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3.7.1
Inheritance
Analytical deep dive — question counts, mark distribution, mastery curves, command-word breakdowns, and examiner narrative analysis.
Questions
30
Total marks
55
Marks / Q
1.8
Accessibility
62.2%
Mastery
40.7%
Student strength
47.9%
01
At a glance
3.7.1 · Inheritance
8YRSYNTHESIS
3.7.1 (Inheritance) appeared in 8 of the 8 years between 2017 and 2024, contributing 30 questions and 55 marks across Papers 1, 2 and 3. APPLICATION dominates the mark distribution at 56.4% of total marks. The accessibility–mastery gap sits at 21.6 percentage points (62.2% vs 40.7%) — most students reach partial credit, but full marks remain harder to secure. Mastery varied year-to-year, lowest in 2019 (27.0%) and highest in 2023 (56.3%). Calculation marks are a small share (3.6%) but typically sit at the lower end of the mastery distribution.
Access–mastery gap
+22 pp
Lowest mastery
2019 · 27.0%
Highest mastery
2023 · 56.3%
02
Question type split
By marks · compound to dominant
55MARKS
55
marks
Application56.4%31 marks
Knowledge40.0%22 marks
Calculation3.6%2 marks
(by marks; compound rows assigned to dominant type):
03
Credit terms — quick reference
Mark scheme tier-locked
19TERMS
Tier 1 · Always credit
epistasis1:1:1:1 ratio
Tier 2 · Sometimes credit
linked genesheterozygousepistatichomozygous recessiveX chromosomeone allele (males)GgX^R X^r
Reject · Never credit
codominancechromosomes (for genotype reference)dihybridincorrect colour assignmentpercentagesfractions (for ratio)9:3:3:1two chromatidsallele causing AD is recessivemutation
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 | 5 | 9 | 68.4% | 50.4% |
| 2018 | 6 | 12 | 64.0% | 36.5% |
| 2019 | 4 | 7 | 43.2% | 27.0% |
| 2020 | 3 | 5 | — COVID | — COVID |
| 2021 | 3 | 6 | — COVID | — COVID |
| 2022 | 3 | 5 | 63.0% | 34.0% |
| 2023 | 3 | 5 | 68.3% | 56.3% |
| 2024 | 3 | 6 | 67.0% | 42.0% |
06
Mark scheme intelligence
2017–2024 mark scheme corpus
24TERMS
Tier 1 — frequently credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| epistasis | 3 | 2017, 2018, 2023 | |
| 1:1:1:1 ratio | 3 | 2019, 2020, 2023 |
Tier 2 — sometimes credited
| Term | Times credited | Years | Notes |
|---|---|---|---|
| linked genes | 3 | 2018, 2021 | |
| heterozygous | 3 | 2022, 2024 | |
| epistatic | 2 | 2017, 2023 | |
| homozygous recessive | 2 | 2018, 2024 | |
| X chromosome | 2 | 2018, 2024 | |
| one allele (males) | 2 | 2018, 2021 | |
| GgX^R X^r | 2 | 2019 |
Commonly rejected language
| Term | Times rejected | Years | Why rejected |
|---|---|---|---|
| codominance | 2 | 2017, 2023 | |
| chromosomes (for genotype reference) | 1 | 2017 | |
| dihybrid | 1 | 2017 | |
| incorrect colour assignment | 1 | 2017 | |
| percentages | 1 | 2017 | |
| fractions (for ratio) | 1 | 2017 | |
| 9:3:3:1 | 1 | 2017 | |
| two chromatids | 1 | 2017 | |
| allele causing AD is recessive | 1 | 2017 | |
| mutation | 1 | 2018 | |
| environmental factors | 1 | 2018 | |
| alleles of the same gene | 1 | 2018 | |
| dominant allele | 1 | 2018 | |
| donor nucleus prevents disease (wrong — child would have donor's DNA) | 1 | 2018 | |
| polar body has NO faulty mitochondria (very few | 1 | 2018 |
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
3CASE STUDIES
Conceptual errors
- Linked genes treated as independently assorting — in 2018, 45% of students scored zero on the linked genes question because they assumed a 9:3:3:1 ratio from independent assortment and then tried to explain departures from this ratio; linked genes on the same chromosome do not assort independently, and the expected ratio from a dihybrid cross between double heterozygotes for linked genes is dominated by parental combinations, not recombinants; students who recognised linkage then frequently omitted crossing over or failed to explain why recombinant phenotypes were rare (2018 P2 Q06.3)
- Crossing over described as occurring in mitosis — in 2018, some students correctly identified linkage and crossing over but then placed crossing over in mitosis; crossing over occurs during meiosis I (prophase I) in the formation of gametes; it does not occur in mitosis (2018 P2 Q06.3)
- Polar body incorrectly described as containing no faulty mitochondria — in 2018, the correct answer was "very few" faulty mitochondria, not "none"; polar bodies receive cytoplasm from the oocyte during meiosis and would contain some mitochondria; stating none was rejected as factually inaccurate (2018 P3 Q03.4)
- Alzheimer's disease allele treated as recessive despite the passage stating otherwise — in 2017, students who suggested the AD allele was recessive to explain why 75 mutations produced only 74 cases scored zero; the passage stated the allele is dominant; the correct explanation was that one individual was homozygous dominant, meaning they carried two copies but could only be counted once (2017 P2 Q10.1)
Vocabulary errors
- "Codominance" named instead of "epistasis" — this error appeared in both 2017 and 2023; epistasis is the interaction where one gene masks the expression of another; codominance is a completely different phenomenon where both alleles contribute to the phenotype equally; students who confused these terms demonstrated a fundamental vocabulary failure (2017 P2 Q07.2, 2023 P2 Q04.1)
- Sex from offspring genotypes omitted in genetic cross — in 2019 and 2023, a common error on sex-linked dihybrid crosses was giving correct offspring genotypes without specifying whether the offspring were male or female; the mark for offspring phenotype included the sex, and omitting it was penalised (2019 P2 Q06.4, 2023 P2 Q04.3)
- "Two chromatids" instead of "two chromosomes" or "two alleles" used to explain why only 204 samples were needed — in 2017, students explained the sample size of 204 using the concept of two chromatids per chromosome rather than two copies of chromosome 14 per individual; chromatids are duplicated chromosome strands within a single chromosome, not the homologous pair, and were rejected (2017 P2 Q10.1)
Application errors
- Grey body used as evidence of dominance incorrectly — in 2019, only 18% scored the mark for identifying which cross showed that grey body colour is dominant; the common error was citing that grey parents produce all grey offspring as evidence of dominance; this only shows that grey breeds true, not that grey is dominant over black; the correct evidence required identifying a cross where grey and black parents produced grey offspring (2019 P2 Q06.2)
- Evidence for X-linkage stated as grey/black ratio rather than sex-specific phenotype — in 2019, only 5% scored both marks on the X-linkage evidence question; students used the relative numbers of grey and black flies rather than identifying the sex-specific pattern; X-linkage evidence requires showing that phenotype frequencies differ between sexes, not just overall (2019 P2 Q06.3)
- Incorrect offspring genotypes in a cross cascade — in 2017, over 25% of students scored zero on the epistasis cross; when the initial parental genotypes were wrong, the entire cross generated incorrect offspring, making it impossible to award subsequent marks; the examiner noted that the question was considered straightforward but errors at the first step propagated through the whole answer (2017 P2 Q07.3)
High-impact failures · examiner narrative
2018 P2 Q06.33 marks
Explanation of results from a dihybrid linked gene cross. 45% scored zero. The question required students to recognise that the data showed linkage, not independent assortment; to state that parental combinations predominate because linked alleles are inherited together; and to explain that rare recombinant phenotypes result from crossing over during meiosis. Students who assumed independent assortment and then tried to reconcile the results with a 9:3:3:1 framework scored nothing. Students who correctly identified linkage but omitted crossing over or stated it occurred in mitosis scored one or two marks.
2019 P2 Q06.32 marks5%full marks
Evidence from pedigree that a gene is not X-linked. Only 5% scored both marks. The mark required identifying which specific flies or cross demonstrated that grey body colour is not X-linked, and explaining why this cross is diagnostic — specifically that a male passing on only his X chromosome to daughters means that if body colour were X-linked, all daughters of a grey-bodied male with homozygous recessive mother would be grey. Students who described the overall ratio of grey to black rather than the sex-specific phenotype pattern were not credited.
2024 P2 Q04.43 marks
Dihybrid cross involving a lethal allele. About 25% scored all three marks; 83% scored at least one. The most common error was including homozygous curly (RR) offspring as viable; the question context established that RR is lethal, producing a 2:1 ratio rather than the standard 3:1; students who generated a standard dihybrid cross without accounting for lethality gave 3:1 or 1:2:1 ratios, neither of which was accepted.
08
Difficulty signals
Performance metric synthesis
22PP GAP
Mean accessibility
62.2%
Mean mastery
40.7%
Mean student strength
47.9%
The accessibility–mastery gap of 21.6 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), 3.7.1 ranks 4 of 4 sub-sections by mean mastery (1 = hardest). Mastery trajectory is broadly flat across the cohort window: 50.4% in 2017 → 42.0% in 2024 (-8.4 percentage points). Mean mastery was lowest in 2019 (27.0%) and highest in 2023 (56.3%).