Population Genetics, Mutations & Evolution – Comprehensive Study Notes

Mutations & Gene Pools

• A population’s collective alleles constitute its gene pool; allele frequency within this pool can be measured, compared, and tracked over time or between locations.

• Gene pools are dynamic—mutation, selection, migration, drift, non-random mating all shift allele frequencies.

Population Genetics – Core Ideas

• Population (genetic definition): an interbreeding group defined by a shared gene pool.

• Field integrates Mendelian inheritance (alleles, segregation) with Darwinian evolution (differential survival).

• Allele frequency = proportion of a specific allele in a gene pool.

• Phenotype surveys (e.g.
  eye colour) can infer underlying allele frequencies.

• Frequencies change via:

  • Mutations (new alleles)

  • Environmental selection pressures

Factors Altering Gene-Pool Composition

• Mutations (ultimate source of new alleles)

• Natural selection

• Random genetic drift (incl. founder effect & bottlenecks)

• Gene flow (migration)

• Non-random mating

Mutations – Source of Variation

• Originate from DNA-replication errors, meiotic mis-segregation, or mutagens (radiation, chemicals, viruses).

• Produce new genotypes → new phenotypes → differential survival.

• May be beneficial, neutral, or harmful.

Base-Pairing Example (Chromosome 11)

• Long DNA sequence shown → transcribed & translated to β-globin chain of haemoglobin.

• Normal mRNA codons: AUG GUG CAC CUG ACU CCU GAG GAG AAG UCU GCC GUU ACU → Amino-acid sequence Met Val His Leu Thr Pro Glu Glu Lys Ser Ala Val Thr.

Point Mutation Illustration

• Single base change in codon 7 (GAG → GUG) converts Glu → Val.

• Alters haemoglobin β-chain → causes sickle-cell disease.

Mutation Classifications

• Gene (point) mutations – small, local changes in nucleotide sequence.

  • Substitutions
    • Missense (different amino acid)
    • Nonsense (STOP codon)
    • Conservative missense = neutral

  • Insertions / Deletions (indels) → Frameshifts

• Chromosome (block) mutations – affect large segments.

  • Deletion

  • Duplication

  • Inversion

  • Translocation

  • Non-disjunction → aneuploidy (nullisomy, monosomy, trisomy, tetrasomy)

Frameshift Consequences

• Ribosome reads mRNA in triplets; adding/removing nucleotides shifts reading frame → alters every downstream amino acid → usually non-functional protein.

Mutation Rates & Arithmetic Example

• Typical human gene mutation rate ≈ $6 \times 10^{4} \div 3 \times 10^{-5} = 0.2$ (illustrative calculation from slide).

• With $3 \times 10^4$ genes × 2 copies → $6 \times 10^4$ loci per cell; ≈1 new mutant allele per $5$ people.

Somatic vs Gametic

• Somatic: occur in body cells; not inherited.

• Gametic: occur in sperm/egg precursors; heritable.

Fitness Classification

• Lethal, Harmful, Neutral, Beneficial.

• Neutral (silent) often involve 3rd-base wobble; important evolutionary reservoir.

• Beneficial seen in rapid-cycling organisms—antibiotic resistance, DDT resistance.

Chromosome-Level (Block) Mutations in Detail

• Deletion (Cri-du-chat, Prader-Willi, Angelman)

• Translocation t(9;22) → Philadelphia chromosome (CML)

• Inversion (chromosome 2 loop) may create supergenes.

• Duplication (evolution of α/β haemoglobin chains).

Aneuploidy & Non-Disjunction

• Failure of homologues (Meiosis I) or sister chromatids (Meiosis II) to separate.

• Produces n+1, n–1 gametes → trisomy 21, 18, 13, etc.

Gene Flow (Migration)

• Movement of alleles between populations via immigration/emigration.

• Introduces variation, homogenises neighbouring populations.

• Barriers: geographical (mountains, oceans), cultural (language, religion), social (caste, status).

Random Genetic Drift

• Definition: stochastic fluctuations of allele frequencies, strongest in small populations.

• Effects:

  • Loss of alleles

  • Fixation of rare/non-adaptive traits

  • Divergence among isolated groups

• Amplified by unequal family sizes, premature deaths.

Founder Effect

• Small group migrates, carries unrepresentative allele sample → new population diverges (e.g.
  Dunkers, Ashkenazi Jews, Linha São Pedro twin village).

Bottleneck

• Catastrophic drop in numbers (wars, typhoons) reduces diversity; increases inbreeding (Pingelap achromatopsia).

Inbreeding & Consanguinity

• Increases homozygosity; exposes recessive disorders (Amish microcephaly, limb-girdle MD, thalassaemia in Mediterranean cousins).

Natural Selection – Mechanism & Types

• Six-stage summary:

  1. Variation (mutations/sexual reproduction)

  2. Overproduction → struggle/competition

  3. Selection pressure acts on phenotypes

  4. Differential survival (fitness)

  5. Differential reproduction passes advantageous alleles

  6. Allele frequencies shift over generations → adaptation/speciation

• Types of selection:

  • Stabilising (intermediate favoured; human birth weight)

  • Directional (one extreme; lactose persistence)

  • Disruptive (both extremes; may begin speciation)

Allele Frequency Concept

• Range $0 \le f_{allele} \le 1$ (or $0\% – 100\%$).

• Selection increases $f$ of favourable, decreases $f$ of unfavourable.

Speciation Process (Allopatric example)

1. Variation

2. Isolation (geographic/social) → separate gene pools

3. Independent natural selection / drift

4. Accumulated differences prevent fertile interbreeding → new species

Case Studies

1. Sickle-Cell Anaemia & Malaria (Heterozygote Advantage)

• Mutation in HBB gene, chromosome 11: GAG→GTG; Glu→Val in β-globin.

• Genotypes:

  • $Hb^{A}Hb^{A}$ normal (susceptible to malaria)

  • $Hb^{A}Hb^{S}$ trait; minor symptoms, strong malaria resistance

  • $Hb^{S}Hb^{S}$ disease; severe anaemia, early mortality

• High $f(Hb^{S})$ correlates with holo-/hyper-endemic malaria zones.

• Parasite struggles to invade/survive in sickling cells; heterozygotes survive & reproduce more → balanced polymorphism.

Equations & Examples in LaTeX

• Mutation-rate example: $\text{Mutation Rate} = \frac{\text{Number of mutant alleles}}{\text{Total alleles}}$

• Non-disjunction gamete outputs:
  $\text{Meiosis I NDJ}:\; n+1,\; n+1,\; n-1,\; n-1$
  $\text{Meiosis II NDJ}:\; n+1,\; n-1,\; n,\; n$

• Frameshift reading: original triplets, insertion shifts by $+1$ base → downstream codons altered.

Examination Tips & Past-Question Themes

• Define gene pool, allele frequency, founder effect, heterozygote advantage.

• Describe processes: mutation → variation → selection → change in allele frequency.

• Distinguish gene vs chromosomal mutations with named examples.

• Apply natural-selection stages to real scenarios (Sentinelese skin colour, Linha São Pedro twins, Tay-Sachs heterozygote TB advantage).

Ethical / Practical Implications Discussed

• Mutagen exposure (mustard gas) → human health, warfare consequences.

• Inbreeding practices (nobility, isolated sects) → elevated genetic-disease burden.

• Antibiotic/insecticide resistance → public-health challenge requiring stewardship.

Summary – Mechanisms Driving Evolutionary Change

• Mutation (introduces alleles)
• Natural selection (fitness-based allele sorting)
• Genetic drift (chance-based allele sorting)
– Founder effect
– Bottlenecks
• Gene flow (allele mixing)
• Non-random mating (inbreeding/outbreeding)
• Over time, combinations of these forces can modify gene pools sufficiently to yield speciation.