Distinguish sister chromosome copies from homologous chromosome pairs
The three-pair diploid model has six chromosome identities and twelve attached DNA copies after replication. Sister copying preserves A/A and a/a separately, so each daughter retains A/a.
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 chromosomes, genes, chromosome sets, selective gene expression and the distinction between a nucleus and a whole cell.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which relationship describes attached sister copies after exact replication?
What you will learn
- Separate two copies from two parental chromosome types
- Preserve different allele forms through copying
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Homologous pair | Separate chromosomes of corresponding type; forms may differ |
| Sister copies | Exact copies of one individual chromosome |
| Diploid model | 3 pairs; 6 chromosomes; 12 DNA molecules after copying |
| A/a outcome | Both completed daughters retain A/a |
Separate two copies from two parental chromosome types
A diploid chromosome pair and the two sister copies of one replicated chromosome are different relationships. The paired chromosomes represent two contributions of the same type and can carry different allele forms. Each of those chromosomes is copied before mitosis, producing two sister copies with the same information under the exact-copying assumption.
An original diploid model has three pairs and six chromosomes: type one carrying A or a, type two carrying B or B, and type three carrying C or c. Replication copies each of the six individual chromosomes. The attached stage therefore contains six replicated chromosomes with twelve DNA molecules; it does not turn the three pairs into only three chromosomes.
Preserve different allele forms through copying
For the A-bearing chromosome, its two sister copies carry A and A. For the paired a-bearing chromosome, its sisters carry a and a. Normal separation supplies one A-bearing chromosome and one a-bearing chromosome to each daughter, preserving the original A/a pair. Copying does not change the a allele to A simply because the chromosome types correspond.
A drawing that uses the same colour for corresponding chromosomes is a type key, not proof of identical allele forms. Identify whether a line joins sister copies or whether a bracket compares homologous types. The lesson uses these relationships to explain the maintained complement; detailed mitotic stage names and the full meiotic behaviour of homologues remain outside this section.
Pause and explain
Six original chromosomes are each replicated. What is the attached-stage DNA-molecule count?
Worked example
A and a are on paired chromosomes. What sister-copy forms appear after exact replication?
Show the worked solution
- Copy A into A and A.
- Copy a into a and a.
Answer The A-bearing chromosome has A/A sister copies; the a-bearing chromosome has a/a sister copies.
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.
Distinguish homologous chromosome pairs and sister copies using the A/a model.
Give me a hint
First pair types, then copy each individual.
Compare my reasoning
- Identify A and a on separate paired chromosomes.
- Copy each chromosome exactly into sisters.
The homologous pair contains separate A-bearing and a-bearing chromosomes; their sister copies are A/A and a/a respectively after exact replication.
Look for these in your work
- Name both relationships
- Preserve the forms
A diploid model has three pairs and six chromosomes. Count the replicated chromosome structures and DNA molecules before separation.
Give me a hint
Replicate all six individuals.
Compare my reasoning
- Retain six joined chromosome structures.
- Multiply six by two DNA molecules.
There are six replicated chromosomes and twelve DNA molecules at the attached-copy stage.
Look for these in your work
- Distinguish pairs and individuals
- Label the counting stage
A claim says copying the A/a pair makes both daughter cells A/A. Repair the explanation under normal accurate mitosis.
Give me a hint
Each daughter receives both original chromosome types and forms.
Compare my reasoning
- Preserve the A and a copies separately.
- Distribute one copy of each original chromosome to each daughter.
Both daughters retain A/a: each receives one A-bearing and one a-bearing chromosome. Exact copying does not replace a with A.
Look for these in your work
- Preserve both parental forms
- Explain complete distribution
Common mistakes
- A homologous pair is not the sister-copy pair of one chromosome.
- Exact replication preserves allele forms rather than replacing them.
What can you explain now?
Six original chromosomes are each replicated. What is the attached-stage DNA-molecule count?
Compare with the explanation
Twelve
Six times two gives twelve DNA molecules.
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Make it stick
Without your notes, separate two copies from two parental chromosome types and preserve different allele forms through copying. Distinguish nuclear division, chromosome copies and completed daughter cells 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.