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3.8.1
Mutations and Gene Control
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
5
Total marks
12
Marks / Q
2.4
Accessibility
83.0%
Mastery
40.0%
Student strength
63.2%
01
At a glance
3.8.1 · Mutations and Gene Control
8YRSYNTHESIS
3.8.1 (Mutations and Gene Control) appeared in 5 of the 8 years between 2017 and 2024, contributing 5 questions and 12 marks across Papers 1, 2 and 3. APPLICATION dominates the mark distribution at 66.7% of total marks. The accessibility–mastery gap sits at 43.0 percentage points (83.0% vs 40.0%) — most students reach partial credit, but full marks remain harder to secure. Mastery varied year-to-year, lowest in 2022 (6.0%) and highest in 2018 (94.0%).
Access–mastery gap
+43 pp
Lowest mastery
2022 · 6.0%
Highest mastery
2018 · 94.0%
02
Question type split
By marks · compound to dominant
12MARKS
12
marks
Application66.7%8 marks
Knowledge33.3%4 marks
Calculation0.0%0 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
11TERMS
Tier 1 · Always credit
Tier 2 · Sometimes credit
substitutiondeletionprimary structureamino acid sequencetertiary structuremutation
Reject · Never credit
active site (for tertiary); amino acids are formed; frame shift (wrong mutation type)different amino acids formedprotein not formed (for mp2)no protein produced; non-functional protein; meiosis'an amino acid is not produced'; 'bases are affected' (vague); 'sequences are changed' (vague); 'frameshift' alone as full explanation
04
Question patterns
Recurring formats & tariff structure
0PARAGRAPHS
05
Year-over-year presence
P1 + P3 · 2017–2024
8YEARS
| Year | Questions | Total marks | Mean accessibility | Mean mastery |
|---|---|---|---|---|
| 2017 | 0 | 0 | — COVID | — COVID |
| 2018 | 1 | 1 | 94.0% | 94.0% |
| 2019 | 1 | 3 | 90.0% | 30.0% |
| 2020 | 0 | 0 | — COVID | — COVID |
| 2021 | 0 | 0 | — COVID | — COVID |
| 2022 | 1 | 3 | 45.0% | 6.0% |
| 2023 | 1 | 3 | 92.0% | 15.0% |
| 2024 | 1 | 2 | 94.0% | 55.0% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
11TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| substitution | 2 | 2018, 2024 | |
| deletion | 2 | 2018, 2024 | |
| primary structure | 2 | 2019, 2022 | |
| amino acid sequence | 2 | 2019, 2022 | |
| tertiary structure | 2 | 2019, 2022 | |
| mutation | 2 | 2022, 2023 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| active site (for tertiary); amino acids are formed; frame shift (wrong mutation type) | 1 | 2019 | |
| different amino acids formed | 1 | 2022 | |
| protein not formed (for mp2) | 1 | 2022 | |
| no protein produced; non-functional protein; meiosis | 1 | 2023 | |
| 'an amino acid is not produced'; 'bases are affected' (vague); 'sequences are changed' (vague); 'frameshift' alone as full explanation | 1 | 2024 |
Marks in this sub-section are typically awarded for precise terminology and correct application of biological principles. Sequential mark schemes — where each mark requires building on the previous one — are common in multi-mark questions; stating the first step without progression rarely earns more than one mark. Calculation marks are typically split between method (correct setup and value extraction) and answer (accurate numerical result), allowing partial credit when arithmetic errors occur.
07
Where students lose marks
Examiner-anchored error patterns
4CASE STUDIES
Conceptual errors
- Treating base substitution as causing a frameshift mutation — frameshifts require insertions or deletions; a single-base swap does not shift the reading frame
- Applying loss-of-function logic to gain-of-function oncogene questions: many students stated that the KRAS mutation produces "no protein" or "a non-functional protein" when the correct answer is an altered or overactive signalling protein
- Reversing causation in tumour development: describing uncontrolled cell division as the trigger for mutation rather than its consequence
- Describing changes in amino acids as occurring "in the gene" rather than in the resulting polypeptide — the mutation alters the base sequence; the amino acid sequence changes are in the protein
- Believing inversion mutations alter the number of DNA bases — an inversion reverses a segment; it does not add or remove bases
Vocabulary errors
- "Active site" in place of tertiary structure when describing polypeptide structure change — haemoglobin and K-Ras are not enzymes; "active site" was explicitly rejected across multiple years
- "Bonds altered" without naming the bond type (hydrogen, ionic, or disulfide) — named bond types are required to access the second mark point in polypeptide structure questions
- "Amino acids are formed" or "amino acids are produced" by the mutation — the mutation changes which amino acid is incorporated, not whether amino acids exist
- "Frameshift" as the full explanation for a shorter polypeptide without linking the mechanism to missing codons or a premature stop codon
- "Epigenetic changes" cited as the cause when the question asks about gene mutation — epigenetics alters expression without changing the base sequence
Application errors
- Stopping at the first mark point ("change in primary structure") without continuing through specific bond disruption to tertiary structure change — these are sequential mark schemes that penalise incomplete chains
- Applying enzyme-language (active site, substrate) to polypeptide structure questions where the protein is not an enzyme
- On shorter-polypeptide questions, restating the stem ("this mutation produces a shorter polypeptide") rather than specifying the codon-level mechanism — missing codons or premature stop codon must be explicitly stated
- On gain-of-function oncogene questions, assuming the protein must be absent because a mutation occurred — the question context (oncogene, K-Ras signalling) implies altered or overproduced function, not absent function
High-impact failures · examiner narrative
2022 P2 Q10.13 marks
Only around 6% of students scored all three marks — the lowest mastery rate in this subtopic. The question required a full sequential chain: mutation changes the DNA base sequence → amino acid sequence (primary structure) of the tumour suppressor protein changes → tertiary structure is altered → the protein can no longer regulate the cell cycle → uncontrolled cell division results. Most students offered vague statements about "the gene not working" or "the protein not forming" without tracing the mechanism through protein structure. The opening mark point — change in DNA base sequence — was commonly omitted as students began their answer at the protein level. "Different amino acids formed" was explicitly rejected.
2023 P2 Q08.13 marks
Approximately 15% scored full marks. The KRAS question caught many students who applied loss-of-function reasoning to a gain-of-function scenario: stating "no protein is produced" or "a non-functional protein is made" was explicitly rejected in both cases. The required answer is that K-Ras becomes altered or overproduced, driving excessive signalling. Additional errors included reversing causation (uncontrolled division described as causing the mutation), citing epigenetic changes rather than a base sequence mutation, and describing amino acid changes in the gene rather than the protein.
2019 P2 Q02.13 marks
Approximately 30% scored full marks. The question required three consecutive steps: change in amino acid sequence (primary structure) → change in named bond types → altered tertiary structure. "Active site" (rejected — not an enzyme) and "bonds altered" without specifying the bond type (rejected — too vague) each blocked access to a mark. Students who described bond changes loosely and students who used "active site" for tertiary structure both lost marks in a scheme where all three points are independently available from a single protein description.
2024 P3 Q05.12 marks
Although 94% named a mutation type, only around 55% secured both marks. MP2 required a codon-level explanation: either that deletion or translocation causes codons to be missing from the reading frame, or that any mutation type can result in a premature stop codon. Students who wrote "frameshift" alone, or who restated the stem ("this produces a shorter polypeptide"), failed to access the second mark — the mechanism must be specified at the triplet or codon level.
08
Difficulty signals
Performance metric synthesis
43PP GAP
Mean accessibility
83.0%
Mean mastery
40.0%
Mean student strength
63.2%
The accessibility–mastery gap of 43.0 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.8 (The control of gene expression), 3.8.1 ranks 4 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is falling across the cohort window: 94.0% in 2018 → 55.0% in 2024 (-39.0 percentage points). Mean mastery was lowest in 2022 (6.0%) and highest in 2018 (94.0%).