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3.7.2
Populations
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
19
Total marks
36
Marks / Q
1.9
Accessibility
57.5%
Mastery
30.5%
Student strength
43.6%
01
At a glance
3.7.2 · Populations
8YRSYNTHESIS
3.7.2 (Populations) appeared in 8 of the 8 years between 2017 and 2024, contributing 19 questions and 36 marks across Papers 1, 2 and 3. CALCULATION dominates the mark distribution at 52.8% of total marks. The accessibility–mastery gap sits at 27.1 percentage points (57.5% vs 30.5%) — most students reach partial credit, but full marks remain harder to secure. Mastery varied year-to-year, lowest in 2023 (14.0%) and highest in 2022 (45.0%).
Access–mastery gap
+27 pp
Lowest mastery
2023 · 14.0%
Highest mastery
2022 · 45.0%
02
Question type split
By marks · compound to dominant
36MARKS
36
marks
Calculation52.8%19 marks
Knowledge25.0%9 marks
Application22.2%8 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
21TERMS
Tier 1 · Always credit
2pqq²
Tier 2 · Sometimes credit
allele frequencyHardy-Weinbergheterozygousgene poolinbreeding2pq = heterozygous frequencyp² + 2pq + q² = 1crossing overnatural selection
Reject · Never credit
treating 36% as homozygous recessivealleles in a species alone (without population)genes instead of allelesinterbreeding within the populationgene instead of allelepq (alone) for heterozygous genotypepq instead of 2pq (zero marks for final answer)crossing overcodominanceindependent assortment
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 | 3 | 5 | 47.7% | 21.3% |
| 2018 | 1 | 2 | 70.0% | 39.0% |
| 2019 | 1 | 2 | 68.0% | 27.0% |
| 2020 | 2 | 4 | — COVID | — COVID |
| 2021 | 2 | 3 | — COVID | — COVID |
| 2022 | 3 | 5 | 56.0% | 45.0% |
| 2023 | 2 | 4 | 30.5% | 14.0% |
| 2024 | 5 | 11 | 70.6% | 32.8% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
26TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| 2pq | 3 | 2017, 2021, 2024 | |
| q² | 3 | 2021, 2023, 2024 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| allele frequency | 3 | 2021, 2023 | |
| Hardy-Weinberg | 2 | 2017, 2023 | |
| heterozygous | 2 | 2017, 2024 | |
| gene pool | 2 | 2017 | |
| inbreeding | 2 | 2017, 2024 | |
| 2pq = heterozygous frequency | 2 | 2018, 2019 | |
| p² + 2pq + q² = 1 | 2 | 2019, 2023 | |
| crossing over | 2 | 2020, 2024 | |
| natural selection | 2 | 2022, 2024 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| treating 36% as homozygous recessive | 1 | 2017 | |
| alleles in a species alone (without population) | 1 | 2017 | |
| genes instead of alleles | 1 | 2017 | |
| interbreeding within the population | 1 | 2017 | |
| gene instead of allele | 1 | 2017 | |
| pq (alone) for heterozygous genotype | 1 | 2018 | |
| pq instead of 2pq (zero marks for final answer) | 1 | 2019 | |
| crossing over | 1 | 2022 | |
| codominance | 1 | 2022 | |
| independent assortment | 1 | 2022 | |
| converse statements | 1 | 2022 | |
| "no births or deaths"; dominant allele will increase in frequency | 1 | 2023 | |
| 2pq² | 1 | 2024 | |
| dividing 2pq by 2 | 1 | 2024 | |
| pq alone | 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
2CASE STUDIES
Conceptual errors
- Wrong phenotype frequency assigned as q² in Hardy-Weinberg calculations — the most consistent calculation error across all years was choosing the wrong value for q²; in 2017, students who assigned q² = 0.36 (the yellow phenotype frequency) obtained the wrong answer (48%) rather than recognising that green, the homozygous recessive phenotype, must equal q² = 0.64; in 2018, students who treated 0.1 as q rather than q² obtained a wrong heterozygote frequency; identifying which phenotype corresponds to q² before squaring is the prerequisite step that most students skipped (2017 P2 Q07.1, 2018 P2 Q10.2)
- Hardy-Weinberg conditions confused with mark-release-recapture assumptions — in 2023, students described "no births or deaths" as a Hardy-Weinberg assumption; this is an assumption of mark-release-recapture methodology; the Hardy-Weinberg principle assumes no natural selection, no mutation, no migration, large population size, and random mating; conflating these two different methodological frameworks was the dominant error (2023 P3 Q06.2)
- "Genes" used instead of "alleles" in population genetics definitions — in 2017, when asked to define gene pool, students frequently used "genes" rather than "alleles"; the gene pool is the total of all alleles in a population; using "genes" instead loses the critical distinction between locus (gene) and variant (allele) (2017 P2 Q09.1)
Vocabulary errors
- pq used for heterozygote frequency instead of 2pq — this was rejected in 2018 and 2019 and results in an answer that is half the correct value; the factor of 2 in 2pq accounts for both allele arrangements in a heterozygote (Aa and aA); students who wrote pq or omitted the factor of 2 scored zero for the final answer even when the understanding of the formula was otherwise demonstrated (2018 P2 Q10.2, 2019 P2 Q06.2)
- Population definition missing either "time" or "interbreeding" — in 2024, only 5% scored both marks on the population definition question; 64% scored zero; the specification definition requires a population to be members of the same species occupying the same area at the same time and able to interbreed; omitting either "time" or "interbreed/fertile offspring" was penalised; students who defined a community (multiple species) rather than a population scored nothing (2024 P3 Q06.1)
- "Frequency" expressed as a decimal when a percentage was required, or vice versa — in 2017, giving 0.32 rather than 32% was penalised; the question specified the answer should be expressed as a percentage; matching the required units and format to the question instruction was the mark (2017 P2 Q07.1)
Application errors
- Degrees of freedom calculated as the number of categories rather than categories minus one — in 2022, many students correctly named the chi-squared test but could not calculate degrees of freedom correctly; for a chi-squared test, degrees of freedom = number of categories − 1; students who gave the number of categories itself as degrees of freedom were not credited (2022 P2 Q06.2)
- Hardy-Weinberg assumptions stated without applying them to why frequencies deviate in a real population — in 2022, students who listed assumptions correctly but did not explain why violating them would cause observed frequencies to differ from Hardy-Weinberg expectations scored partial credit only; the mark required connecting the assumption to the real-world mechanism of deviation (2022 P2 Q06.3)
- Nuclear DNA percentage not factored into mutation accumulation calculation — in 2024, 25% of students scored two marks rather than three because they calculated the number of mutations without accounting for the 80% of mutations that occur in nuclear DNA; the question required multiplying by 0.80 at one stage and by 0.15 at another; omitting either factor produced a wrong final answer (2024 P3 Q05.2)
High-impact failures · examiner narrative
2017 P2 Q07.12 marks
Hardy-Weinberg calculation of heterozygote frequency. Only about one in six students scored both marks. The calculation required recognising that green phenotype (64%) = q², not yellow (36%); taking the square root to get q; using p = 1 − q; and then calculating 2pq. Most students used yellow as q² = 0.36, giving q = 0.6, p = 0.4, and 2pq = 0.48 — a plausible but wrong answer. The examiner reported this as a case where mathematical ability was not the limiting factor; students who recognised the green/recessive assignment completed the calculation correctly.
2023 P3 Q06.22 marks
Conditions of Hardy-Weinberg not met in a real population. Mastery 14.0% — the lowest in any year for this sub-section. Students described "no births or deaths" as a Hardy-Weinberg condition, confusing it with mark-release-recapture. Others stated that the dominant allele would increase in frequency without selection acting on it — which is not a Hardy-Weinberg principle. The correct conditions — no selection, no mutation, no migration, random mating, large population — were rarely stated with the additional requirement of explaining why each violation causes deviation from equilibrium.
08
Difficulty signals
Performance metric synthesis
27PP GAP
Mean accessibility
57.5%
Mean mastery
30.5%
Mean student strength
43.6%
The accessibility–mastery gap of 27.1 percentage points characterises this sub-section's difficulty profile. Most students reach partial credit; full marks remain harder to achieve. Within 3.7 (Genetics, populations, evolution and ecosystems), 3.7.2 ranks 2 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is broadly flat across the cohort window: 21.3% in 2017 → 32.8% in 2024 (+11.5 percentage points). Mean mastery was lowest in 2023 (14.0%) and highest in 2022 (45.0%).