Distinguish haploid and diploid nuclei
Explain chromosome-set number using n and 2n and connect it to gametes and 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
What does n represent in a haploid nucleus?
What you will learn
- Define haploid (n) as one complete chromosome set
- Define diploid (2n) as two chromosome sets
Original teaching model. Counts, source combinations and sample records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Reference | n = 3 means one complete set of three types |
| Diploid precursor | 2n = 6 chromosomes in one nucleus |
| Normal haploid product | n = 3 chromosomes in each product nucleus |
| Across separate products | A combined total is not the complement of one nucleus |
Build the genetic explanation
A haploid nucleus has one complete set of chromosomes, written n. A diploid nucleus has two sets, written 2n, typically one homologous set from each parent. Gametes are haploid; fertilisation fuses nuclei and restores the diploid number in a zygote. The value of n is a number of chromosome types, not the total number of DNA molecules in every cell state.
Check the evidence and limits
A replicated chromosome still counts as one chromosome until sister chromatids separate at the centromere. Do not confuse DNA molecule number with chromosome-set number.
Pause and explain
A fertilised egg receives one haploid set from each parent. State its ploidy and why.
Worked example
A species has n=4. How many chromosomes are in a diploid nucleus?
Show the worked solution
- Diploid means two complete sets.
- Calculate 2n=2×4.
Answer 2n=8 chromosomes.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A species has n=7. State the diploid chromosome number and explain the calculation.
Give me a hint
Set out the alleles or steps before deciding the outcome.
Compare my reasoning
- Diploid has two chromosome sets.
- Multiply n by two.
The diploid number is 2n=14 because a diploid nucleus contains two complete sets of seven chromosomes.
Look for these in your work
- Genetic terms used correctly
- Steps linked
A haploid gamete contains one chromosome of each type. Is it necessarily a cell with half the DNA molecules of every diploid cell? Explain.
Give me a hint
Use the given evidence and state your assumptions.
Compare my reasoning
- Haploid and diploid refer to chromosome sets.
- DNA replication and cell-cycle stage affect DNA-molecule number.
No. Haploid describes one chromosome set, while DNA-molecule number depends on replication and cell state. A replicated chromosome still belongs to one set until its chromatids separate.
Look for these in your work
- Prediction or interpretation supported
- Limits stated
A fertilised egg receives one haploid set from each parent. State its ploidy and why.
Give me a hint
Apply the inheritance or gene-control model to the new context.
Compare my reasoning
- The two haploid nuclei fuse.
- One set plus one set makes two sets.
The zygote is diploid (2n), because fertilisation combines one haploid set from each parent.
Look for these in your work
- Model applied accurately
- Unsupported certainty avoided
Common mistakes
- Treating n as a count of chromatids
- Calling a diploid nucleus four sets because each chromosome has replicated
What can you explain now?
What does n represent in a haploid nucleus?
Compare with the explanation
One complete set of chromosomes
It is one representative of each chromosome type.
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Make it stick
What do n and 2n count?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.