32.2
Hardy-Weinberg Principle
Diploid organisms have two alleles of each gene, one from each parent, in their somatic cells. Therefore, each individual contributes two alleles to t…
The Hardy–Weinberg Principle states that in a large, randomly mating population, allele frequencies stay the same over time if the population is not evolving.
When a gene has two alleles at one locus, such as red and brown coat alleles in squirrels, their frequencies—represented by p and q—add up to one.
We can calculate the frequency of each genotype. The frequency of homozygous red and homozygous brown individuals is the square of the allele frequency—p² and q²—which gives the probability of inheriting the same allele from both parents.
Heterozygous individuals with red-brown coats can form in two ways: the egg can carry the red allele and the sperm the brown allele, or the egg can carry the brown allele and the sperm the red allele. So, the frequency of heterozygous individuals equals two times the product of the allele frequencies, or 2pq.
Together, these genotype frequencies sum to one. This principle is true only under specific, non-evolving conditions.
There must be no natural selection, and mating must be random with no preference for particular genotypes. There must be no gene flow from outside the population and no mutations within the population.
Finally, the population must be very large because random events can strongly change allele frequencies in small populations.
Although no real-world population can satisfy all of these conditions, the principle still offers a useful model for population analysis.
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Q1: What does the Hardy-Weinberg Principle tell us about allele frequencies in populations?
The Hardy-Weinberg Principle states that in a large, randomly mating population, allele frequencies remain constant over time if the population is not evolving. When two alleles exist at a locus, their frequencies—represented by p and q—always add up to one. This principle provides a null model for studying evolution and detecting when populations deviate from genetic equilibrium.
Q2: How do you calculate genotype frequencies using the Hardy-Weinberg equation?
The Hardy-Weinberg equation (p² + 2pq + q² = 1) relates allele frequencies to genotype frequencies. Homozygous individuals have frequencies of p² and q², representing the probability of inheriting the same allele from both parents. Heterozygous individuals occur with frequency 2pq because the dominant allele can come from either parent, creating two possible crosses.
Q3: What five conditions must a population meet to maintain Hardy-Weinberg equilibrium?
Hardy-Weinberg equilibrium requires an infinite population size, random mating, and an absence of genetic mutations, natural selection, and gene flow. When these conditions are met, allele frequencies remain constant and the population does not evolve. Most natural populations violate at least one condition, making true equilibrium rare in nature.
Q4: Why doesn't population size matter under Hardy-Weinberg conditions?
The Hardy-Weinberg Principle requires an infinite or very large population size because random events can strongly change allele frequencies in small populations. In large populations, random sampling errors are negligible, so allele frequencies remain stable across generations. Small populations experience unpredictable fluctuations that violate equilibrium assumptions.
Q5: How does random mating affect genotype frequencies in Hardy-Weinberg populations?
Random mating ensures that alleles combine by chance alone, without preference for particular genotypes. This produces predictable genotype frequencies based on allele frequencies. When mating is non-random or selective, genotype frequencies deviate from Hardy-Weinberg predictions, indicating the population is evolving or under different genetic conditions.
Q6: Why is the Hardy-Weinberg Principle useful if no real populations meet all its conditions?
The Hardy-Weinberg Principle serves as a null model or baseline for population genetics studies. By comparing observed genotype frequencies to Hardy-Weinberg predictions, scientists can identify which evolutionary forces—such as mutation, gene flow, and genetic drift—are acting on a population. This helps detect genetic associations and genotyping errors in research.
Q7: What historical question led scientists to develop the Hardy-Weinberg Principle?
In the early 20th century, scientists wondered why rarely-observed dominant traits did not increase in frequency across generations in randomly-mating populations. For example, polydactyly (extra fingers or toes) remained rare despite being dominant. In 1908, Wilhelm Weinberg and G. H. Hardy independently demonstrated that genetic variation could remain unchanged, establishing the principle of Hardy-Weinberg equilibrium.