Meiosis explained: how gametes form and why it matters
See how meiosis makes four different gametes, how it differs from mitosis and why sexual reproduction creates variation, with practice questions.
GCSEHigh SchoolAPCurriculum fit: GCSE Biology and Combined Science (meiosis; Higher content in some boards). US: high school life science (inheritance and variation); also AP Biology.
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What is it?
Meiosis is the type of cell division that produces gametes: sperm and egg cells in animals. Each gamete has half the normal number of chromosomes, so when two gametes join at fertilisation the full number is restored.
Because each gamete gets a different mix of genes, meiosis helps explain why siblings are not identical. This variation is the raw material for natural selection.
Key formulas and ideas
- Meiosis: one diploid cell (46 chromosomes in humans) makes four genetically different haploid cells (23 chromosomes).
- Fertilisation: haploid sperm (23) + haploid egg (23) = diploid zygote (46).
- Sources of variation: independent assortment of chromosomes, crossing over, and random fertilisation.
Worked examples
Example 1: Chromosome numbers
Question: A human sperm cell has 23 chromosomes. How many chromosomes are in a fertilised egg?
- The egg cell also has 23 chromosomes.
- Add them at fertilisation: 23 + 23 = 46.
Answer: 46 chromosomes
Example 2: How many gametes?
Question: How many gametes form when one cell undergoes meiosis?
- Meiosis has two divisions.
- One cell becomes two, then each becomes two again.
Answer: Four gametes
Example 3: Compare with mitosis
Question: State two differences between meiosis and mitosis.
- Mitosis makes 2 identical cells; meiosis makes 4 different cells.
- Mitosis keeps the chromosome number; meiosis halves it.
Answer: Mitosis: 2 identical diploid cells. Meiosis: 4 different haploid cells
Common mistakes
- Confusing the two types of division.
- Saying meiosis produces identical cells.
- Forgetting that gametes are haploid.
- Not linking meiosis and fertilisation to the maintenance of the chromosome number.
Practice questions
Frequently asked questions
Why do gametes need half the chromosomes?
If they had the full number, the chromosome number would double in every generation.
Where does meiosis happen in humans?
In the ovaries and testes.
Related guides
Mitosis and the cell cycle: stages explained step by step
Learn what happens at each stage of mitosis, why cells divide and how to count cells after division, with worked examples and practice questions.
GCSEHigh SchoolAPDNA, genes and inheritance: Punnett squares explained
Understand genes, alleles, dominant and recessive traits and how to complete a Punnett square, with worked examples for GCSE and high school.
GCSEA-LevelHigh SchoolAPNatural selection and evolution: how it works, with examples
Understand variation, competition and inheritance, with examples such as peppered moths and antibiotic resistance, and practice questions.
GCSEHigh SchoolAPSources and review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.