The Hardy-Weinberg equilibrium is a cornerstone of population genetics. It describes a theoretical state where allele frequencies remain constant across generations, assuming specific conditions are met. This concept helps geneticists understand how populations evolve and what factors can disrupt genetic stability.

The equation p^2 + 2pq + q^2 = 1 is central to Hardy-Weinberg equilibrium. It allows scientists to calculate expected genotype frequencies and compare them to observed frequencies. Deviations from equilibrium can indicate evolutionary forces at work, such as mutation, migration, or natural selection.

Hardy-Weinberg Equilibrium

Assumptions of Hardy-Weinberg equilibrium

• No mutation occurs allele frequencies remain constant as no new alleles are introduced or removed from the population
• No migration (gene flow) individuals neither enter nor leave the population, preventing the influx or outflow of alleles
• Large population size minimizes the effect of genetic drift and random fluctuations in allele frequencies
• Random mating individuals mate randomly within the population ensuring no preferential mating that could alter allele frequencies (panmixis)
• No natural selection all genotypes have equal survival and reproductive success maintaining stable allele frequencies across generations

Application of Hardy-Weinberg equation

• The Hardy-Weinberg equation: $p^2 + 2pq + q^2 = 1$
  • $p$ represents the frequency of the dominant allele (A)
  • $q$ represents the frequency of the recessive allele (a)
  • $p + q = 1$ the sum of allele frequencies equals 1
• Genotype frequencies:
  • $p^2$ = frequency of homozygous dominant individuals (AA)
  • $2pq$ = frequency of heterozygous individuals (Aa)
  • $q^2$ = frequency of homozygous recessive individuals (aa)
• Calculating allele frequencies from genotype frequencies:
  1. $p = \sqrt{AA} + \frac{1}{2}Aa$
  2. $q = \sqrt{aa} + \frac{1}{2}Aa$

Determination of equilibrium status

• Calculate expected genotype frequencies using the Hardy-Weinberg equation and observed allele frequencies
• Compare observed genotype frequencies to expected frequencies
  • If observed frequencies closely match expected frequencies, the population is likely in Hardy-Weinberg equilibrium
  • Significant differences between observed and expected frequencies indicate the population is not in equilibrium (evolution is occurring)

Factors disrupting Hardy-Weinberg equilibrium

• Mutation introduces new alleles or alters existing ones changing allele frequencies over time (sickle cell anemia)
• Migration (gene flow)
  • Influx of individuals with different allele frequencies can change the population's allele frequencies (admixture)
  • Outflow of individuals can also alter allele frequencies (founder effect)
• Small population size genetic drift has a more significant impact causing random fluctuations in allele frequencies due to chance events (bottleneck effect)
• Non-random mating
  • Assortative mating (mating based on similar phenotypes) can increase the frequency of certain alleles (sexual selection)
  • Disassortative mating (mating based on dissimilar phenotypes) can decrease the frequency of certain alleles (heterosis)
• Natural selection differential survival and reproduction of genotypes
  • Favored genotypes increase in frequency, while less favored genotypes decrease in frequency (antibiotic resistance)