Unit 4 · Theme D: Continuity and Change
The Complete IB Biology Guide · IB BiologyUnit 4 · Theme D: Continuity and Change
- The Complete IB Biology Guide
- IB Biology
- 4 sections
International Baccalaureate does not publish a per-unit weighting for this exam. Exam structure (official): SL: Paper 1 (90 min, 36%) · Paper 2 (90 min, 44%) · Internal Assessment (20%); HL: Paper 1 (120 min, 36%) · Paper 2 (150 min, 44%) · Internal Assessment (20%). Be ready to carry the same unit skills and representations into a complete answer on every paper. Theme D: Continuity and Change links molecular detail to patterns at cellular, organismal, and ecological scales. The Theme D: Continuity and Change table identifies three high-value routes for revision. In Theme D: Continuity and Change, start with the biological object, then name the comparison or mechanism, and only then select a calculation or diagram.
- How IB Biology assesses this International Baccalaureate publishes no per-unit percentage for IB Biology; Paper 1 accounts for 36% and Paper 2 for 44% of the total at both SL and HL. · SL: Paper 1 (90 min, 36%) · Paper 2 (90 min, 44%) · Internal Assessment (20%) | HL: Paper 1 (120 min, 36%) · Paper 2 (150 min, 44%) · Internal Assessment (20%) · show the model with inheritance table or statistical test, cell-cycle or gene-expression diagram, frequency, water-potential, or climate graph
- Key skills DNA Replication and Cell and Nuclear Division, Protein Synthesis, Mutation, and Gene Editing, Water Potential and Homeostasis, Reproduction, Inheritance, Natural Selection, Stability and Change in Ecosystems and Climate Change, Gene Expression (HL)
- How to study for Unit 4 This page turns inheritance table or statistical test, cell-cycle or gene-expression diagram, frequency, water-potential, or climate graph into one route: Identify the scale of change and write the null model, probability model, or rate relationship before calculating..
- The organizing decision Use inheritance, cell, and change models only after stating the assumptions and scale that make each model meaningful.
What IB Biology Unit 4 covers
Use this map to connect each assessed skill to the relationship or representation that makes it visible.
DNA Replication and Cell and Nuclear Division
DNA replication rate; mitotic index; semi-conservative replication02Protein Synthesis, Mutation, and Gene Editing
protein-coding length; mutation; gene editing03Water Potential and Homeostasis
water potential; solute potential; pressure potential04Reproduction
meiosis; gamete; fertilization05Inheritance
chi-square test; dihybrid probability; independent assortment06Natural Selection
allele frequency; selection pressure; Hardy–Weinberg equilibrium07Stability and Change in Ecosystems and Climate Change
climate-change rate; ecosystem stability; resilience08Gene Expression (HL)
gene expression; transcription factor; epigenetic modificationInternational Baccalaureate does not publish a per-unit weighting for this exam.
Official unit name and weighting: IB subject guide / subject brief.
Theme D: Continuity and Change
Connect the published share to the unit model
Theme D: Continuity and Change links molecular detail to patterns at cellular, organismal, and ecological scales. The Theme D: Continuity and Change table identifies three high-value routes for revision. In Theme D: Continuity and Change, start with the biological object, then name the comparison or mechanism, and only then select a calculation or diagram.
The Theme D: Continuity and Change examples use independently created measurements. Those Theme D: Continuity and Change settings are practice material, not past-paper questions. The purpose in Theme D: Continuity and Change is to make units, assumptions, and limits visible while keeping the biological interpretation attached to every number.
For qualitative prompts in Theme D: Continuity and Change, predict the biological relationship before reading any options. For Theme D: Continuity and Change, reject a choice that reverses cause and effect, changes the stated scale, or claims more certainty than the evidence supports. For quantitative work in Theme D: Continuity and Change, list values with units and estimate the likely magnitude before calculating.
A complete Theme D: Continuity and Change response distinguishes observation from explanation. Within Theme D: Continuity and Change, a pattern states what changes; a mechanism names how a biological process produces that change; an evaluation identifies the design feature that limits confidence. These Theme D: Continuity and Change roles should not be blended into a single vague sentence.
The decision that organizes this unit
Define the system and choose the route before calculating
Use inheritance, cell, and change models only after stating the assumptions and scale that make each model meaningful.
Identify the scale of change and write the null model, probability model, or rate relationship before calculating.
Mechanism route and repair branches
Relationships to preserve
- Inheritance probabilities depend on assumptions such as independence or equilibrium.
- A cell index is a snapshot proportion and does not by itself measure phase duration.
- A rate summarizes an interval and does not alone establish the cause of change.
Representations to read
- inheritance table or statistical test
- cell-cycle or gene-expression diagram
- frequency, water-potential, or climate graph
Branches to reject
- using observed counts in the denominator of a chi-square term
- treating a test statistic as though it were a probability
- ignoring a negative sign when comparing water potentials
| Key concept | Why it is hard | What scores |
|---|---|---|
| Inheritance | Probabilities depend on assumptions | State independence or equilibrium |
| Cells | Indices are snapshot proportions | Separate fraction from duration |
| Change | Rates summarize an interval | Limit the causal claim |
How IB Biology assesses Theme D: Continuity and Change
What a complete response must make visible
Match the task to evidence that a reader can audit, then check the most likely reasoning failure before finalizing the response.
| Task | Evidence to show | Hurdle |
|---|---|---|
| DNA Replication and Cell and Nuclear Division | DNA replication rate; mitotic index; semi-conservative replication | using observed counts in the denominator of a chi-square term |
| Protein Synthesis, Mutation, and Gene Editing | protein-coding length; mutation; gene editing | treating a test statistic as though it were a probability |
| Water Potential and Homeostasis | water potential; solute potential; pressure potential | ignoring a negative sign when comparing water potentials |
| Reproduction | meiosis; gamete; fertilization | using observed counts in the denominator of a chi-square term |
| Inheritance | chi-square test; dihybrid probability; independent assortment | treating a test statistic as though it were a probability |
| Natural Selection | allele frequency; selection pressure; Hardy–Weinberg equilibrium | ignoring a negative sign when comparing water potentials |
| Stability and Change in Ecosystems and Climate Change | climate-change rate; ecosystem stability; resilience | using observed counts in the denominator of a chi-square term |
| Gene Expression (HL) | gene expression; transcription factor; epigenetic modification | treating a test statistic as though it were a probability |
Chi-square test of inheritance
Read each question as it appears on the exam paper, choose your answer, then tap it to see the feedback.
Practice 1: Calculating chi-square for inheritance data
Observed offspring counts are 82, 28, 31 and 9; expected counts for a 9:3:3:1 ratio are 84.375, 28.125, 28.125 and 9.375.
Calculate the chi-square statistic and state the number of degrees of freedom.
Show your working.
A. Add (observed − expected)² ÷ expected for all four classes to obtain approximately 0.377, with 4 − 1 = 3 degrees of freedom.
✅ Correct! 😊 This uses each matching expected count in the denominator, includes every offspring class and calculates degrees of freedom from the four classes.
B. Add (observed − expected)² ÷ observed for all four classes and report approximately 0.377 with 3 degrees of freedom.
❌ Not quite 😭 This puts observed counts below the fraction bar. Chi-square weights each squared deviation using its expected count, so this set-up does not earn the method mark.
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C. Add observed − expected for all four classes to obtain 0, with 4 degrees of freedom.
❌ Not quite 😭 Unsigned deviations are not the chi-square terms and positive and negative differences cancel. Degrees of freedom are also one fewer than the number of classes.
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D. Calculate one term using the largest deviation only and report 0.294 as the chi-square statistic, with 1 degree of freedom.
❌ Not quite 😭 This omits three offspring classes. Every class contributes to the statistic, and four classes give 3 degrees of freedom rather than 1.
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Full explanation
- Step 1Pair each observed offspring count with its corresponding expected count.
- Step 2For each class, calculate (observed − expected)² ÷ expected, keeping the expected value in the denominator.
- Step 3Add the four contributions: 0.067 + 0.001 + 0.294 + 0.015 ≈ 0.377.
- Step 4Calculate degrees of freedom as the number of classes minus one: 4 − 1 = 3.
Practice 2: Interpreting a mitotic index
A microscope image contains 200 counted cells, of which 36 are observed in mitosis.
Which conclusion is supported by these observations?
Select the best answer.
A. The mitotic index is 18%, so 18% of the counted cells were observed in mitosis in this snapshot.
✅ Correct! 😊 This calculates 36 ÷ 200 × 100 and describes the result as a snapshot proportion. It makes no unsupported claim about how long mitosis lasts.
B. The mitotic index is 18%, so every cell spends exactly 18% of its cell cycle in mitosis.
❌ Not quite 😭 The percentage calculation is correct, but the conclusion converts a snapshot into an exact phase duration. The observations do not establish that timing.
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C. The mitotic index is 82% because cells not observed in mitosis must be dividing too slowly to detect.
❌ Not quite 😭 This reports the complement rather than the fraction observed in mitosis. It also invents a cause for cells outside the observed mitotic group.
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D. The mitotic index is 5.6 because the total cell count is divided by the number in mitosis.
❌ Not quite 😭 This reverses the required fraction. A mitotic index is the number observed in mitosis divided by the total number counted, not the reciprocal.
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Full explanation
- Step 1Identify the relevant counts: 36 cells in mitosis out of 200 cells observed.
- Step 2Calculate the snapshot proportion: 36 ÷ 200 = 0.18, or 18%.
- Step 3State exactly what was measured: the fraction of counted cells observed in mitosis.
- Step 4Do not convert this snapshot into an exact claim about phase duration without further evidence.
Key terms for Unit 4: Theme D: Continuity and Change
Models, uses, and boundaries
- Chi-square statistic
- For each class, square observed minus expected, divide by expected, then add all class values. Use when observed counts are compared with expected counts from a stated inheritance model.
- Two-sample t statistic
- Divide the difference between two sample means by the standard error of that difference. Use when independent sample means and the standard error of their difference are given.
- Hardy–Weinberg carrier frequency
- Take the square root of the recessive phenotype frequency to find q, set p to one minus q, then calculate two times p times q. Use when a recessive phenotype frequency is given and equilibrium assumptions are stated.
- Water potential
- Add solute potential and pressure potential to find total water potential. Use when both potential components are supplied in matching pressure units.
IB Biology Unit 4 FAQ
How much of IB Biology does Unit 4 carry?
International Baccalaureate does not publish a per-unit weighting for this exam.
What is the first move on a Theme D: Continuity and Change problem?
Identify the scale of change and write the null model, probability model, or rate relationship before calculating.
Which relationships should I preserve?
Inheritance probabilities depend on assumptions such as independence or equilibrium. A cell index is a snapshot proportion and does not by itself measure phase duration. A rate summarizes an interval and does not alone establish the cause of change.
Which representations should I practice?
Practice moving among inheritance table or statistical test, cell-cycle or gene-expression diagram, frequency, water-potential, or climate graph.
What error should I check before submitting an answer?
Check for using observed counts in the denominator of a chi-square term; treating a test statistic as though it were a probability; ignoring a negative sign when comparing water potentials.
Evidence workshop
Continue from the free model into complete practice
The full unit guide continues with the chapter’s worked examples, figures, scoring tables, and answer checks.
- Chi-square test of inheritance
- Two-sample t statistic
- Hardy–Weinberg carrier frequency
- Water potential
Full unit practice. Open the complete guide for the full evidence workshop and synthesis.
Related IB Biology unit guides
IB Biology Exam Guide & Review
The whole exam and its official unit sequence.01Theme A: Unity and Diversity
International Baccalaureate does not publish a per-unit weighting for this exam.02Theme B: Form and Function
International Baccalaureate does not publish a per-unit weighting for this exam.03Theme C: Interaction and Interdependence
International Baccalaureate does not publish a per-unit weighting for this exam.How to study IB Biology Unit 4
Start with the organizing decision
Before solving, restate the decision in operational terms: Use inheritance, cell, and change models only after stating the assumptions and scale that make each model meaningful.. Your first written move should be to Identify the scale of change and write the null model, probability model, or rate relationship before calculating..
Practice the same idea in several representations
Rotate through inheritance table or statistical test, cell-cycle or gene-expression diagram, frequency, water-potential, or climate graph. Use each representation to practice DNA Replication and Cell and Nuclear Division, Protein Synthesis, Mutation, and Gene Editing, Water Potential and Homeostasis, Reproduction, Inheritance, Natural Selection, Stability and Change in Ecosystems and Climate Change, Gene Expression (HL), and explain what stays invariant when the surface form changes.
Turn each error into a repair check
After every attempt, audit the response for using observed counts in the denominator of a chi-square term; treating a test statistic as though it were a probability; ignoring a negative sign when comparing water potentials. Then redo only the first step that made the reasoning diverge, keeping units, direction, and model conditions visible.
Confirm current course details in the official IB subject guide / subject brief for the 2027 administration.