Read chromosome pairing evidence without equating pairs with identical genes
Corresponding autosome positions can have different allele forms. Changing a form does not change chromosome-set counts, and the usual XY sex-chromosome pair is not two identical copies.
Section 17.1 · Chromosomes, genes and proteins
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
What can corresponding positions on a supplied autosome pair show?
What you will learn
- Use supplied corresponding types to recognise a pair arrangement
- Retain allele and sex-chromosome differences
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Autosome correspondence | Supplied type and gene-position key |
| Possible information difference | Different allele forms |
| Unchanged small-model counts | Two sets, three pairs, six individuals |
| Sex-pair distinction | X and Y are not identical copies |
Use supplied corresponding types to recognise a pair arrangement
A pair arrangement groups two representatives of the same supplied chromosome type in the simple autosome model. Corresponding positions can carry the same gene identities without carrying the same allele forms. Pairing is therefore a relationship between supplied chromosome types, not a statement that every DNA base on both objects must be identical.
The original small model shows three paired autosome types, with one contribution from each stipulated gamete set. At the marked gene position on type 2, one chromosome carries form A and the other form a. There are still three chromosome pairs and six individuals; changing the allele labels does not change the number of sets or create an extra whole chromosome.
Retain allele and sex-chromosome differences
The usual human count adds a sex-chromosome pair to 22 autosome pairs. In the XX/XY model, X and Y are not identical copies and differ in information. Keep this distinction when using the syllabus shorthand about pairs: an autosome pairing key should not be used to claim every sex-chromosome pair has identical shapes, sizes or genes.
A drawn resemblance is not a complete chromosome-identification test. The practice supplies a key giving corresponding types and explicitly labels unreplicated individuals, so counts can be checked. An unkeyed image alone may not establish exact type, genotype or a diagnosis. Keep what the key supplies separate from observations it does not support.
Pause and explain
What remains unchanged when one allele label changes on the same six objects?
Worked example
Does changing one allele label from A to a add a chromosome pair?
Show the worked solution
- Retain the same chromosome objects and positions.
- Identify the changed information form rather than a new chromosome.
Answer No. The allele form changes in the model, but the chromosome objects, pair count and set number remain the same.
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.
Interpret A and a at a corresponding position on a supplied autosome pair.
Give me a hint
Use identity, form and pair count separately.
Compare my reasoning
- Keep one gene identity at that position.
- Identify two forms on the same chromosome pair.
The pair carries the same supplied gene identity at corresponding positions but different allele forms A and a; it remains one chromosome pair.
Look for these in your work
- Distinguish gene and form
- Keep pair count
A three-pair six-individual model changes only one allele label. Count sets pairs and individuals after the change.
Give me a hint
No chromosome object has been added or removed.
Compare my reasoning
- Retain two supplied sets.
- Retain three pairs and six individuals.
The model still has two sets, three pairs and six individual chromosomes; only the supplied information form changed.
Look for these in your work
- Give all counts
- Explain unchanged objects
Evaluate the claim that a usual human XY sex-chromosome pair must contain identical information because it is called a pair.
Give me a hint
Use the specific X/Y distinction.
Compare my reasoning
- Identify X and Y as different sex chromosomes in the model.
- Reject identical information from the pair label.
The claim is inaccurate: X and Y differ in size and information in the specified model, so calling them a sex-chromosome pair does not make them identical copies.
Look for these in your work
- Name the X/Y distinction
- Reject the identical-copy inference
Common mistakes
- Allele labels do not create whole chromosome objects.
- X and Y should not be called identical copies.
What can you explain now?
What remains unchanged when one allele label changes on the same six objects?
Compare with the explanation
Three pairs and six individuals in the supplied model
Information labels do not add objects.
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Make it stick
Explain use supplied corresponding types to recognise a pair arrangement and retain allele and sex-chromosome differences. 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.