Preparatory Mindset
Hardy-Weinberg is the foundation. It's a null hypothesis — it tells you what to expect if nothing is changing. Deviations from HWE tell you what IS happening (selection, drift, etc.). Master the formula: q² = disease frequency, 2pq = carrier frequency.
Core Concepts
- Hardy-Weinberg equilibrium: p + q = 1 (allele frequencies); p² + 2pq + q² = 1 (genotype frequencies)
- HWE assumptions: Random mating, no mutation, no natural selection, infinite population size, no gene flow (migration)
- Disease frequency AR: q² = incidence → q = √incidence → carrier frequency = 2pq ≈ 2q (when p≈1)
- Example: If AR disease incidence = 1/10,000 → q = 1/100 → carrier frequency = 1/50
- Genetic drift: Random fluctuations in allele frequency (stronger in smaller populations)
- Founder effect: High frequency of mutation in population descended from few founders (e.g., Tay-Sachs in Ashkenazi Jews)
- Consanguinity: Mating between relatives → increased risk of AR disorders in offspring
- Fitness: Reproductive success relative to population average. Selection reduces frequency of deleterious alleles
- Mutation-selection balance: Equilibrium between new mutations entering and selection removing them
- Heterozygote advantage: Carriers have survival benefit (e.g., sickle cell trait → malaria resistance)
- Assortative mating: Non-random mating (e.g., deaf individuals marrying each other) → changes genotype frequencies
High-Yield Points
- q² = disease incidence → q = √incidence → carrier freq = 2pq
- HWE assumptions: random mating, no mutation/selection/drift/migration
- Founder effect explains high frequency of certain AR diseases in specific populations
- Consanguinity → ↑ AR risk in offspring
Topic Summary
Hardy-Weinberg equilibrium describes stable populations. q² gives disease frequency; 2pq gives carrier frequency. Deviations reveal evolutionary forces at work.