Principles of Population Genetics – Comprehensive Study Notes

Session Objectives

• Define and accurately use core population-genetic terms (allele frequency, genotype frequency, phenotype frequency, Hardy–Weinberg equilibrium).

• Master the Hardy–Weinberg (HW) model:

  • List its five assumptions.

  • Apply $p^2 + 2pq + q^2 = 1$ to estimate disease risk, carrier status, and population structure.

• Recognize the evolutionary forces that can disrupt HW expectations:

  • Mutation, natural selection, migration (gene flow), genetic drift (incl. bottlenecks & founder effects), non-random mating.

• Relate these forces to observed inter-population differences in allele frequencies and disease prevalence.

• Appreciate clinical limitations of using average population risk, especially given the over-representation of European ancestry in genomic data.

Historical Background

• 1775 – Johann Blumenbach proposed five human “races” based on cranial morphology.

• Early 1900s – Franz Boas measured >$18{,}000$ immigrant skulls, showing skull shape changed within a generation → challenged fixed racial categories.

• 1970s – Richard Lewontin compared blood groups, serum proteins, RBC enzymes; concluded (in these data) more genetic variation across populations than within. (NB: modern re-analyses generally show the reverse; >85 % variation within groups.)

Biological Context & Human Genetic Variation

• In biology, “race” ≈ subspecies; requires both measurable variation and clear clustering.

• Most human traits follow clinal (geographic) gradients rather than discrete boundaries (e.g., skin pigmentation intensity correlates with latitude).

• Variation is often non-concordant: traits/alleles vary independently across geography (e.g., ABO B allele map differs from skin-color map).

• Illustration: a Hadza and a Maasai individual (both East African) can be genetically more distinct than a Celt and a Mongolian → intra-African diversity is the greatest worldwide.

Key Terminology

• Allele frequency (p or q): proportion of a specific allele at a locus in a population.

• Genotype frequency: proportion of a given diploid combination (e.g., $AA, Aa, aa$).

• Phenotype frequency: percentage showing a given observable trait.

• Population genetics: quantitative study of how allele distributions change across time/space.

• Hardy–Weinberg Equilibrium (HWE): theoretical state in which allele and genotype frequencies remain constant from generation to generation (serves as a null model).

Hardy–Weinberg Equilibrium – Core Concepts

• Assumptions (no forces acting):

  1. No selection (alleles equally fit).

  2. No mutation.

  3. No migration (gene flow).

  4. Random mating (panmixia).

  5. Infinitely large population (no genetic drift).

• Under these, genotype distribution follows $p^2 + 2pq + q^2 = 1$ where:

  • $p^2$ = freq. homozygous dominant.

  • $2pq$ = freq. heterozygotes (carriers for recessive traits).

  • $q^2$ = freq. homozygous recessive (often the clinically manifest genotype).

• Interpreting deviations → evidence that at least one assumption is violated (evolutionary force acting).

Worked Example – CFTR ΔF508 in Northern Europeans

• ΔF508 deletion removes phenylalanine 508 → defective chloride channel → cystic fibrosis (CF).

• Observed recessive disease allele frequency: $q = 0.02$.

• Calculate $p = 1 - q = 0.98$.

• Apply HW: $p^2 + 2pq + q^2 = 0.98^2 + 2(0.98)(0.02) + 0.02^2 = 0.9604 + 0.0392 + 0.0004 = 1$.

  • Genotype frequencies

  • Homozygous WT: $96.04\%$.

  • Heterozygotes (carriers): $3.92\%$.

  • Homozygous ΔF508: $0.04\%$ ≈ 1 in $2{,}500$ births.

  • Phenotype frequencies mirror above (carriers usually asymptomatic).

Mechanisms Affecting Allele Frequencies

1. Mutation

• Definition: heritable change in DNA sequence (germ-line required for evolutionary impact).

• Random with respect to fitness; source of all novel variation; can be deleterious, neutral, or beneficial.

• Clinical spotlight – FGFR3 & Achondroplasia

  • Autosomal dominant FGFR3 mutations.

  • Incidence $1$ in $15{,}000$–$25{,}000$ births; $\approx80\%$ are de novo.

2. Natural Selection

• Preconditions: variation + heritability + differential survival/reproduction.

• Only mechanism that produces adaptation.

• Orchid mantis (Hymenopus coronatus) showcase – camouflage shaped by selection.

• Clinical spotlight – Sickle Cell & Malaria

  • Hemoglobin alleles:

  • $Hb^A$ (normal β-globin).

  • $Hb^S$ (E6V missense) → sickle shape.

  • Genotypes & phenotypes:

  • $Hb^A Hb^A$ – normal RBCs.

  • $Hb^S Hb^S$ – sickle cell disease; often fatal before reproduction in pre-modern contexts.

  • $Hb^A Hb^S$ – sickle-cell trait (codominant expression: mixed normal & sickled RBCs).

  • Plasmodium falciparum life cycle localized in RBCs; cannot replicate efficiently in sickled cells.

  • Heterozygote advantage in malaria-endemic zones ➔ maintains $Hb^S$ at appreciable frequencies:

  • $\sim5\text{–}10\%$ African-American carriers.

  • $\sim20\%$ Central African carriers.

  • US regional variation: $15\%$ in Southeast, $3\%$ in NYC.

  • Females with trait exhibit higher fertility; adaptive payoff offsets disease cost.

3. Gene Flow / Migration

• Movement and interbreeding of individuals redistributes alleles.

• Example – Duffy Antigen (FY) in the Americas

  • Alleles $FY^A, FY^B, FY^{BES}$ show mosaic frequencies reflecting trans-Atlantic slave trade & European colonization.

  • FY^{BES} confers resistance to Plasmodium vivax; high in West/Central Africans, introgressed into New-World populations.

4. Genetic Drift

• Random fluctuation in allele frequencies, strongest in small populations.

• Types:

  • Bottleneck: sharp reduction to few breeders (e.g., natural disaster).

  • Founder effect: small subgroup colonizes new area, carrying non-representative allele sample.

• Examples

  • Dutch founders in South Africa → elevated Huntington’s disease in Afrikaners.

  • Old Order Amish (~$200$ founders):

  • Ellis-van Creveld syndrome (EVC).

  • Maple-Syrup Urine Disease (MSUD).

  • Cartilage-hair hypoplasia.

  • Polydactyly.

  • Amish lethal microcephaly.

  • Angelman syndrome.

Integration: Health Disparities & Clinical Caveats

• Most genomic research cohorts are >$80\%$ European ancestry → risk estimates, variant annotations, polygenic scores less accurate in other groups.

• Population averages can mislead individual risk assessment:

  • Example: using CF carrier screens designed on Northern-European frequencies underestimates risk in Ashkenazi Jews; overestimates in East Asians.

• Ethical imperative: diversify genetic databases to reduce disparities in diagnosis, treatment, and pharmacogenomics.

Quick HW Practice (Answers Provided on Slides)

• HW assumptions: no mutation, no selection, random mating, no migration, large $N$.

• If $p = 0.7$, then $2pq = 2(0.7)(0.3) = 0.42$ heterozygotes.

• Disorder allele $q = 0.1$ ⇒ carrier frequency $2pq = 2(0.9)(0.1) = 0.18$ (18 %).

Concept Application Questions & Solutions

• Rare disorder high on isolated island: Genetic drift → Founder effect.

• Hb^S frequency after migration to malaria-free zone: declines (selection lifted).

• Sickle-cell advantage violates which HW assumption? No selection.

• Bird populations mixing after corridor restored: Gene flow.

• Rapidly mutating virus strains: Mutation produces new surface proteins.

Key Equations & Numerical Tools

• Allele relations: $p + q = 1$ (for two-allele locus).

• Genotype expectations: $p^2 + 2pq + q^2 = 1$.

• Carrier frequency (autosomal recessive): $2pq$.

• Disease incidence (AR): $q^2$.

Suggested Resources for Deeper Study

• Relethford JH et al., Human Biological Variation.

• Molnar S., Human Variation: Races, Types, and Ethnic Groups (6th ed.).

• HHMI BioInteractive “Population Genetics Explorer”.

• Nina Jablonski TED Talk – “Skin color adaptations in humans”.

These bullet-point notes are designed to substitute for the entire slide deck: all definitions, numerical examples, clinical correlations, and evolutionary principles are embedded for rapid exam review.