Lecture: Mendel, Darwin, and Population Genetics

Introduction

• Gregor Mendel: Discovered basic principles of genetics.
  • Parents contribute alleles to offspring.
  • Dominant vs. recessive alleles.
• Charles Darwin: Developed theory of natural selection.
  • No understanding of trait inheritance.

Population Genetics

• Study of how species' populations change genetically over time.
• Definition of a Population: A group of interbreeding individuals of the same species.
• Uses genetic testing to study changes over generations.

Factors Affecting Allele Frequency

1. Natural Selection: Alleles for fitter organisms become more frequent.
2. Sexual Selection: Alleles for more sexually attractive organisms become more frequent.
3. Mutation: New alleles arise due to DNA copying errors.
4. Genetic Drift: Random changes in allele frequency (more significant in small populations).
5. Gene Flow: Introduction of new alleles from other populations (immigration/emigration).

Hardy-Weinberg Principle

• Developed by Godfrey Hardy and Wilhelm Weinberg in 1908.
• Describes allele frequency in a non-evolving population.
• Five Assumptions for Equilibrium:
  1. No natural selection.
  2. No sexual selection (random mating).
  3. No mutations.
  4. Large population size.
  5. No gene flow.

Hardy-Weinberg Equation

• Relationship between phenotype and allele frequency.
• Equation: p^2 + 2pq + q^2 = 1
  • p^2: Frequency of homozygous dominant genotype (WW).
  • 2pq: Frequency of heterozygous genotype (Ww).
  • q^2: Frequency of homozygous recessive genotype (ww).

Calculation Example

• Trait: Earwax type (wet - dominant, dry - recessive).
• Assume a population of 100 people, 9% have dry earwax (ww).
• q^2 = 0.09 → q = 0.30
• p + q = 1 → p = 0.70
• Genotypes:
  • WW (p^2) = 0.49
  • Ww (2pq) = 0.42
  • ww (q^2) = 0.09

Application

• Hardy-Weinberg equilibrium helps identify when evolutionary factors are at work.
• Example: Introduction of new individuals (gene flow) disrupting equilibrium.

Conclusion

• Population genetics allows observation of genetic changes over time.
• Hardy-Weinberg principle provides a baseline for studying these changes.