = two divisions. Mitosis: growth, repair, asexual reproduction; meiosis: gamete production in reproductive organs.
Answer
Question
Explain why meiosis produces genetic variation but mitosis does not.
Step-by-step solution
Step 1
In meiosis, crossing over occurs during the first division — homologous chromosomes pair up and exchange segments of DNA, producing new combinations of alleles on each chromosome.
Step 2
Independent assortment: chromosomes of each homologous pair separate independently → different combinations of maternal and paternal chromosomes in each gamete.
Step 3
Fertilisation (random fusion of two gametes) further increases variation.
Step 4
In mitosis, DNA is copied exactly before division. Each daughter cell receives an identical set of chromosomes — no new allele combinations are formed.
Answer
Meiosis produces variation through: (1) crossing over — new allele combinations on chromosomes; (2) independent assortment — random separation of homologous pairs. Mitosis produces exact DNA copies — no new combinations, so no variation.
Question
A person has 46 chromosomes in their skin cells. State how many chromosomes would be found in their egg cells.
Step-by-step solution
Step 1
Skin cells are body (somatic) cells — they are diploid (2n = 46).
Step 2
Egg cells are gametes, produced by meiosis. Meiosis halves the chromosome number → egg cells are haploid (n = 23).
Answer
23 chromosomes. Egg cells are haploid, produced by meiosis, which halves the chromosome number from 46 to 23.
Question
Explain why Down syndrome occurs in some individuals.
Step-by-step solution
Step 1
During meiosis in the formation of gametes, chromosome 21 (or another pair) may fail to separate correctly — this is called non-disjunction.
Step 2
One gamete ends up with two copies of chromosome 21 (instead of one). The other gamete has none.
Step 3
If the gamete with two copies of chromosome 21 is fertilised by a normal gamete (one copy), the resulting zygote has three copies of chromosome 21 (trisomy 21).
Step 4
The individual has 47 chromosomes instead of 46. This causes the characteristic features of Down syndrome.
Answer
Non-disjunction during meiosis causes chromosome 21 to fail to separate; a gamete with two copies of chromosome 21 is fertilised by a normal gamete; the zygote has three copies of chromosome 21 (trisomy 21) — resulting in Down syndrome (47 chromosomes total).
Definitions to memorise and the exact keywords mark schemes credit for cell division answers — sharpened from recent examiner reports for the 2026 0654 sitting.
A type of cell division producing two genetically identical diploid daughter cells from one parent cell. Used in growth, repair, and asexual reproduction.
A type of cell division that produces four genetically non-identical haploid daughter cells. Occurs in reproductive organs to produce gametes. Introduces genetic variation.
A cell or organism containing two complete sets of chromosomes (one from each parent). Human body cells: 2n = 46.
A cell containing one set of chromosomes. Gametes are haploid. Human gametes: n = 23.
A random, spontaneous change in the DNA base sequence (gene mutation) or chromosome structure/number (chromosomal mutation). Can be caused by radiation or mutagens.
The failure of homologous chromosomes (or chromatids) to separate correctly during meiosis, resulting in gametes with abnormal chromosome numbers.
The traps other students keep falling into on cell division questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2022
Why it happens
Students know mitosis is the common cell division and apply it to gamete production.
How to avoid it
Gametes are produced by meiosis (to halve chromosome number). Mitosis produces somatic (body) cells for growth and repair.
Why it happens
Students confuse the two types of cell division.
How to avoid it
Mitosis = identical cells. Meiosis = genetically varied cells (due to crossing over and independent assortment).
Why it happens
The word 'mutation' has negative connotations in everyday language.
How to avoid it
Most mutations are neutral (no effect on phenotype). Some are harmful; a small number can be beneficial (e.g. antibiotic resistance in bacteria, sickle cell trait protecting against malaria).
Why it happens
Students know it involves an extra chromosome but do not explain non-disjunction.
How to avoid it
Down syndrome = trisomy 21 caused by non-disjunction during meiosis. Always explain the mechanism: chromosome pair fails to separate → gamete with two chromosomes 21 → trisomy after fertilisation.
The things students keep getting wrong in this sub-topic, answered.