Gene Mutations

Spec Mapping — OCR H420 Module 6.1.1 — Cellular control, content statements covering the types and causes of gene mutations and their consequences for protein structure and function (refer to the official OCR H420 specification document for exact wording). This lesson supplies the molecular foundation on which every later lesson in the inheritance module — sex linkage, Mendelian disease genetics, Hardy-Weinberg disease-allele frequency, the role of mutation as the ultimate source of variation under natural selection — depends.

A gene mutation is a random, spontaneous change in the base sequence of DNA. Because proteins are made by translating mRNA transcripts of genes, a change in DNA sequence can change the sequence of amino acids in a polypeptide, and hence the structure and function of a protein. Mutations are therefore the ultimate source of all genetic variation; without them, evolution by natural selection could not occur. However, most mutations that affect a protein are deleterious or neutral — truly beneficial mutations are rare. OCR A-Level Biology A specification module 6.1.1(a) requires you to understand the types, causes and consequences of gene mutations.

The intellectual story is also part of A-Level depth. Hugo de Vries (1901) coined the word Mutation to describe sudden heritable changes he observed in the evening primrose Oenothera. Hermann Muller (1927, Nobel 1946) showed that X-rays induce heritable mutations in Drosophila — paraphrasing his school of thought, this established for the first time that the genome is vulnerable to external physical agents. Barbara McClintock (Nobel 1983) discovered transposable elements ("jumping genes") in maize and showed that the genome is far from a static blueprint: pieces of DNA can move from one locus to another, often disrupting genes en route. Her work was overlooked for decades, then vindicated when molecular biology found transposons in every domain of life. Linus Pauling and Harvey Itano (1949) showed that sickle-cell anaemia is caused by a single amino-acid substitution in haemoglobin, making it the first "molecular disease". Vernon Ingram (1957) pinpointed the responsible Glu6→Val change. These paraphrased schools of thought give the historical scaffolding for the molecular detail you will be examined on.

Key Definitions:

• Gene mutation — a change in the base sequence of DNA (strictly speaking, any heritable change at a single locus).
• Point mutation — a mutation affecting a single base (substitution, insertion or deletion of one nucleotide).
• Mutagen — a physical, chemical or biological agent that increases the natural mutation rate.
• Silent mutation — a base change that does not alter the encoded amino acid.
• Missense mutation — a base change that alters one amino acid in the polypeptide.
• Nonsense mutation — a base change that creates a premature stop codon.
• Frameshift mutation — an insertion or deletion (not a multiple of three) that shifts the reading frame.

---

Why Mutations Happen

DNA replication is remarkably accurate — DNA polymerase has a proofreading activity and the post-replication mismatch-repair system removes most errors. Even so, roughly 1 in 10⁹ nucleotides is mis-incorporated per cell division in humans, which over a 3 × 10⁹ bp genome gives a handful of new mutations per cell generation. Mutations arise:

• Spontaneously during replication (tautomeric shifts in bases, slippage at repeat sequences).
• Induced by mutagens (UV light, ionising radiation, chemicals such as benzene, alkylating agents, base analogues, reactive oxygen species).
• Transposon activity — "jumping genes" that insert into other loci.

---

Types of Point Mutation

flowchart TB
    A[Original DNA: ATG TTC GCA]
    A --> B[Substitution: ATG TAC GCA - missense Tyr replaces Phe]
    A --> C[Insertion: ATG TTA CGC A - frameshift]
    A --> D[Deletion: ATG TCG CA - frameshift]

1. Substitution

One base is replaced by another.

• Transition — a purine → purine (A ↔ G) or pyrimidine → pyrimidine (C ↔ T).
• Transversion — a purine ↔ pyrimidine.

Substitutions produce three possible outcomes at the protein level:

Outcome	Meaning	Example	Effect
Silent	New codon codes for the same amino acid	GAA → GAG (both Glu)	None — degeneracy of the code absorbs the change
Missense	New codon codes for a different amino acid	GAG → GTG (Glu → Val)	One amino acid substitution; effect depends on position
Nonsense	New codon is a stop codon	CAG → TAG	Premature termination — usually severe

2. Insertion

One or more extra bases are inserted into the sequence. If the number is not a multiple of three, the reading frame shifts downstream of the insertion — a frameshift. Every subsequent codon is changed, and usually a premature stop codon is encountered within a few dozen bases.

3. Deletion

One or more bases are removed. Again, a frameshift occurs unless the number removed is a multiple of three. An in-frame deletion of 3 bases removes one amino acid without shifting the frame — the resulting protein may still function partially (e.g. the common ΔF508 deletion in the CFTR gene causes cystic fibrosis by removing a single phenylalanine).

---

Why the Genetic Code Softens the Blow

The genetic code is degenerate — most amino acids are encoded by more than one codon, and the "wobble" rules mean that many third-position changes are silent. For example:

Codon	Amino acid
GCU	Ala
GCC	Ala
GCA	Ala
GCG	Ala

A mutation at the third position of any of these codons does not change the amino acid. This degeneracy acts as a buffer against mutation.

---

Named Examples of Gene Mutations

Disease	Gene	Mutation type	Effect
Sickle-cell anaemia	HBB (β-globin)	Missense (GAG → GTG, Glu6Val)	Hydrophobic valine replaces glutamate, haemoglobin polymerises in low O₂, sickles erythrocytes
Cystic fibrosis	CFTR	In-frame 3-bp deletion (ΔF508)	Misfolded CFTR chloride channel is degraded before reaching cell surface
Duchenne muscular dystrophy (DMD)	DMD (dystrophin)	Frameshift deletion	Little or no dystrophin protein; progressive muscle wasting
Huntington's disease	HTT	Expansion of CAG trinucleotide repeat	Toxic gain of function; polyglutamine tract aggregates in neurons
Phenylketonuria (PKU)	PAH	Missense, nonsense or splice site	No functional phenylalanine hydroxylase; Phe accumulates

---

Effect on Phenotype

Whether a mutation actually changes the phenotype depends on:

• Where in the gene it falls — mutations in the active site of an enzyme are usually catastrophic; mutations in surface loops may have little effect.
• Nature of the substitution — swapping glycine for alanine is usually tolerated (both small, non-polar), but swapping glycine for arginine (a large, charged residue) is often not.
• Whether the gene is haplosufficient — if one functional copy is enough, a recessive loss-of-function mutation will be silent in heterozygotes.
• Whether the mutation is in a coding, regulatory or intronic region — regulatory mutations can change how much protein is made rather than what it is.
• Cell type — a mutation in a gene expressed only in the liver will not affect the brain.

Mutations may be beneficial (a new function, e.g. lactase persistence in adult humans), neutral (most silent and many intronic mutations) or deleterious (most amino-acid-changing mutations in essential genes).

---

Mutagens

Mutagens increase the mutation rate. OCR expects you to know several examples:

• Physical mutagens: UV light (causes pyrimidine dimers — adjacent T–T bonds), ionising radiation (X-rays, γ-rays: cause double-strand breaks), heat (accelerates depurination).
• Chemical mutagens: nitrous acid (deaminates C → U and A → hypoxanthine), alkylating agents (mustard gas, ethyl methanesulfonate — add alkyl groups to bases), base analogues (5-bromouracil — mimics T but pairs with G), reactive oxygen species.
• Biological mutagens: certain viruses can insert their DNA into host genes; transposable elements.

Many mutagens are also carcinogens because mutations in proto-oncogenes or tumour suppressor genes can lead to cancer.

---

Somatic vs Germ-Line Mutations

• Somatic mutations occur in body cells after fertilisation. They are only passed on to the daughters of that one cell by mitosis, and cannot be inherited. They are, however, the cause of almost all cancers.
• Germ-line mutations occur in the cells that give rise to gametes. They can be inherited by offspring and by all their descendants.

---

Exam Tip

Be precise with your language. "A substitution of one base by another" is not the same as "the swapping of an amino acid". The mutation is at the DNA level; its consequence (silent, missense, nonsense) is at the protein level. Always link cause to effect in your answers: "a substitution of A for T in the second base of codon 6 of the β-globin gene changes the codon from GAG to GTG, so the amino acid changes from glutamate to valine, producing haemoglobin S, which polymerises in low-oxygen conditions and distorts erythrocytes into a sickle shape."

---

Common Exam Mistakes

• Saying "all mutations are harmful". Many are silent; some are neutral; a few are beneficial.
• Forgetting that a substitution at the third base of a codon is often silent because of the degeneracy of the code.
• Claiming that frameshifts change one amino acid. They change every amino acid downstream.
• Confusing an in-frame deletion (3 bp) with a frameshift deletion. An in-frame deletion removes one amino acid but leaves the rest unchanged.
• Saying UV light causes substitutions. UV causes pyrimidine dimers, which during replication lead to errors but the primary damage is a covalent cross-link.
• Forgetting that regulatory mutations exist. Mutations in promoter or enhancer regions can change transcription rates without changing the protein sequence at all.

---

Quick Recap

• A gene mutation is a change in base sequence: substitution, insertion or deletion.
• Substitutions may be silent, missense or nonsense.
• Insertions and deletions that are not multiples of three cause frameshifts.
• The degeneracy of the genetic code makes many substitutions silent.
• Named examples: sickle-cell (missense), cystic fibrosis (in-frame deletion), DMD (frameshift deletion), Huntington's (CAG repeat expansion).
• Mutagens (UV, ionising radiation, chemicals) increase mutation rates and many are carcinogens.
• Somatic mutations affect one cell line; germ-line mutations are inherited.

---

Visualising Substitution, Insertion and Deletion

The three classes of point mutation produce strikingly different consequences for the polypeptide. The SVG below tracks the same starting codon block under each mutation type.

Wild type: ATG GAG TTC GCA Met–Glu–Phe–Ala

Silent (synonymous): ATG GAA TTC GCA Glu → Glu (no aa change)

Missense: ATG GTG TTC GCA Glu → Val (sickle-cell)

Nonsense: ATG TAG --- --- Premature STOP — truncated protein

Insertion (1 bp): ATG CGA GTT CGC FRAMESHIFT — all downstream codons changed

Deletion (1 bp): ATG AGT TCG CA- FRAMESHIFT — usually premature STOP within a few dozen bases

The crucial principle: substitutions affect at most one amino acid; frameshifts (non-multiple-of-three indels) corrupt every downstream codon. The same insertion of three bases would be in-frame and add only one amino acid — far less catastrophic than a 1- or 2-bp insertion at the same point.

---

Triplet Repeat Expansions and Anticipation

A specialised class of mutation deserves separate treatment because it explains a whole family of late-onset neurological diseases. Triplet repeat expansion occurs at unstable regions of the genome where a short motif (e.g. CAG, CGG, CTG) is repeated many times in tandem. During replication, slippage of DNA polymerase can add (or, more rarely, remove) repeat units. Below a threshold the protein is normal; above it, the protein either misfolds or is toxic in its expanded form.

Disease	Repeat	Location	Pathological threshold	Mechanism
Huntington's disease	CAG	HTT exon 1	>36 repeats	Polyglutamine tract aggregates in neurons (gain of function)
Fragile X syndrome	CGG	FMR1 5' UTR	>200 repeats	Hypermethylation silences FMR1; loss of FMRP protein
Myotonic dystrophy type 1	CTG	DMPK 3' UTR	>50 repeats	Toxic mRNA aggregates sequester splicing factors
Friedreich's ataxia	GAA	FXN intron 1	>200 repeats	Frataxin gene silenced → mitochondrial iron handling impaired

Anticipation is the clinical observation that the disease appears at an earlier age, or more severely, in successive generations. Mechanistically this reflects the tendency for repeat tracts to expand further during gametogenesis (especially in male meiosis for CAG, female meiosis for CGG). Anticipation is direct evidence that the mutation itself is unstable across generations — something Mendel could not have imagined.

---

Mutation Rates and the Molecular Clock

Spontaneous mutation rates per nucleotide per generation are surprisingly consistent across organisms: roughly 10⁻⁸ to 10⁻⁹ for replicating DNA in mammals. With a 3 × 10⁹ bp diploid genome, that is ~30–100 new mutations per germ-line cell per generation. The "molecular clock" hypothesis (Zuckerkandl and Pauling, 1962, paraphrased) uses this near-constancy to estimate divergence times between species from the number of accumulated neutral substitutions — independent of fossil evidence.

A typical pitfall is conflating somatic and germ-line mutation rates. Somatic mutation rates can be far higher because of tissue-specific replication stress, oxidative damage and replication errors in long-lived stem cells. Cancer genomes routinely accumulate thousands of point mutations per cell — a clear sign that the cell of origin had a defect in one or more DNA-repair pathways (mismatch repair, base-excision repair, nucleotide-excision repair, or double-strand-break repair).

---

Repair Pathways That Resist Mutation

Mutations are rare partly because cells invest enormously in fixing them. OCR does not require the molecular detail but the conceptual hierarchy is important for synoptic links.

flowchart TB
    A[DNA damage] --> B{Type of damage}
    B -->|Single base mismatch| C[Mismatch repair MMR]
    B -->|Single damaged base| D[Base excision repair BER]
    B -->|Bulky lesion, dimer| E[Nucleotide excision repair NER]
    B -->|Double-strand break| F{Cell cycle phase}
    F -->|G1| G[Non-homologous end joining]
    F -->|S, G2| H[Homologous recombination]
    C --> I[Restored sequence]
    D --> I
    E --> I
    G --> I
    H --> I

Defects in repair pathways are direct causes of cancer-predisposition syndromes: xeroderma pigmentosum (NER defect → catastrophic UV sensitivity), hereditary non-polyposis colorectal cancer (HNPCC / Lynch syndrome) (MMR defect), and BRCA1/BRCA2 (homologous-recombination defect → familial breast/ovarian cancer). The takeaway: mutations are mostly invisible because cells fix them — and when fixing fails, mutation rate explodes and cancer often follows.

---

Synoptic Links

Synoptic Links — Connects to:

• ocr-alevel-biology-nucleic-acids-enzymes / dna-replication (replication fidelity — proofreading by DNA polymerase ε/δ — sets the baseline mutation rate; this lesson explains what happens when proofreading fails).
• ocr-alevel-biology-nucleic-acids-enzymes / transcription-translation (a missense or nonsense mutation only matters because translation faithfully reads the codon and changes the polypeptide — link cause to effect).
• ocr-alevel-biology-genetics-inheritance / phenotypic-variation-monogenic-inheritance (named diseases — sickle cell, cystic fibrosis, Huntington's — re-appear with their mutation types and inheritance patterns).
• ocr-alevel-biology-genetics-inheritance / hardy-weinberg-speciation (mutation is the ultimate source of new alleles and one of the four forces that disrupt Hardy-Weinberg equilibrium).
• ocr-alevel-biology-diseases-immunity / pathogen-defence (HbS heterozygote advantage against Plasmodium falciparum — a missense mutation maintained by balancing selection because it confers partial malaria resistance).

---

Practical Activity Group anchor

Practical Activity Group anchor: PAG 12 — Research skills (reporting) and PAG 11 — Research skills (planning). While mutations themselves are not directly observable at A-Level lab scale, the analysis of published genetic data — using chi-squared to compare observed offspring counts of an affected vs unaffected ratio against a Mendelian expectation, or interpreting pedigree diagrams — is the practical application of this lesson. PAG 11 anchors the design of such an analysis (state H₀, choose an appropriate test, identify confounders); PAG 12 anchors the reporting (clear tables, correct statistical inference, appropriate biological conclusion).

---

Specimen question modelled on the OCR H420 paper format

Question (9 marks): Sickle-cell anaemia is caused by a missense mutation in the HBB gene that changes codon 6 from GAG to GTG, replacing glutamate with valine. Explain how this single-base substitution leads to the clinical features of sickle-cell anaemia, and discuss why the allele is maintained at high frequencies in some human populations despite being deleterious in homozygotes.

AO breakdown

Mark	AO	Awarded for
1	AO1	Identifying the mutation as a missense substitution / point mutation
2	AO1	Stating the amino acid change Glu6 → Val and that valine is hydrophobic / glutamate is polar/charged
3	AO2	Linking the hydrophobic substitution to polymerisation of deoxy-haemoglobin S into fibres
4	AO2	Explaining how polymerisation distorts erythrocytes into a sickle shape
5	AO2	Linking sickled erythrocytes to vaso-occlusion and tissue ischaemia
6	AO1	Stating that sickle cell anaemia is autosomal recessive — only homozygotes show full disease
7	AO3	Recognising heterozygote advantage / balancing selection
8	AO3	Linking heterozygote advantage to malaria resistance from Plasmodium falciparum infection of erythrocytes
9	AO3	Evaluative synthesis — explicit Hardy-Weinberg / population-genetics framing of allele maintenance

AO split: AO1 = 3, AO2 = 3, AO3 = 3.

Grade C model answer (~170 words)

A missense substitution changes one base in the DNA, which changes one codon. Here the codon GAG (glutamate) becomes GTG (valine), so one amino acid in the beta-globin chain is different. Valine is hydrophobic but glutamate is polar, so the new haemoglobin (HbS) sticks together when oxygen is low. The red cells become sickle-shaped and block blood vessels, causing pain and damaging organs. Sickle-cell anaemia is recessive, so only people with two copies of the mutated allele have the full disease. People with one copy are carriers and are usually healthy. The allele is common in some African and Asian populations because people with one copy are more resistant to malaria. The malaria parasite cannot grow as well in sickle-shaped red cells, so heterozygotes survive malaria better than people with two normal alleles. This is why natural selection keeps the allele at a high frequency in malaria areas, even though homozygotes have a serious disease.

Examiner commentary: M1 (missense identification), M1 (amino acid change with hydrophobic/polar contrast), M1 (polymerisation), M1 (sickle shape), M1 (vaso-occlusion implied), M1 (autosomal recessive), M1 (heterozygote advantage), M1 (malaria mechanism). Around 7/9 — strong AO1 and AO2; AO3 is descriptive rather than formally evaluative, with no explicit Hardy-Weinberg / balancing-selection framing. Solid Grade C.

Grade B model answer (~230 words)

The HBB GAG → GTG substitution is a missense point mutation, replacing the polar negatively-charged glutamate at residue 6 of the β-globin chain with the hydrophobic valine. In deoxygenated haemoglobin S, the new hydrophobic surface fits into a complementary pocket on adjacent β-globin chains, allowing HbS molecules to polymerise into long fibres that distort the erythrocyte into the characteristic sickle shape. Sickled cells are inflexible, occlude capillaries, lyse prematurely (haemolytic anaemia), and cause painful vaso-occlusive crises and chronic organ ischaemia. The disease is autosomal recessive: only HbS/HbS homozygotes show the full phenotype, while HbA/HbS heterozygotes (sickle-cell trait) have a mixture of HbA and HbS, polymerise only under extreme hypoxia, and are usually clinically well.

The allele is maintained at frequencies up to ~0.15 in equatorial Africa by heterozygote advantage (balancing selection). HbA/HbS individuals are partially protected against severe Plasmodium falciparum malaria: infected sickle-trait erythrocytes are more readily destroyed by phagocytes, limiting the parasite's expansion. In a Hardy-Weinberg framework, the increased relative fitness of heterozygotes compared with both homozygotes maintains both alleles in the population despite the homozygous disadvantage — the equilibrium frequency reflects the relative fitness costs of malaria mortality (HbA/HbA) versus sickle-cell mortality (HbS/HbS).

Examiner commentary: M1–M5 fully secured (mechanistic explanation of pathology); M6 (recessive); M7+M8 (heterozygote advantage + malaria mechanism); M9 partially secured via the Hardy-Weinberg framing. Around 8/9 — a strong Grade B answer; lacks the explicit fitness-cost trade-off algebra of the A* response.

Grade A* model answer (~310 words)

The GAG → GTG substitution at codon 6 of HBB is a transversion missense mutation: a purine-to-pyrimidine swap (A → T on the coding strand) that replaces glutamate (polar, negatively charged at physiological pH) with valine (hydrophobic, neutral). The substitution introduces a hydrophobic patch on the β-globin surface; in deoxy-HbS, this patch docks into a complementary pocket on the β1 chain of an adjacent tetramer, nucleating polymerisation into 14-stranded helical fibres that mechanically distort the erythrocyte. Distorted (sickled) cells exhibit reduced deformability, premature splenic clearance (haemolytic anaemia), increased adhesion to endothelium and vaso-occlusion of post-capillary venules — driving the characteristic pain crises, splenic infarction, and chronic ischaemic damage to bone, kidney, lung and brain.

Sickle-cell anaemia is autosomal recessive. HbA/HbS heterozygotes are usually asymptomatic at normal oxygen tensions because their erythrocytes contain ~40% HbS and ~60% HbA — too dilute to nucleate polymerisation efficiently except under extreme hypoxia. The HbS allele is therefore strongly selected against in homozygotes (relative fitness w(SS) ≈ 0.1–0.3 historically), yet remains at substantial frequencies (q ≈ 0.05–0.15) across malaria-endemic equatorial Africa, the Mediterranean and South Asia.

This is the textbook case of balancing selection. Heterozygotes enjoy ~10× protection against severe Plasmodium falciparum malaria, plausibly because parasitised HbA/HbS erythrocytes preferentially sickle under the low oxygen tensions of deep vasculature, accelerating their phagocytic clearance before the parasite completes its schizogony. With w(AA) < w(AS) > w(SS), the equilibrium HbS allele frequency is q* = (1 − w(AA))/((1 − w(AA)) + (1 − w(SS))) — i.e. determined by the ratio of malaria-driven mortality on AA homozygotes to sickle-driven mortality on SS homozygotes. The diaspora-tracked decline of HbS frequencies in African-American populations over generations (where malaria pressure is absent) is a clean empirical confirmation: drop the selective pressure and the allele frequency decays under purifying selection alone.

Examiner commentary: Full 9/9. M1–M5 demonstrate complete mechanistic AO2 mastery (with the transversion subtlety and the deoxy-HbS fibre-nucleation detail signalling top-band molecular precision). M6 secured. M7–M9 fully secured by the explicit fitness-coefficient framing — the A* discriminators are (i) naming balancing selection rather than just describing it, (ii) giving the algebraic equilibrium-frequency expression, and (iii) the diaspora-test empirical argument. A model A* synoptic response.

---

A-Level-depth misconceptions

The errors that distinguish A from A*:

1. Confusing "mutation" with "phenotype change". A silent mutation changes the DNA but not the protein; many missense mutations change the protein but not the function. An A* candidate is explicit that mutation operates on DNA, with downstream consequences at the protein and phenotype levels filtered by the genetic code, protein chemistry and physiological context.
2. Believing all mutations are random and rare. The base rate is low (~10⁻⁹ per nucleotide per generation), but at the genome level there are ~30–100 new mutations per gamete in humans — mutation is a common fact at population scale.
3. Treating frameshifts as "small errors". A 1-bp insertion or deletion typically obliterates downstream protein sequence and introduces a premature STOP, giving a much more severe loss-of-function than most missense mutations.
4. Failing to distinguish gain-of-function from loss-of-function. Huntington's is a gain-of-function (toxic polyQ aggregate); cystic fibrosis is loss-of-function (no functional CFTR). The distinction explains why Huntington's is autosomal dominant (one bad allele is enough to introduce toxicity) but cystic fibrosis is autosomal recessive (one working allele is enough to provide function).
5. Saying UV light causes "substitutions". UV's primary damage is pyrimidine dimers — covalent cross-links between adjacent T-T or C-T bases. These distort the helix; subsequent error-prone bypass during replication is what produces substitutions.

Common errors and mark-loss patterns

Pedagogical observations — not fabricated statistics:

• A typical pitfall is forgetting that the genetic code's degeneracy makes many substitutions silent. Examiners reward candidates who flag this and explicitly distinguish third-base wobble from first- or second-base substitution.
• Candidates frequently mix up in-frame deletions (multiples of 3 bp — remove whole amino acids) with frameshift deletions (non-multiples — corrupt everything downstream). The ΔF508 cystic-fibrosis deletion is in-frame; most disease-causing deletions are not.
• A common pitfall in describing Huntington's is omitting the threshold (~36 CAG repeats) — without it, the dominant inheritance pattern and the anticipation phenomenon make no sense.
• Candidates often write that "mutations cause cancer" without specifying which kinds of genes — gain-of-function in proto-oncogenes (RAS, MYC) or loss-of-function in tumour-suppressor genes (TP53, RB1). Mark schemes reward precision here.

Going further

• Strachan & Read, Human Molecular Genetics — full undergraduate treatment of mutation classes, repair pathways, and disease-allele frequencies.
• Oxbridge interview-style prompt: "Why aren't all CGG triplet expansions disease-causing? What buffers the genome against repeat-instability mutations?" (Answer touches mismatch-repair surveillance of slipped intermediates and the fact that below ~50 repeats CGG is normally polymorphic without pathology.)
• Oxbridge interview-style prompt: "If mutation rates were halved by some technological intervention, would human evolution stop? Slow down? Accelerate?" (Answer requires unpacking standing genetic variation vs new mutation as substrates for selection, and the asymmetry between adaptive sweep and purifying selection.)
• Undergraduate analogue: the medical genetics course in most UK medical schools revisits this material from a clinical genetics angle in year 1, then returns to it in oncology / pharmacogenomics in year 3.

Reference: OCR A-Level Biology A (H420) specification 6.1.1(a) (refer to the official OCR H420 specification document for exact wording).