explain the genetic basis of discontinuous variation and continuous variation

Variation – Cambridge A‑Level Biology (9700)

Objective: Explain the genetic basis of discontinuous (qualitative) and continuous (quantitative) variation, describe the sources of genetic variation, and relate these concepts to inheritance patterns, population genetics, natural selection, speciation and conservation (AO1‑AO3).

1. Sources of Genetic Variation

• Mutation

  • Point mutations – base substitution, insertion or deletion (e.g., sickle‑cell allele).
  • Frameshift mutations – insertions/deletions that alter the reading frame.
  • Chromosomal alterations – duplications, deletions, inversions, translocations.

• Genetic recombination – crossing‑over and independent assortment during meiosis create new allele combinations.
• Gene flow – movement of individuals or gametes between populations introduces new alleles.

  • Rate of allele change: \(\Delta p = m (pm - p)\) where m = proportion of migrants, pm = migrant allele frequency.

• Genetic drift – random changes in allele frequencies, especially in small populations.

  • Effective population size (Ne) determines magnitude of drift.
  • Bottleneck: a temporary reduction in Ne → loss of alleles.
  • Founder effect: a small group colonises a new area → allele frequencies may differ markedly from the source.
  • Variance of allele frequency change per generation: \(\operatorname{Var}(\Delta p)=\frac{p q}{2N_e}\).

2. Mendelian Foundations (Inheritance – Syllabus Topic 16)

2.1 Basic Principles

• Dominance & recessivity – complete, incomplete, codominance.
• Segregation – each parent contributes one allele per locus (Mendel’s 1st law).
• Independent assortment – genes on different chromosomes segregate independently (Mendel’s 2nd law).

2.2 Linkage & Recombination

• Genes on the same chromosome tend to be inherited together.
• Recombination frequency (RF) = \(\frac{\text{Number of recombinants}}{\text{Total progeny}} \times 100\%\).
• Map distance (cM) ≈ RF (for RF < 20 %).
• Example: In peas, seed‑shape (R) and seed‑colour (Y) are linked (RF ≈ 10 %).

2.3 Sex‑Linked Inheritance

• Genes on the X or Y chromosome show characteristic patterns (e.g., red‑green colour blindness, hemophilia).
• Male genotype: XY; female genotype: XX → predict phenotypic ratios using sex‑specific Punnett squares.

2.4 Epistasis & Pleiotropy

• Epistasis – interaction where one gene masks or modifies the effect of another (e.g., coat‑colour in Labrador retrievers: B (black) is epistatic to E (brown)).
• Pleiotropy – one gene influences multiple traits (e.g., Marfan syndrome gene affecting skeleton, eye and cardiovascular system).

2.5 Practice Question (AO3)

Cross a heterozygous red‑flowered Petunia (Rr) with a white‑flowered plant (rr). Predict the F₂ phenotypic ratio if the R gene is linked to a second locus (A) with a recombination frequency of 15 %.

3. Types of Variation

3.1 Discontinuous (Qualitative) Variation

Traits that fall into distinct phenotypic classes; usually controlled by one or few genes with large effects.

Genetic basis

• Complete dominance, incomplete dominance, codominance.
• Multiple alleles (e.g., human ABO blood groups).
• Sex‑linked genes.
• Epistatic interactions.

Examples

Key characteristics

• Clear, separate phenotypic classes.
• Often a single locus with a large effect.
• Environmental influence is minimal.
• Predictable with Mendelian ratios (e.g., 3:1, 1:2:1).

3.2 Continuous (Quantitative) Variation

Traits that display a continuous range of phenotypes, usually forming a normal (bell‑shaped) distribution.

Genetic basis – polygenic inheritance

• Many loci (polygenes) each contribute a small additive effect.
• Allelic effects are primarily additive; dominance and epistasis may modify the total.
• Environmental factors (nutrition, climate, etc.) interact with genotype.

Variance components

The total phenotypic variance (\(V_P\)) is partitioned as:

\[

VP = VA + VD + VI + VE + V{GE}

\]

• \(V_A\) – additive genetic variance.
• \(V_D\) – dominance variance.
• \(V_I\) – epistatic (gene‑gene) variance.
• \(V_E\) – environmental variance.
• \(V_{GE}\) – genotype × environment interaction.

Heritability

Broad‑sense: \(H^2 = \dfrac{VG}{VP}\) where \(VG = VA + VD + VI\).

Narrow‑sense (most useful for selection): \(h^2 = \dfrac{VA}{VP}\).

Response to selection – Breeder’s equation

\[

R = h^2 \, S

\]

where R = response (change in mean), S = selection differential.

Examples

Key characteristics

• Phenotypic distribution approximates a normal curve.
• No discrete categories; intermediates are common.
• Both genotype and environment contribute substantially.
• Statistical analysis (regression, ANOVA, heritability estimates) is required.

4. Hardy–Weinberg Equilibrium (HWE) – Baseline for Evolutionary Change

4.1 Equation

\[

p^2 + 2pq + q^2 = 1

\]

• p = frequency of allele A (dominant).
• q = frequency of allele a (recessive), with \(p+q=1\).
• Genotype frequencies: AA = \(p^2\), Aa = \(2pq\), aa = \(q^2\).

4.2 Assumptions

1. Infinite (very large) population.
2. Random mating.
3. No mutation.
4. No migration (gene flow).
5. No selection.
6. No genetic drift.

4.3 Example – Directional selection on a colour allele

Initial allele frequency of dark colour D: \(p = 0.6\). Fitnesses: DD = 0.9, Dd = 0.8, dd = 0.5.

1. Genotype frequencies (HWE): DD = 0.36, Dd = 0.48, dd = 0.16.
2. Weighted frequencies after selection: DD = 0.324, Dd = 0.384, dd = 0.08.
3. Normalise (total = 0.788): DD = 0.411, Dd = 0.487, dd = 0.102.
4. New allele frequency: \(p' = 0.411 + \frac{1}{2}(0.487) = 0.655\).

Selection increased the frequency of the dark allele.

5. Natural Selection & Evolution (Syllabus Topic 17)

5.1 Types of selection

• Directional – favours one extreme; shifts the mean (e.g., larger beaks during drought).
• Stabilising – favours intermediate phenotypes; reduces variance (e.g., optimal birth weight).
• Disruptive – favours both extremes; can split a population (e.g., peppered‑moth morphs).

5.2 Fitness & Adaptive Landscape

• Fitness (\(w\)) = reproductive success of a genotype.
• Adaptive landscape visualises fitness peaks and valleys; movement across the landscape is driven by mutation, recombination, drift and selection.

5.3 Link to variation type

• Discontinuous traits often experience strong directional or disruptive selection (e.g., fixation of a disease‑resistance allele).
• Continuous traits are classic targets of all three selection modes, producing measurable shifts in mean or variance.

6. Genetic Drift & Gene Flow – Quantitative Treatment

6.1 Genetic drift

• Change in allele frequency per generation: \(\Delta p \approx 0\) on average, but variance \(\operatorname{Var}(\Delta p)=\dfrac{p q}{2N_e}\).
• Probability of fixation of a neutral allele: \(P{\text{fix}} = \dfrac{1}{2Ne}\) (for a new mutation).
• Bottleneck example: a population reduced from 10 000 to 200 individuals loses ~90 % of heterozygosity.

6.2 Gene flow

• Change in allele frequency due to migrants: \(\Delta p = m (p_m - p)\).
• High migration rates homogenise allele frequencies among populations, counteracting drift and selection.

6.3 Practical illustration

Simulate two islands (Island A: \(p=0.7\), Island B: \(p=0.3\)). If 5 % of individuals each generation migrate from A to B and vice‑versa, calculate the new allele frequencies after one generation using the migration equation.

7. Speciation & Phylogeny (Syllabus Topic 17 – Extension)

7.1 Modes of speciation

• Allopatric – geographic isolation leads to divergence (e.g., Darwin’s finches on different islands).
• Sympatric – reproductive isolation evolves without physical separation (e.g., polyploidy in plants, host‑shift in insects).
• Key mechanisms: pre‑zygotic (temporal, behavioural, mechanical) and post‑zygotic (hybrid sterility, inviability).

7.2 Phylogenetic trees & cladograms

• Branches represent lineages; nodes represent common ancestors.
• Cladograms illustrate shared derived characters (synapomorphies).
• Use of molecular data (DNA sequencing) to infer relationships and estimate divergence times.

7.3 Example

Construct a simple cladogram for the mammals Homo sapiens, Pan troglodytes, Canis lupus and Felis catus based on the presence/absence of the following characters: (1) opposable thumb, (2) carnassial teeth, (3) marsupial pouch. Discuss the inferred evolutionary relationships.

8. Classification, Biodiversity & Conservation (Syllabus Topic 18)

8.1 Taxonomic hierarchy

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species. Binomial nomenclature (Genus species) is italicised, genus capitalised.

8.2 Biodiversity importance

• Genetic diversity underpins adaptability to changing environments.
• Species diversity contributes to ecosystem stability and services.
• Conservation of rare alleles can prevent inbreeding depression.

8.3 IUCN Red List categories

8.4 Conservation genetics

• Maintain effective population size (\(N_e\) > 500) to preserve heterozygosity.
• Use genetic markers (microsatellites, SNPs) to monitor gene flow and inbreeding.
• Ex situ conservation (seed banks, captive breeding) should aim to capture maximal genetic variation.

9. Practical / Experimental Skills (AO3)

• Design a breeding experiment to distinguish Mendelian from polygenic inheritance (record F₁/F₂ ratios, perform chi‑square test).
• Measure a quantitative trait in a large sample, construct a frequency distribution, calculate mean, variance, standard deviation and estimate narrow‑sense heritability using parent–offspring regression.
• Test Hardy–Weinberg equilibrium by sampling a natural population, converting phenotypes to genotype frequencies, and applying a chi‑square goodness‑of‑fit test.
• Simulate selection in the laboratory (e.g., select the largest beans for several generations; compare observed response with \(R = h^2 S\)).
• Assess genetic drift using a small laboratory population of fruit flies; track allele frequency changes over generations and compare with the expected variance \(\frac{pq}{2N_e}\).
• Construct a cladogram from morphological or molecular data and interpret evolutionary relationships.

10. Comparison of Discontinuous and Continuous Variation

11. Summary

• Genetic variation arises from mutation, recombination, gene flow and drift.
• Discontinuous variation is usually Mendelian; continuous variation results from polygenic inheritance plus environmental effects.
• Hardy–Weinberg provides a null model; departures indicate selection, drift, migration or non‑random mating.
• Selection (directional, stabilising, disruptive) acts on the phenotypic distribution produced by genetic variation.
• Genetic drift and gene flow are quantified by effective population size and migration rate, respectively, and can counter‑ or reinforce selection.
• Speciation (allopatric, sympatric) and phylogenetic analysis explain the origin of new taxa.
• Understanding classification, biodiversity and conservation genetics links variation to real‑world management of species.
• Practical skills—crosses, trait measurement, HWE testing, selection simulations, and cladogram construction—demonstrate mastery of AO1‑AO3.