AGB 121: Principles of Animal Genetics and Population Genetics (2+1)

HARDY- WEINBERG LAW

• Hardy-Weinberg law was proposed independently in 1908 by Wilhelm Weinberg, a German physician, and Godfrey Harold Hardy, a British mathematician.

Both gene and genotype frequencies in a population remain constant generation after generation when the population is large; mating is at random and in the absence of selection, mutation and migration.

• When the gene frequency remains constant generations after generations, the population is in genetic equilibrium or Hardy-Weinberg equilibrium non-evolutionary model.
• When the population is in genetic equilibrium, the rate of evolution is zero. That is, when a population obeys, hardy-Weinberg law the population will not undergo evolution. So evolution occurs only when Hardy-Weinberg equilibrium is altered.
• The Hardy-Weinberg law is represented by a simple formula.

  • For 2 alleles (A₁ and A₂) of one gene

    p =	f(A) Frequency of 'A1' gene
    q =	f(a) Frequency of 'A2' gene

  • Then the next generation will have:

    • The frequency of homozygotes is equal to the gene frequency squared
      • The frequency of the A₁A₁ genotype = p²
      • The frequency of the A₂A₂ genotype = q²
    • The frequency of heterozygotes is equal to twice the product of the two gene frequencies
      • The frequency of the A₁A₂ genotype = 2pq
  • For a dimorphic gene the Hardy-Weinberg equation is based on the binomial distribution:
    • p² + 2pq + q² = 1
  • This formula is used to find out the frequency of dominant gene and recessive gene in a population.

p =	Frequency of dominant gene
q =	Frequency of recessive gene
p2 =	Frequency of dominant homozygote
2pq =	Frequency of heterozygote
q2 =	Frequency of recessive homozygote

• Hardy-Weinberg law lays the foundation for the study of population genetics.
• It gives a mathematical approach for genetics and evolution.
• The relationship between gene (allele) frequencies and genotype frequencies expressed by the H-W equation only holds if these 5 conditions are met
  • no new mutations
  • no migration in or out of the population
  • no selection (all genotypes have equal fitness)
  • random mating
  • very large population