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Hardy-Weinberg and Allele Frequency Changes

Hardy-Weinberg equilibrium is a foundational concept in population genetics that describes how allele and genotype frequencies remain stable in a population over generations when no evolutionary forces are acting. Named after G. Hardy and Wilhelm Weinberg, the principle states that in the absence of factors like mutation, migration, genetic drift, or natural selection, allele frequencies will not change. This stability arises because random mating and the laws of probability ensure that the distribution of genotypes follows predictable patterns. For a gene with two alleles, A and a, the expected genotype frequencies are p² for AA homozygotes, 2pq for Aa heterozygotes, and q² for aa homozygotes, where p and q are the frequencies of the A and a alleles, respectively. Understanding Hardy-Weinberg equilibrium is crucial for identifying when evolutionary forces are at work in a population. Deviations from these expected frequencies can signal the presence of factors like selection, inbreeding, or genetic drift. This principle is widely used in genetic studies to test for non-random mating, population stratification, and other disruptions to genetic stability.

Figures (10)

Hardy–Weinberg proportions for two alleles: the horizontal axis shows the two allele frequencies p and q and the vertical axis shows the expected genotype frequencies. Each line shows one of the three possible genotypes.
Length of p, q corresponds to allele frequencies (here p = 0.6, q = 0.4). Then area of rectangle represents genotype frequencies (thus AA : Aa : aa = 0.36 : 0.48 : 0.16).
Punnett square for three-allele case (left) and four-allele case (right). White areas are homozygotes. Colored areas are heterozygotes.
The logarithm of fitness as a function of the number of deleterious mutations. Synergistic epistasis is represented by the red line - each subsequent deleterious mutation has a larger proportionate effect on the organism's fitness. Antagonistic epistasis is in blue. The black line shows the non-epistatic case, where fitness is the product of the contributions from each of its loci.
Current tree of life showing vertical and horizontal gene transfers
Founder effect: The original population (left) could give rise to different founder populations (right).
A de Finetti diagram representing a distribution of genotype frequencies
Drosophila melanogaster
The Great Wall of China is an obstacle to gene flow of some terrestrial species.[51]
The small founding population experiences a loss of heterozygosity after multiple generations. ("Genetic Drift" by Boundless, 2015.[15])

This happens because random mating and probability rules keep the distribution of genotypes predictable. The Hardy-Weinberg principle helps scientists understand when evolutionary forces are acting on a population. If the observed genotype frequencies differ from the expected ones, it suggests that factors like selection, inbreeding, or genetic drift are at play.

In real populations, deviations from Hardy-Weinberg expectations can signal the presence of evolutionary influences. If something changes—like a new mutation or a shift in environment—the frequencies will shift too. Scientists use this idea to study how populations change and what forces are driving those changes.

Key Points

  • Hardy-Weinberg equilibrium is a principle in population genetics stating that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences.
  • Allele frequency is the proportion of a particular allele in a population, calculated by dividing the number of copies of the allele by the total number of alleles at that locus in the population.
  • Genotype frequency is the proportion of individuals in a population that have a specific genotype, calculated by dividing the number of individuals with that genotype by the total number of individuals in the population.
  • Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology.
  • Genetic drift is a change in the frequency of an existing allele in a population due to chance, which may cause alleles to disappear completely or become much more frequent, possibly leading to fixation.
  • The founder effect is a type of genetic drift occurring when a small group in a population splinters off from the original population and forms a new one, leading to a loss of genetic variation and potentially distinct genetic makeup in the new population.

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