The Hardy-Weinburg Principle

• A species is defined as a group of similar organisms that can reproduce to give fertile offspring
• A population is a group of organisms of the same species living in a particular area at a particular time - so they have the potential to interbreed. Species can exist as one or more populations e.g. there are populations of the American black bear in America and Canada
• The gene pool is the complete range of alleles present in a population.
• Allele frequency is is how often an allele occurs in a population. It is usually given as a percentage of the total population or a decimal.
• What is the Hardy-Weinberg principle?

  A mathematical model that predicts the frequencies of alleles in a population won't change from one generation to the next. This prediction is only true under certain conditions
• What are the conditions for the Hardy-Weinberg principle?

  1. It has to be a large population where there is no immigration, emigration, mutations or natural selectin
  2. There needs to be random mating - all possible genotypes can breed with all others
• The Hardy-Weinberg equations are based on the principle and can be used o estimate the frequency of particular alleles, genotypes and phenotypes in a population. It can also be used to test whether or not the Hardy-Weinberg principle applies to particular alleles in particular populations e.g. to test if selection or other factors are influencing allele frequencies. If frequencies do change between generations in a large population then there's an influence of some kind.
• Allele frequency: the total frequency of all possible alleles for a characteristic in a certain population is 1.0 (100%). So the frequencies of individual alleles (e.g. dominant and recessive) must add up to 1
• Allele frequency equation?


  p + q = 1

  p = frequency of dominant allele

  q = frequency of recessive allele
• Genotype frequency: the total frequency of all possible genotypes for one characteristic in a certain population is 1.0. So the frequencies of the individual genotypes must add up to 1.0 - there are three genotypes:

  1. homozygous recessive
  2. homozygous dominant
  3. heterozygous
• p2 + 2pq + q2 = 1

  assuming p is dominant and q is recessive

  p2 = frequency of homozygous dominant genotype

  2pq = frequency of heterozygous genotype

  q2 = frequency of homozygous recessive genotype