Meiosis — Reduction Division and Continuity Across Generations
If fertilisation joins two gametes together, chromosome number would double every generation unless gametes were made differently from ordinary body cells. Meiosis solves that problem by halving chromosome number before fertilisation restores it.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
A student says, "Gametes should be made by mitosis, because mitosis keeps cells stable and accurate. Then fertilisation would just make the organism stronger by doubling the DNA."
Before reading on, explain why this reasoning is wrong. What would happen to chromosome number across generations if gametes were produced by mitosis instead of meiosis?
Know
- The difference between diploid and haploid cells.
- The broad sequence of meiosis I and meiosis II.
- That meiosis halves chromosome number.
Understand
- Why meiosis is essential for chromosome-number stability across generations.
- How crossing over and independent assortment generate variation.
- Why meiosis has a different purpose from mitosis.
Can Do
- Explain meiosis as a reduction division.
- Link meiosis to fertilisation and continuity of species.
- Identify sources of variation introduced during meiosis.
Core Content
Chromosome logic · diploid → haploid
To understand meiosis, start with chromosome sets. Diploid cells have paired chromosomes; gametes must end with only one set.
A diploid cell has two sets of chromosomes. In sexually reproducing organisms, these occur as homologous chromosome pairs, with one chromosome of each pair inherited from each parent. Gametes, however, must be haploid, containing only one set of chromosomes.
If gametes were diploid and then fused at fertilisation, chromosome number would double each generation. Meiosis prevents this by reducing the chromosome number before fertilisation. Fertilisation can then restore the diploid state.
The stages of meiosis showing two divisions and reduction to haploid.
What to write in your book
- Diploid = two chromosome sets (homologous pairs, one from each parent).
- Haploid = one chromosome set (gametes).
- Meiosis reduces chromosome number before fertilisation.
- Meiosis + fertilisation together maintain continuity across generations.
A cell with two complete sets of chromosomes is described as _____.
Two divisions · reduction then separation
Meiosis is not one ordinary division. It involves two linked divisions with different outcomes.
| Step | What happens |
|---|---|
| Before meiosis | DNA replicates so each chromosome is duplicated. |
| Meiosis I | Homologous chromosome pairs separate. Chromosome number is reduced. |
| Meiosis II | Sister chromatids separate in a division similar in outline to mitosis. |
| Outcome | Four haploid cells are produced. |
In meiosis I, homologous chromosomes pair and are separated into different cells. This is the reduction division because the chromosome number is halved. In meiosis II, sister chromatids separate. The result is four haploid daughter cells.
At HSC depth, the main distinction from mitosis is that meiosis separates homologous chromosomes first and reduces chromosome number, whereas mitosis maintains chromosome number in somatic cells.
What to write in your book
- DNA replicates before meiosis.
- Meiosis I = reduction division: homologous pairs separate (number halved).
- Meiosis II = sister chromatids separate (like mitosis in outline).
- Outcome = four haploid cells.
Which division of meiosis reduces the chromosome number by separating homologous pairs?
Variation · new combinations of alleles
Meiosis is not only about reducing chromosome number. It also helps generate genetically varied gametes.
Crossing over occurs when homologous chromosomes exchange corresponding segments. This produces new combinations of alleles on chromosomes. Independent assortment occurs because homologous pairs line up randomly before separation, so different combinations of maternal and paternal chromosomes enter the resulting gametes.
These processes increase variation between gametes. That matters because after fertilisation, offspring are more likely to differ genetically from one another. This contributes to the variation that can later affect survival in changing environments.
Crossing over
- Exchange of segments between homologous chromosomes.
- Creates new combinations of existing alleles.
- Occurs during meiosis I.
Independent assortment
- Random arrangement of homologous pairs.
- Changes which chromosomes enter each gamete.
- Also increases variation between gametes.
What to write in your book
- Crossing over (meiosis I): homologous chromosomes exchange segments → new allele combinations.
- Independent assortment: random orientation of pairs → different chromosome mixes in gametes.
- Both increase variation between gametes (and so between offspring).
- Crossing over makes new COMBINATIONS, not new alleles (mutation does that).
Crossing over creates brand-new alleles that did not exist before.
Meiosis involves two rounds of nuclear division but only one round of DNA replication.
Independent assortment creates new alleles that did not exist in the parents.
Continuity · stable chromosome number + variation
Meiosis supports continuity of species across generations because it produces haploid gametes. When two haploid gametes fuse at fertilisation, the diploid chromosome number is restored rather than doubled. This keeps chromosome number stable from one generation to the next.
| Process | Effect on chromosome number | Why it matters |
|---|---|---|
| Meiosis | Halves chromosome number to produce haploid gametes. | Prevents chromosome number from doubling each generation. |
| Fertilisation | Restores diploid chromosome number. | Combines genetic information from two parents while preserving species chromosome number. |
| Crossing over and independent assortment | Do not change the count; change the combinations. | Increase variation between gametes and future offspring. |
This is why meiosis is essential not only for gamete formation, but for the long-term maintenance of species identity through stable chromosome number and varied offspring.
What to write in your book
- Meiosis → haploid gametes; fertilisation → diploid restored (not doubled).
- Chromosome number stays stable from one generation to the next.
- Crossing over + independent assortment change combinations, not the count.
- Meiosis supports both stable chromosome number AND varied offspring.
How many haploid cells does one complete meiosis produce?
Activities
Compare and Calculate
A species has body cells with 12 chromosomes. Answer each part, then explain why this sequence matters for continuity of species.
1. How many chromosomes would a diploid body cell have?
2. How many chromosomes would each haploid gamete have after meiosis?
3. How many chromosomes would a zygote have after fertilisation?
Identify the Source of Variation
For each example, decide whether it best illustrates crossing over, independent assortment or fertilisation restoring chromosome number.
| Item | Answer | Justification |
|---|---|---|
| Homologous chromosomes exchange segments during meiosis. | ||
| One gamete receives a different mix of maternal and paternal chromosomes from another gamete. | ||
| Two haploid gametes fuse to form a diploid zygote. |
Core idea
- Meiosis is a reduction division that produces haploid gametes and helps maintain chromosome-number stability across generations.
Mechanism / process
- Meiosis I separates homologous chromosomes, meiosis II separates sister chromatids, and crossing over plus independent assortment increase variation.
Common mistake
- Confusing meiosis with mitosis or saying crossing over creates new alleles.
Exam sentence starter
- "Meiosis is essential for continuity across generations because it..."
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Explain the difference between diploid and haploid cells, and state the role of meiosis in producing gametes.
AnalyseBand 4(4 marks) 2. Explain how meiosis and fertilisation together maintain chromosome-number stability across generations.
EvaluateBand 5–6(5 marks) 3. Evaluate the statement: "Meiosis is valuable not only because it reduces chromosome number, but also because it increases variation."
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1 — Compare and Calculate
1. A diploid body cell has 12 chromosomes.
2. Each haploid gamete has 6 chromosomes after meiosis.
3. The zygote has 12 chromosomes after fertilisation.
Why this matters: Meiosis halves chromosome number and fertilisation restores it, preventing chromosome number from doubling each generation.
Activity 2 — Identify the Source of Variation
1. Crossing over.
2. Independent assortment.
3. Fertilisation restoring chromosome number.
Short Answer Model Responses
Q1 (3 marks): Diploid cells contain two sets of chromosomes, while haploid cells contain one set [1]. Body cells are usually diploid and gametes are haploid [1]. Meiosis produces haploid gametes by halving chromosome number from the diploid state [1].
Q2 (4 marks): Meiosis halves chromosome number to produce haploid gametes [1]. This prevents gametes from carrying a full diploid set [1]. At fertilisation, two haploid gametes fuse and restore the diploid chromosome number [1]. Together, meiosis and fertilisation keep chromosome number stable across generations [1].
Q3 (5 marks): The statement is correct because meiosis is important for two major reasons [1]. First, it is a reduction division that halves chromosome number, which is essential so fertilisation restores rather than doubles the diploid number each generation [1]. Second, meiosis increases variation because crossing over creates new combinations of existing alleles and independent assortment produces different chromosome combinations in gametes [1]. This variation contributes to genetic differences among offspring [1]. Therefore, meiosis is valuable both for continuity of species through chromosome-number stability and for variation among offspring [1].
Meiosis
Produces haploid gametes by halving chromosome number.
Meiosis I
Separates homologous chromosomes and reduces chromosome number.
Variation
Crossing over and independent assortment change allele combinations in gametes.
Exam trap
Crossing over creates new combinations of alleles, not new alleles.
Rapid-fire questions on reduction division, meiosis I and II, haploid gametes and sources of variation. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
You should now be able to reject the idea that gametes should be made by mitosis. Meiosis is essential because it halves chromosome number before fertilisation restores it, and it also helps generate variation among offspring.