Lec 9

• Population Genetics


  • Concerns with genetic properties of a population
  • Gene and genotypic frequencies at the population level
• Hardy-Weinberg Equilibrium


  • Factors that keep gene and genotypic frequencies constant over many generations
  • Factors that tend to change gene frequencies in the population
• Random mating is assumed occurring in a large population
• Hardy-Weinberg Equilibrium


  When gene and genotypic frequencies are kept constant from generation to generation
• Assumptions for Hardy-Weinberg Equilibrium

  • Infinite or large population
  • Random mating
  • No forces changing gene frequency (selection, migration and mutation)
• Hardy-Weinberg Equilibrium/Law


  An equilibrium state of a single locus in a randomly mating diploid population that is free of other evolutionary forces: mutation, migration, selection and genetic drift
• Genetic drift


  • Causes random changes in the allele frequency
  • Alleles are lost from the population
  • Direction of random drift is neutral
• Small populations


  • Subject to random drift: a higher likelihood that gene frequencies fluctuate from one generation to another just by chance
  • Less likelihood of random mating to occur
• Large populations

  • Random drift is not an issue
  • A higher possibility of random mating occurring
• Founder effect


  • When the population grew from a few founding individuals
  • A few individuals cannot represent all the genomes of a founding population
• Random genetic drift


  Changes gene and genotype frequencies in small isolated populations
• Random mating

  • Individuals have equal chance to mate among them
  • Required to maintain a constant state of unchanged gene and genotypic frequencies in an infinitely large population
  • Individuals contribute proportionately to the generation of offspring in the next generation
• Seldom occurring in livestock populations
• Non-random mating in livestock populations

  • Animals are assigned mates in mating systems
  • Use of reproductive biotechnologies such as artificial insemination and multiple ovulation and embryo transfer techniques would dictate which animals are selected as parents
  • A small number of males are selected as parents to be mated to a larger number of females in the population
• Possible parent mating combinations in random mating

  • AA x AA
  • Aa x AA
  • aa x AA
  • AA x Aa
  • Aa x Aa
  • aa x Aa
  • AA x aa
  • Aa x aa
  • aa x aa
• Genotypic frequency in random mating

  25% AA: 50% Aa: 25% aa
• Possible parent mating combinations and expected offspring genotypes in random mating

  • AA x AA: 4 AA

  AA x Aa: 2 AA, 2 Aa

  AA x aa: 4 Aa

  Aa x AA: 2 AA, 2 Aa

  Aa x Aa: 1 AA, 2 Aa, 1 aa

  Aa x aa: 2 Aa, 2 aa

  aa x AA: 4 Aa

  aa x Aa: 2 Aa, 2 aa

  aa x aa: 4 aa
• Positive assortative mating

  • Mating between individuals of similar phenotypes for selected traits
  • Increases the number of homozygotes and decreases the number of heterozygotes
• Possible parent mating combinations in positive assortative mating

  • AA x AA

  Aa x Aa

  aa x aa
• Expected offspring genotypes in positive assortative mating

  • AA x AA: 4 AA

  Aa x Aa: 1 AA, 2 Aa, 1 aa

  aa x aa: 4 aa
• Consanguineous mating

  Mating of closely related individuals

  Results in a high inbreeding coefficient and less genetic diversity among descendants

  Can increase frequency of deleterious or advantageous alleles if present in the family
• Negative assortative mating

  Mating between individuals of dissimilar phenotypes for selected traits

  Increases the number of heterozygotes and decreases the number of homozygotes
• Possible parent mating combinations in negative assortative mating

  • AA x Aa, Aa x AA, aa x AA

  AA x aa, Aa x aa, aa x Aa
• Equilibrium distribution of genotypes not possible with non-random mating
• Non-random mating used in genetic improvement programmes of livestock to increase the frequency of desirable alleles
• Non-random mating used in developing purebred lines of laboratory animals, dogs, horses and farm animals
• Non-random mating in small isolated populations may increase frequency of genetically inherited abnormalities and diseases
• Non-random mating normally happening in small populations such as commercial livestock herds/flocks
• Genetic drift

  Random changes in allele frequencies due to small populations

  Dispersive force removing genetic variation from the population

  Probability of survival of new mutations is quite independent of population size
• Neutral theory

  Most allelic substitutions are neutral

  Most mutations have no influence on survival of genotypes

  Mutations that do affect survival are subjected to natural selection
• Natural populations are dynamic and always fluctuate in size