population genetics

• Population Genetics

  The study of allele frequencies in populations, not offspring of single matings
• Gene pool

  The set of genetic information (all alleles) carried by the members of a sexually reproducing population
• Populations

  • Humans not distributed randomly across the world; clustered into discrete populations
  • More diverse than individuals
  • Can be described by age structure, geography, birth and death rates, and allele frequencies
• Allele frequency in a population changes from generation to generation

  Can cause change in phenotype frequency; long-term change in allele frequency is evolutionary change
• Monomorphic

  Gene with 1 allele
• Dimorphic

  Gene with 2 alleles
• Polymorphic

  A gene is "polymorphic" if there is more than 1 allele present in at least 1% of the population
• Study of white people from New England

  • 122 human genes that produced enzymes were examined, 51 were monomorphic and 71 were polymorphic
• Genotype frequencies and allele frequencies can be used to describe the genetic structure of a population
• Allele frequency

  The proportion of a specific allele in a population
• Hardy-Weinberg equilibrium

  A mathematical formula to determine frequency of alleles when one or more alleles are recessive, under certain specified conditions
• Hardy-Weinberg equilibrium

  • Used to describe a non-evolving population
  • Shuffling of alleles by meiosis and random fertilization have no effect on the overall gene pool
  • Natural populations are NOT expected to actually be in Hardy-Weinberg equilibrium
  • Deviation from Hardy-Weinberg equilibrium usually results in evolution
  • Understanding a non-evolving population helps us to understand how evolution occurs
• Necessary conditions for Hardy-Weinberg equilibrium

  • No new mutations
  • No migration in or out of the population
  • No selection (all genotypes have equal fitness)
  • Random mating
  • Very large population
• Hardy-Weinberg mathematics

  1. Allele frequency: p = frequency of A, q = frequency of a, p + q = 1

  2. Genotype frequency: p^2 = frequency of AA homozygotes, 2pq = frequency of Aa heterozygotes, q^2 = frequency of aa homozygotes, p^2 + 2pq + q^2 = 1
• Genetic equilibrium

  When the allele frequency for a particular gene remains constant from generation to generation
• Using the Hardy-Weinberg law in human genetics

  • Estimate frequencies of autosomal dominant and recessive alleles in a population
  • Detect when allele frequencies are shifting in a population (evolutionary change)
  • Measure the frequency of heterozygous carriers of deleterious recessive alleles in a population
• Factors affecting allele frequency

  • Mutation
  • Migration
  • Genetic drift
  • Natural selection
  • Non-random mating
• Mutation


  Spontaneous change in DNA that creates new alleles
• Migration

  Movement of organisms into or out of the population, introduces new alleles - "gene flow"
• Genetic drift

  Random change in allele frequencies, occurs mostly in small, isolated populations, loss of genetic variation, increase in homozygotes and decrease in heterozygotes
• Natural selection

  Environmental pressures; competition; climate change, certain genotypes produce more offspring, differences in survival or reproduction - "fitness", leads to adaptation
• Non-random mating

  Preferences in selection of a mating partner, increases homozygotes and decreases heterozygotes
• Factors affecting allele frequency

  Mutation, migration, natural selection, genetic drift, and non-random mating
• Non-random mating

  Alters genotype frequencies, leaves allele frequencies unchanged
• Human populations carry a large amount of genetic diversity
• Bottleneck effect

  May lead to reduced genetic variability following some large disturbance that removes a large portion of the population
• Founder effect

  May lead to reduced variability when a few individuals from a large population colonize an isolated habitat