BIOL 2200 Lecture 4: Genetic variation & the Hardy-Weinberg equation

Page 1: Introduction

• Lecture Title: Genetic variation & the Hardy-Weinberg equation

Page 2: Learning Objectives

• Compare Phenotypic Plasticity to Evolution

  • Phenotypic Plasticity is when a genotype produces various phenotypes for an organism during a single generation, this is not heritable

  • Evolution is the acquisition of traits and passing down the most favorable to offspring to increase fitness which causes changes in variation or adaptability, this is heritable

• Characterize Sources of Genetic Variation

  • Point Mutations

    • Deletions, Duplications, Inversions

  • Chromosomal Mutations

    • Insertions

  • Crossing Over

    • Translocations

• Describe Heritability of Alleles

  • Dominant alleles determine phenotype

    • Doesn’t mean the most common or adaptive

  • The Recessive allele is masked by the phenotype

• Calculate Frequencies of Alleles Using Hardy-Weinberg Equation

  • p^2+2pq+q^2=1

  • p^2 = A1= .7, A1 A1 = 49%

  • 2pq= A1 A2 (heterozygous)= 42%

  • q^2 = A2= .3, A2 A2= 9%

• Understand Assumptions of Hardy-Weinberg

  • It assumes that there won’t be any selection, mutation, migration, large populations, or random mating which is not plausible

• Examine Hardy-Weinberg as a Null Hypothesis for Evolution

  • This equation only shows frequencies of alleles and genotypes in the absence of evolution

    • Deviations from the equilibrium indicate that one of the assumptions have been violated

PollEV

Page 3: Phenotypic Plasticity

• Definition: Genotype produces different phenotypes depending on environmental responses, during single generation

  • Example: Variations in phenotype with different predator environments.

• Heritability: The phenotype resulting from plasticity is not heritable

  • Environment drives phenotype not parental phenotype

  • Not all organisms have it

Page 4: Comparison: Phenotypic Plasticity and Evolution

• Responses to Predators: Observational comparisons between predator-present and predator-absent environments highlighting variations over generations.This demonstrates how phenotypic traits can adapt in response to selective pressures, leading to evolutionary changes that are distinct from those arising solely from genetic inheritance.

Page 5: Types of Phenotypic Plasticity

• Forms of Plasticity:

  • Developmental: The environment you’re born in develops phenotype

  • Seasonal: In certain environments like summer or spring phenotypes can change throughout their lifetime

Page 6: Examples of Plasticity vs. Evolution

• Chameleons changing color when moving is phenotypical plasticity

Page 7: Natural Selection

• Commonality: Not all offspring survive.

• Variation: Traits vary within a population; some traits may be heritable.

• Fitness Advantage: Heritable traits that provide an advantage tend to become more common due to higher fitness.

Page 8: Genetic Variation

• Definition: Differentiation among individuals in gene composition or DNA sequences.

• Noteworthy Point: Genetic variation does not necessarily lead to observable phenotypic differences.

• Phenotypic Plasticity vs. Genetic Variation: Not all phenotypic variation signifies genetic variation.

Page 9: Sources of Genetic Variation

• Types of Variations:

  • Point mutations: random errors

  • Chromosomal mutations: insertions, inversions, deletions, duplications

  • Crossing over during meiosis: translocation

Page 10: Heritability in Genetics

• Mendel's Observations: Noted inheritance patterns using pea plants demonstrating 3:1 ratio.

• 2 Alleles at a Locus:

  • Dominant: Determines the phenotype

  • Recessive: Masked in phenotype

• Note: Dominance does not imply commonality or adaptiveness.

  • They can have incomplete dominance or co-dominance

Page 11: Mendelian Inheritance Example

• Cross-Fertilization: between Purple and White parents leads to a uniformly purple F1 generation.

• F2 Generation Ratio: 3:1 phenotypic ratio

Page 12: Population Level Genetics

• Population Definition: A group of individuals of the same species, living in the same area and interbreeding leaving viable offspring

• Alleles: Different gene variants (e.g., P/p, A1/A2)

• Gene Pool: All copies of all alleles at every locus present within a population.

• Evolution: Change in gene frequencies of a population over generations.

Page 13: Population-Level Approach

• Example: A diploid population with 2 alleles.

• Hardy-Weinberg Principle: In absence of evolution, allele frequencies remain constant and do not change over time

Page 14: Hardy-Weinberg Equation Overview

• Basic Principles:

  • Alleles A1 and A2 represented by frequencies p(dom) and q(rece.).

  • Evolution does not occur in populations in Hardy-Weinberg equilibrium.

Page 15: Allele Frequency Calculation

• Frequencies:

  • Dominant allele: A1 (p = 0.7)

  • Recessive allele: A2 (q = 0.3)

• Genotypic Frequencies: Calculated using Hardy-Weinberg equation:

  • p² + 2pq + q² = 1

Page 16: Genetic Frequencies Continued

• Parental Frequencies and Offspring Genotype Frequencies reiterated to ensure understanding of allele distribution (p and q).

Page 17: Assumptions of Hardy-Weinberg Equilibrium

1. No selection

2. No mutation

3. No migration

4. Large population size

5. Random mating

Page 18: Hardy-Weinberg in Practice

• Example in Humans: MN blood-type allele frequencies.

  • Codominant alleles affecting phenotype.

• Population Data: Observed frequencies from Caucasians and Aboriginal populations.

Page 19: More on Hardy-Weinberg Frequencies

• Expected Genotype Frequencies: Comparison of observed frequencies in different human populations (Caucasians vs. Aborigines).

Page 20: Assessing Hardy-Weinberg Equilibrium

• Equilibrium Status: The population is in Hardy-Weinberg equilibrium if genotype frequencies match expected frequencies derived from allele calculations.

Page 21: Hardy-Weinberg as a Null Hypothesis

• Null Hypothesis: Establishing baseline frequencies of alleles and genotypes in the absence of evolutionary change.

• Deviation Interpretation: Indicates a violation of assumptions possibly leading to evolutionary mechanics.