Hardy-Weinberg Principle and Effects of Evolutionary Forces

Hardy-Weinberg Principle and Effects of Evolutionary Forces

• 1.
  Hardy-Weinberg Principle and Effectsof Evolutionary Forces Understanding Equilibrium in Population Genetics Submitted by - Group 6
• 2.
  Prerequisite Concepts 1. PopulationGenetics: • Population: A group of individuals of the same species that interbreed. • Gene Pool: The total collection of alleles in a population. 2. Allele Frequency: The proportion of a specific allele among all alleles for a gene in a population. 3. Genotype Frequency: The proportion of a specific genotype among all genotypes in a population. 4. Equilibrium: A state where allele and genotype frequencies remain constant over time in the absence of evolutionary forces. 5. Migration (Gene Flow): Movement of individuals and their alleles from one population to another. 6. Genetic Drift: Random changes in allele frequencies, especially significant in small populations.
• 3.
  Statement • Definition: TheHardy-Weinberg principle states that allele and genotype frequencies in a population will remain constant from generation to generation in the absence of evolutionary influences. • This principle provides a mathematical baseline for studying genetic variation in populations. It's essentially a null hypothesis for detecting whether evolution or other factors are acting on a population.
• 4.
  Hardy-Weinberg Equation 1. AlleleFrequencies: p + q = 1 In a diploid, p and q represent the frequency of allele A and allele a. 2. Genotype Frequencies: p² + 2pq + q² = 1 • If alleles in the gamete pool exactly mirror those in the parent generation, and if they meet up randomly (in an infinitely large number of events), there is no reason—in fact, no way—for allele and genotype frequencies to change from one generation to the next.
• 5.
  Conditions for theprinciple • Large population size - The population must be sufficiently large to prevent random changes in allele frequencies (genetic drift) • Random mating- It is to ensure that gene frequencies remain stable and that there is no bias in mate selection, which could otherwise alter the genetic makeup of the population. • No mutation-There must be no new alleles introduced by mutation. Mutations alter allele frequencies by introducing new alleles or converting one allele to another. • No migration- No new alleles can enter the population through migration, and none can leave. Migration can introduce or remove alleles, changing the genetic structure of the population. • No natural selection- All genotypes must have equal chances of surviving and reproducing. Natural selection changes allele frequencies by favoring some genotypes over others.
• 6.
  Effect of NaturalSelection • Natural selection is a process in which heritable variations enabling better survival are enabled to reproduce and leave greater number of progeny. • It disrupts Hardy-Weinberg equilibrium by favoring certain alleles over others, causing allele frequencies to change. • Example - selection due to presence of antibiotic resistance gene in bacteria
• 7.
  Effect of Mutation •Mutation is a random change in the DNA sequence of an organism, which can introduce new alleles into a population. • It alters allele frequencies by creating new alleles or converting one allele to another, disrupting Hardy- Weinberg equilibrium. • Example - Sickle Cell Anemia Mutation In areas with high malaria prevalence, individuals heterozygous for the sickle cell allele (HbA/HbS) have a survival advantage because they are more resistant to malaria. This demonstrates how a mutation can persist in a population when it provides a selective advantage, despite being harmful in homozygous form (HbS/HbS).
• 8.
  Effect of Migration(gene flow) • Migration, or gene flow, is the movement of alleles between populations due to the movement of individuals or gametes. • It can alter allele frequencies in a population, introducing new alleles or changing the frequencies of existing alleles, thereby disrupting Hardy- Weinberg equilibrium. Convergence at Gen 3 - indicating significant gene flow and interbreeding between the populations. Post convergence - slight fluctuations are observed, indicating continued but balanced gene flow between the populations.
• 9.
  Effect of GeneticDrift • Genetic drift is the random fluctuation of allele frequencies in a population due to chance events, especially significant in small populations. • It can cause allele frequencies to change unpredictably over time, leading to deviations from Hardy-Weinberg equilibrium. • Types- Founder effect and Bottleneck effect Random sampling of gametes for reproduction can result in some alleles being overrepresented or lost by chance. Over successive generations, these chance events can lead to the fixation of one allele or the loss of genetic variation.
• 10.
  Testing for Hardy–Weinberg Proportions •Calculate observed genotype frequencies. • Calculate allele frequencies. • Calculate expected genotype frequencies under Hardy-Weinberg equilibrium. • Compute the chi-square statistic. • Compare the chi-square value to the critical value from the chi-square distribution table. • Example Data: Genotype counts in a population: AA = 40, Aa = 50, aa = 10 Total individuals (N) = 40 + 50 + 10 = 100 1. Observed Frequencies of : AA = 40/100 = 0.4 Aa = 50/100 = 0.5 aa = 10/100 = 0.1 2. Calculate Allele Frequencies: A (p) = = 0.65 a (q) = 1 - p = 1 - 0.65 = 0.35
• 11.
  3. Calculate Expectednumber of Genotype Frequencies under Hardy-Weinberg Equilibrium: AA (p²) × N = (0.65)² × 100 = 40.225 Aa (2pq) × N = 2 × 0.65 × 0.35 × 100 = 40.55 aa (q²) × 100 = (0.35)² × 100 = 12.25 4. Compute the Chi-Square Statistic: χ² = Σ After putting the values, χ² 0. ≈ 9784 5. Compare the Chi-Square Value to the Critical Value: - Degree of Freedom (df) = number of genotypes - number of alleles = 3 - 2 = 1. - Critical Value: For df = 1 and α = 0.05, the critical value is 3.841. - Decision: If χ² < 3.841, the population is in Hardy-Weinberg equilibrium. In this example, 0.964 < 3.841, so the population is in equilibrium.
• 13.
  Thank You:)