Distinguish chromosome-set number from total chromosome count
Use total and supplied n together: 12/n gives two sets when n = 6 and one set when n = 12. An unlabelled total alone does not determine ploidy across different models.
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall the nucleus, DNA base pairing, gametes, fertilisation, enzymes, membrane carriers and chemical synapses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which information helps interpret a chromosome total as sets?
What you will learn
- Use a supplied set size to interpret totals
- Recognise when an unlabelled total is insufficient
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Set size | n: chromosomes in one set |
| Total | Number of individual chromosomes |
| Model A | Total 12; n 6 → two sets |
| Model B | Total 12; n 12 → one set |
Use a supplied set size to interpret totals
Chromosome-set number and total chromosome count are different quantities. If a model’s single set contains three chromosomes, one set has 3 and two sets have 6. The relationship uses the specified set size; a total without information about the organism or its chromosome types may be insufficient to determine how many sets it represents.
Consider two stipulated models both with a total of 12 chromosomes. Model A has n = 6, so its total represents two sets. Model B has n = 12, so its total represents one set. Equal totals therefore do not establish equal ploidy across models with different set sizes. The numbers are invented teaching cases, not counts assigned to a named natural species.
Recognise when an unlabelled total is insufficient
A labelled chromosome drawing can provide the missing type information. One representative of each supplied type represents one set; two representatives across the corresponding types represent two sets in this simple model. An unlabelled rod collection does not reveal that relationship merely through its overall size or colour.
The original comparison table asks for both n and set number before giving a total. Keep the categories explicit in calculations: chromosome individuals are counted as objects, while sets are groups containing the supplied types. These models use unreplicated chromosomes and do not ask learners to infer a DNA-copy stage or count sister chromatids.
Pause and explain
A model has n = 6 and total 12. How many sets does that represent?
Worked example
Why can a total of 12 chromosomes represent different set numbers in two models?
Show the worked solution
- Identify the different supplied n values.
- Divide the total by each stated set size.
Answer With n = 6, 12 represents two sets; with n = 12, it represents one set. The total alone does not fix ploidy across different models.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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.
Calculate totals for one and two sets in an n = 3 model.
Give me a hint
Multiply the given size by the number of sets.
Compare my reasoning
- Use one times three.
- Use two times three.
One set has 3 chromosomes; two sets have 6 chromosomes in the stipulated model.
Look for these in your work
- Give both totals
- Distinguish set number
Compare total 12 with n = 6 and total 12 with n = 12.
Give me a hint
Use the total-to-set-size ratio separately.
Compare my reasoning
- Calculate 12/6.
- Calculate 12/12.
The n = 6 model has two sets; the n = 12 model has one set despite the equal total.
Look for these in your work
- Use both supplied n values
- Explain equal-total limitation
An unlabelled drawing has twelve rods but gives no species or type key. Can it establish haploid or diploid?
Give me a hint
The one-set size is missing.
Compare my reasoning
- Identify the total shown.
- State that n or corresponding-type evidence is absent.
No. The total is twelve, but without n or chromosome-type information it does not establish whether the drawing represents one set or two.
Look for these in your work
- Keep the visible count
- State missing set evidence
Common mistakes
- An unlabelled total does not give every model’s ploidy.
- Do not count sets and individuals as the same quantity.
What can you explain now?
A model has n = 6 and total 12. How many sets does that represent?
Compare with the explanation
Two sets
12 divided by 6 is 2.
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Make it stick
Explain use a supplied set size to interpret totals and recognise when an unlabelled total is insufficient. Use the stated cell, chromosome sets and information pathway before opening the solutions.
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.