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LESSON C2 · EXPLAINER
Meiosis and Gamete Formation
Two sisters with the same parents can look completely different. Meiosis is the reason.
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Explore in 3D
Meiosis in 3D
Step through meiosis and compare it with mitosis.
Step 2 · Read
Key notes
Copy these into your notebook in your own words — that is how they stick.
Meiosis
Reduction division: one diploid cell → four genetically different haploid gametes.
Sites
Testes and ovaries; anthers and ovules in flowering plants.
Halving
Ensures the diploid number is restored at fertilisation and stays constant between generations.
Sources of variation
Crossing over, independent assortment, random fertilisation (and mutation).
Mitosis vs meiosis
Mitosis: 2 identical diploid cells. Meiosis: 4 different haploid cells.
Errors
Non-disjunction in meiosis → abnormal chromosome number (e.g. Down syndrome: 47 chromosomes).
Plant gametes
Male nucleus in pollen grain; egg cell (ovum) in the ovule.
EXAM TIPS & COMMON MISTAKES
When completing a meiosis diagram, each gamete gets ONE chromosome from each pair — never both of a pair.
Gametes are HAPLOID. Say so explicitly.
“Why do sisters differ?” — independent assortment and crossing over in meiosis, plus random fertilisation.
Step 2 · Learn from examples
Worked examples & study resources
The kind of support a teacher gives after the lesson: worked examples to attempt first, a board illustration, key terms, common misconceptions and memory aids.
Worked example 1
Why gametes must be haploid
Human body cells have 46 chromosomes. (a) How many chromosomes are in a sperm cell? (b) What would happen to the chromosome number over generations if gametes were made by mitosis?
Try it in your notebook first, then reveal the solution one step at a time.
Gametes are made by meiosis, which halves the chromosome number: 46 ÷ 2 = 23 — one chromosome from each pair.
At fertilisation, a sperm (23) fuses with an egg (23) to form a zygote with 46 — the normal diploid number is restored.
If gametes were made by mitosis they would each have 46 chromosomes, and the zygote would have 92.
The number would double every generation (46 → 92 → 184 …), which would not produce a normal organism.
Answer. (a) 23 (haploid). (b) The chromosome number would double in every generation. Meiosis keeps the number constant from one generation to the next.
Examiner’s tip. State both halves of the idea: meiosis HALVES the number; fertilisation RESTORES it.
Worked example 2
How many different gametes?
An organism has 3 pairs of chromosomes (2n = 6). Ignoring crossing over, how many genetically different gametes can it make by independent assortment? How many can a human make?
Try it in your notebook first, then reveal the solution one step at a time.
In meiosis each pair lines up independently, so for each pair there are 2 choices of which chromosome goes into a gamete.
With 3 pairs: 2 × 2 × 2 = 2³ = 8 different gametes.
Humans have 23 pairs: 2²³ = 8 388 608 — over 8 million combinations from one person.
Crossing over (swapping sections between paired chromosomes) and random fertilisation increase the variety even more.
Answer. 8 different gametes for 2n = 6; over 8 million (2²³) for a human — before crossing over is even counted.
Examiner’s tip. This is why brothers and sisters with the same parents look different — only identical twins come from the same egg and sperm.
On the board · Mitosis and meiosis compared
Mitosis
Meiosis
Purpose
Growth, repair, asexual reproduction
Making gametes (sex cells)
Where
All body cells
Testes and ovaries (anthers and ovules in plants)
Number of divisions
One
Two
Cells produced
2
4
Chromosome number
Same as parent (diploid)
Halved (haploid)
Genetically
Identical to the parent
Different from each other and from the parent
Mitosis is for growth and repair; meiosis is for making gametes.
Key terms
Meiosis
Cell division that halves the chromosome number, producing four genetically different cells.
Haploid (n)
Having one set of chromosomes, e.g. 23 in human gametes.
Homologous chromosomes
A matching pair of chromosomes, one from each parent.
Crossing over
Exchange of sections between homologous chromosomes during meiosis.
Independent assortment
Random arrangement of chromosome pairs, giving new combinations in gametes.
Gamete
A haploid sex cell — sperm, egg, pollen nucleus or ovule nucleus.
Common misconceptions
✗ Meiosis produces two cells.✓ Meiosis has two divisions and produces four cells.
✗ Meiosis happens all over the body.✓ Meiosis only happens in the sex organs; other cells divide by mitosis.
✗ Children of the same parents get the same genes.✓ Each gamete carries a different combination of alleles, so siblings differ (except identical twins).
Memory aid
Mi-TWO-sis makes two identical cells; Mei-FOUR-sis makes four different sex cells. Meiosis = Making Eggs (and sperm).
Real-life link
Down syndrome usually results from a mistake in meiosis: one pair of chromosome 21 fails to separate, so a gamete carries an extra copy, and the child has three copies of chromosome 21 in every cell.
Try this
Model independent assortment: use two colours of paper strips for three pairs of chromosomes. Find all the different ways one strip from each pair can be chosen. Do you get 8?
Play & practise
Games
3 quick games on this lesson. Earn XP and badges as you play — your best score for each game is saved. Games are for practice and do not change your exam readiness.
Step 3 · Practise
Paper 1 practice — multiple choice
8 items modelled on real CSEC Paper 1 questions. Choose one answer for each, then check. Aim for 80% or better.
Step 4 · Apply
Paper 2 practice — structured questions
Write full answers, then reveal the mark scheme and tick the points you earned (16 marks in total). Your answers are saved on this device.
Based on CSEC P2 May/June 2023, Question 2
Question 1 [11 marks]
A partial diagram shows meiosis in an animal cell producing four genetically different daughter cells, P, Q, R and S.
(a) State whether the daughter cells are haploid or diploid, name ONE body part where meiosis occurs, and name the structure in the nucleus that carries genetic material. [3]
MARK SCHEME — (a) (max 3)
(b) Khara and Keturah have the same parents but look very different. Use genetics to explain why, giving TWO reasons. [4]
MARK SCHEME — (b) (max 4)
(c) State TWO reasons meiosis is important, and name the TWO gametes produced in flowering plants. [4]
MARK SCHEME — (c) (max 4)
Based on CSEC P2 January 2024, Question 5(b)–(d)
Question 2 [5 marks]
A diagram of meiosis shows a cell with two homologous pairs.
(a) Describe the events of meiosis that halve the chromosome number and create variation. [3]
MARK SCHEME — (a) (max 3)
(b) Suggest ONE consequence if meiosis does not occur correctly, and ONE reason genetic variation is important to speciation. [2]
MARK SCHEME — (b) (max 2)
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Sources
Where this topic appears in CSEC papers
Find these in your own past-paper booklets for extra timed practice. The practice above is adapted from them.