Population Genetics Part 1

Introduction to Population Genetics

• Examines the genetic basis of evolutionary change, focusing on genetic variation within and between populations.

• Emphasizes that evolution is best understood in light of genetics.

Definitions in Population Genetics

• Alleles at a Locus: Gene variants at a specific chromosome position.

• Genotype: Genetic constitution; Phenotype: Observable characteristics (genetic + environmental).

• Dominant Allele: Expresses phenotype even with a recessive allele; Recessive Allele: Must be homozygous to express phenotype.

• Frequency: Proportion of a specific allele or genotype in a population.

Hardy-Weinberg Equilibrium (HWE)

• Describes a hypothetical non-evolving population where allele frequencies remain constant across generations.

• Serves as a 'null hypothesis' to test for evolutionary forces.

• Founded by W. Weinberg and G. H. Hardy.

Key Assumptions of Hardy-Weinberg Equilibrium

1. Random Mating: Individuals pair by chance.

2. Infinite Population Size: No genetic drift.

3. No Mutation: No new alleles.

4. No Selection: All genotypes have equal reproductive success.

5. No Migration: No allele movement in or out.

• Violations of these assumptions indicate active evolutionary forces.

Allele and Genotype Frequency Calculations

• For two alleles A (frequency p) and a (frequency q):

  • Relation: $p + q = 1$

  • Expected genotype frequencies: $AA: p^2$, $Aa: 2pq$, $aa: q^2$

  • Overall Equation: $(p + q)^2 = p^2 + 2pq + q^2 = 1$

Conclusions from Hardy-Weinberg Equilibrium Data

• Allele frequencies are constant across generations in HWE, demonstrating no evolutionary influences from random assortment.

Demonstrating Hardy-Weinberg Equilibrium

• Involves calculating allele frequencies from observed genotypes, predicting expected genotype counts using HWE formulas, and comparing observed versus expected values to assess if HWE assumptions are met.

• Differences suggest HWE violation.

Applications of Hardy-Weinberg Equilibrium

• Used to calculate allele frequencies for disorders or determine heterozygote numbers in a population, assuming HWE.

Addressing Assumptions Violations

• Deviations from HWE indicate evolutionary processes like non-random mating, mutation, migration, selection, or genetic drift are occurring.

Impact of Selection on Allele Frequencies

• Selection directly modifies allele frequencies by altering the survival or reproductive success of different genotypes across generations, moving the population out of HWE. For example, differential survival rates of genotypes B1B1, B1B2, and B2B2 will change the overall B1 and B2 allele frequencies.