Trace chromosome contributions into a zygote model
Follow both contributor sets with an original diagram key.
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 DNA, inherited characteristics, nuclei and the distinction between growth and reproduction.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What do blue and gold show in the schematic?
What you will learn
- Read sets and corresponding chromosome types
- Separate contributor colour from a new allele
Trace chromosome contributions into a zygote model
| Feature | Meaning |
|---|---|
| Blue set | First gamete contribution |
| Gold set | Second compatible gamete contribution |
| Fused nucleus | Two sets, six chromosomes |
| Limit | Types and counts do not specify a full genotype |
Read sets and corresponding chromosome types
The original schematic uses an invented model with three chromosome types in a complete set. One gamete nucleus contains one chromosome of each type, and a second compatible gamete nucleus contains the corresponding types. Blue and gold distinguish the two contributors; shapes distinguish chromosome types. The colours are a teaching key, not natural chromosome colours.
After the supplied nuclear fusion, the zygote nucleus contains six chromosomes organised as two sets of three. Corresponding chromosomes carry the same types of genes, but may carry different versions. The diagram does not specify every gene or allele, so it cannot establish exact genotype or a visible trait. Matching shape does not require identical genetic information.
Separate contributor colour from a new allele
Trace each contribution rather than multiplying the number of drawn nuclei. Before fusion, there are two nuclei with one set each; after fusion, the supplied zygote has one nucleus with two sets. Counting one final nucleus and calling it haploid would confuse organelle number with chromosome-set number. Likewise, the second set is contributed, not a new empty set created from nothing.
This diagram models fertilisation, not the chromosome movements of meiosis or a measurement of specimen size. Its three-type example is distinct from the larger counts in the later data table. Use the stated complete-set definition and the colour key consistently, and keep fusion of two inherited contributions separate from later copying for cell division.
Pause and explain
Does a pair of corresponding chromosome shapes require identical alleles?
Worked example
A keyed diagram shows three blue chromosomes plus three gold chromosomes in a fused nucleus. Give sets and total.
Show the worked solution
- Each contributor supplies a complete set of three.
- The combined nucleus contains both sets.
Answer Two sets; six chromosomes in the diploid zygote nucleus.
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Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
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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.
Count sets and chromosomes after two model gametes each contribute three chromosomes.
Give me a hint
Combine complete sets.
Compare my reasoning
- One set has three chromosomes.
- Two sets contain six.
The zygote nucleus is diploid, with two sets and six chromosomes.
Look for these in your work
- Count both contributions
- Name diploid
Explain why one final nucleus can be diploid.
Give me a hint
Nuclei and chromosome sets are different counts.
Compare my reasoning
- Nuclear fusion produces one nucleus.
- It contains two contributed sets.
One fused nucleus can be diploid because it contains two chromosome sets, one contributed by each gamete.
Look for these in your work
- Separate nucleus and set counts
- Trace both contributors
The schematic uses matching shapes and different contributor colours. Does it establish every allele or inherited trait?
Give me a hint
Use only what the diagram key supplies.
Compare my reasoning
- Shapes and colours track types and contributions.
- Exact alleles and trait outcomes are not supplied.
No. The key tracks chromosome types and contributor sets; it does not specify every allele or inherited trait.
Look for these in your work
- Read the key accurately
- State the missing genetic detail
Common mistakes
- Calling one fused nucleus one chromosome set.
- Reading model colours as natural colours or exact genotypes.
What can you explain now?
Does a pair of corresponding chromosome shapes require identical alleles?
Compare with the explanation
No; matching types can carry different versions
Shape is not a full genotype record.
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Make it stick
Trace both sets and explain why one fused nucleus is diploid.
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.