Distinguish genetic identity from identical appearance or function
Matching supplied genetic records can support identity in an exact-copying model. Equal chromosome counts or similar appearance alone do not establish every DNA base.
Section 17.2 · Mitosis
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
Two normal mitotic daughters have the same genes but different protein expression. Which conclusion fits?
What you will learn
- Compare chromosome information
- Separate genotype from visible features
Original teaching model. Counts and records are stipulated; drawings are schematic.
| Feature | Meaning |
|---|---|
| Parent record | A, B and C |
| Daughter 1 | A, B and C under exact copying |
| Daughter 2 | A, B and C under exact copying |
| Limit | Same count or appearance is not a full sequence test |
Compare chromosome information
The normal mitotic model gives each daughter the same genetic information as the parent and the other daughter. In an original three-label record, the parent carries chromosome records A, B and C, and each daughter receives A, B and C. Genetic identity is checked by comparing the supplied information, not by expecting the daughters to have identical measured sizes at every later time.
A chromosome count is useful but does not establish a complete DNA sequence. Two nuclei can each contain three chromosomes while differing in a chromosome identity or a DNA change. In a teaching model that explicitly supplies exact copying and complete matching labels, the intended identity follows from those assumptions and records. A photograph with three rods in each nucleus gives less information.
Separate genotype from visible features
Genetically identical daughter cells can later grow under different conditions or express different genes. The preceding expression pathway explained that shared genes do not require the same proteins to be produced in every cell. A daughter becoming specialised therefore need not mean mitosis deliberately supplied it with a different set of genes.
Do not turn the normal model into a promise that every cell division is error-free. Mutations or abnormal separation can disrupt the expected result, but those possibilities are outside a question that stipulates accurate copying and normal mitosis. State which evidence is supplied, which conclusion follows and which additional information a real observation would need.
Pause and explain
What does an unkeyed image of three rods in each nucleus establish most directly?
Worked example
Parent A B C gives daughters A B C and A B C. What is preserved?
Show the worked solution
- Compare every named entry.
- Separate the record from later appearance.
Answer Both daughters retain the supplied chromosome information; later appearance need not remain identical.
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.
Compare parent labels A B C with daughter labels A B C and A B C under exact copying.
Give me a hint
Check each entry in each daughter.
Compare my reasoning
- List the parent information.
- Compare both daughter lists.
Each daughter retains A, B and C, so both have the same supplied chromosome information as the parent.
Look for these in your work
- Compare all entries
- Keep the exact-copying assumption
Daughter P is larger than daughter Q after growth, but their supplied DNA records match. Is genetic identity contradicted?
Give me a hint
Separate size from DNA information.
Compare my reasoning
- Identify the matching DNA records.
- Identify size as a different observation.
No. The supplied genetic records match; different later sizes do not by themselves contradict genetic identity.
Look for these in your work
- Use the DNA evidence
- Distinguish size and information
Two nuclei each contain four unlabelled chromosomes. Explain why their equal counts do not prove exact genetic identity.
Give me a hint
Ask what the drawing does not supply.
Compare my reasoning
- Recognise the equal number.
- Identify missing identities and base sequences.
Equal counts show the same represented number, but without chromosome identity and DNA-sequence evidence they do not prove identical genetic information.
Look for these in your work
- Acknowledge the count
- State the evidence limit
Common mistakes
- Equal counts alone do not prove identical DNA sequences.
- Different expression does not require removal of genes.
What can you explain now?
What does an unkeyed image of three rods in each nucleus establish most directly?
Compare with the explanation
The represented counts, not every DNA base
The image does not supply a sequence record.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Without your notes, compare chromosome information and separate genotype from visible features. 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.