Intro to Population Genetics

Intro to Population Genetics

• Overview of the field and its importance in studying genetic variation.

Learning Objectives

By the end of the section, you should be able to:

• Describe how each of the forces of evolution can affect allele frequencies.
• Define Hardy-Weinberg Equilibrium and identify the conditions required to maintain it.
• Identify the evolutionary forces that create, maintain, or eliminate genetic variation.
• Differentiate between genetic drift and gene flow, and describe their effects on allele frequencies and genetic variation in populations of different sizes.

Definition of Population Genetics

• Population genetics is the study of genetic structure, which includes:
  • Allele frequencies: The relative frequencies of different alleles of a gene in a population.
  • Genotype frequencies: The relative frequencies of different genotypes in a population.
  • Changes over time: How genetic structures change in response to various evolutionary forces.
• The five processes that affect population genetics are not detailed in this transcript but may include natural selection, genetic drift, mutation, gene flow, and non-random mating.

Characteristics of the Gene Pool

• A gene pool represents the set of genetic information (alleles and genes) within a population.
  • Key considerations:
  • Number of individuals (N): Total count of organisms in the population.
  • Size of the gene pool: The total number of alleles available within the population.

Measuring Allele and Genotype Frequencies

Definitions

• Allele Frequencies:
  • Defined as: $ext{Allele Frequency} = \frac{ ext{Number of specific allele} }{ ext{Total number of alleles for that gene in a population} }$
• Genotype Frequencies:
  • Defined as: $ext{Genotype Frequency} = \frac{ ext{Number of individuals with specific genotype} }{ ext{Total number of individuals in a population} }$

Total Frequencies

• In any given population, the sum of all allele frequencies must equal 1, which can be expressed as:
  • For two alleles (A and a): $p + q = 1$
  • Where
  • $p$ = frequency of allele A
  • $q$ = frequency of allele a
  • Relationships include:
  • $q = 1 - p$

Types of Allelic Frequency

• Monomorphic: Only one allele at a locus, frequency = 1 (allele is fixed).
• Polymorphic: More than one allele at a locus.

Measuring Genotype Frequencies

• For a population with two alleles (A and a), the possible genotypes are:
  • AA
  • Aa
  • aa
• The frequencies can be calculated as:
  • Frequency of genotype AA: $\frac{N_{AA}}{N}$
  • Frequency of genotype Aa: $\frac{N_{Aa}}{N}$
  • Frequency of genotype aa: $\frac{N_{aa}}{N}$
  • Where:
  • $N$ = Total number of individuals in the population
  • $N_{AA}$ = Number of individuals with AA genotype
  • $N_{Aa}$ = Number of individuals with Aa genotype
  • $N_{aa}$ = Number of individuals with aa genotype

Hardy-Weinberg Equilibrium

• Definition:
  • The Hardy-Weinberg equilibrium is a model in which allele frequencies do not change across generations; therefore, genotype frequencies can be predicted from allele frequencies.
  • At equilibrium, allele and genotype frequencies remain constant, suggesting that no evolutionary forces are acting (no evolution).

Conditions Needed for Hardy-Weinberg Equilibrium

• Conditions required to maintain Hardy-Weinberg equilibrium include:
  • No mutation
  • No selection
  • Random mating
  • No gene flow
  • Infinite population size (no genetic drift)

Predictions Based on Hardy-Weinberg Equilibrium

• Genotype frequencies can be predicted based on laws of probability and the allele frequencies in the population.
  • For every population with two alleles:
  • $p + q = 1$
  • $q = 1 - p$ or $p = 1 - q$
  • The squared relationship:
  • $(p + q)(p + q) = 1$
  • $p^2 + 2pq + q^2 = 1$
• Specific frequencies derived:
  • $ext{frequency of } AA = p^2$
  • $ext{frequency of } Aa = 2pq$
  • $ext{frequency of } aa = q^2$

Implications of Hardy-Weinberg Equilibrium

• In nature, populations rarely meet the strict conditions necessary for Hardy-Weinberg equilibrium, indicating that all populations are subject to evolutionary change.
• The model provides a useful framework for predicting approximate genotype frequencies and observing specific patterns that deviate from equilibrium, which reflects the processes of evolutionary change.
• Hardy-Weinberg equilibrium serves as a null model for understanding evolution.