Population Genetics
Overview
Population genetics describes how allele frequencies are distributed and change within populations. This chapter covers Hardy-Weinberg equilibrium and mutation-selection balance, two foundational concepts that connect population-level allele frequencies to individual-level risk assessment. Deriving carrier frequencies from disease incidence data is the starting point for many risk calculation problems in clinical practice.
The Hardy-Weinberg equilibrium provides a mathematical model for relating genotype frequencies to allele frequencies in an idealized population. While real populations never perfectly meet all Hardy-Weinberg assumptions, the model is remarkably useful in clinical genetics. The most common application is calculating the carrier frequency (2pq) for an autosomal recessive condition when you know the disease frequency (q-squared). This calculation is the first step in countless risk assessment problems.
Mutation-selection balance explains why deleterious alleles persist in populations despite natural selection acting against them. This concept is important for understanding why certain genetic conditions remain at stable frequencies across generations, and it provides the theoretical basis for observed carrier frequencies of conditions like cystic fibrosis, sickle cell disease, and Tay-Sachs disease.
Key Concepts
- Hardy-Weinberg equation: p-squared + 2pq + q-squared = 1 and how to apply it to calculate carrier frequencies
- Hardy-Weinberg assumptions: random mating, no selection, no mutation, no migration, large population size, and what happens when each assumption is violated
- Founder effect and genetic drift: why certain populations have unusually high carrier frequencies for specific conditions
- Heterozygote advantage: how carriers for certain conditions (e.g., sickle cell trait and malaria resistance) can have a selective advantage that maintains the allele in the population
- Consanguinity and population stratification: how departures from random mating affect genotype frequencies
Allele Frequencies
Hardy-Weinberg Equilibrium is the central quantitative tool in population genetics: the derivation and application of $p^2 + 2pq + q^2 = 1$, the conditions required for equilibrium, and the common violations in human populations. The most frequent application in clinical genetics is computing carrier frequency (2pq) from disease incidence ($q^2$): the starting point for countless recessive-risk problems.
Mutation-Selection Balance explains why allele frequencies for deleterious conditions reach a stable equilibrium between the introduction of new mutations and removal by selection. The classic clinical implications are that severe dominant disorders are typically driven by new mutations (high selection pressure) while severe recessive disorders persist at higher carrier frequencies than selection alone would predict.
Evolutionary Forces
The leaves in this section cover the forces that perturb allele frequencies away from Hardy-Weinberg equilibrium. Fitness and selection quantifies how selection pressure changes allele frequencies over generations and introduces concepts like heterozygote advantage (e.g., sickle-cell trait and malaria). Founder effect explains why isolated populations carry unusually high frequencies of specific variants (Ashkenazi Jewish carrier frequencies for BRCA founder variants, Tay-Sachs; Finnish heritage disorders). Genetic drift and gene flow cover the stochastic and migration-driven contributions to allele frequency change, which jointly shape the geographic distribution of genetic conditions and rationalize population-specific screening programs.