Explain why meiosis reduces chromosome number
Relate reduction division in meiosis to haploid gametes and restoration of diploidy at fertilisation.
A Level; Cambridge International Biology 9700, 2028–2030. Written answers are self-checked; this is not practical-skills certification.
Before you begin
- Genes are DNA sequences that can be inherited; meiosis and fertilisation link parental genomes.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which meiotic division separates homologous chromosomes?
What you will learn
- Explain why meiosis reduces chromosome-set number during gamete production
- Relate the reduction to maintenance of a stable species chromosome number after fertilisation
Original teaching model. Counts, source combinations and sample records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Before copying | 6 chromosomes represented by 6 DNA molecules |
| Attached copies | 6 replicated structures represented by 12 DNA molecules |
| Four final nuclei | 3 individual chromosomes in each haploid nucleus |
| Combined final count | 4 × 3 = 12 across four nuclei; not per nucleus |
Build the genetic explanation
Meiosis includes one DNA replication followed by two nuclear divisions. Homologous chromosomes separate in meiosis I, reducing the chromosome-set number from diploid to haploid. Meiosis II separates sister chromatids without another replication. Haploid gametes then fuse at fertilisation, restoring diploidy; without reduction, chromosome-set number would double in each generation.
Check the evidence and limits
Meiosis does not halve the number of chromosome types in every daughter nucleus by random loss; it separates homologous pairs so each haploid nucleus receives one of each type.
Pause and explain
A proposal says that two rounds of DNA replication are needed because meiosis has two divisions. Correct it.
Worked example
Why must gametes be haploid if fertilisation restores the diploid number?
Show the worked solution
- Each parent contributes one set.
- Two haploid sets restore two sets.
Answer Gametes must be haploid so that fusion of two gamete nuclei restores the diploid chromosome number rather than doubling it.
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Explain how meiosis and fertilisation together maintain chromosome-set number across generations.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- Meiosis produces haploid nuclei by separating homologues.
- Fertilisation fuses two haploid nuclei.
Meiosis separates homologous chromosomes in meiosis I to produce haploid nuclei. Fertilisation combines two haploid gametes and restores the diploid number, maintaining the species chromosome-set number.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A diploid species has 2n=12. State the number of chromosome types in a normal gamete.
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- Gametes are haploid.
- n is half of 2n.
The gamete contains n=6 chromosomes, one of each chromosome type.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A proposal says that two rounds of DNA replication are needed because meiosis has two divisions. Correct it.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- There is one replication before meiosis I.
- Meiosis II separates sister chromatids without another replication.
DNA replicates once before meiosis I. The two divisions then separate homologous chromosomes and sister chromatids; DNA does not replicate again between the divisions.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Saying meiosis has two DNA replications
- Saying meiosis II halves chromosome-set number again
What can you explain now?
Which meiotic division separates homologous chromosomes?
Compare with the explanation
Meiosis I
Homologues separate in the reduction division.
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Make it stick
Which division causes reduction, and what restores diploidy?
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