7.2 Populations

• What is a population?

  • A group of organisms of the same species in a particular space at a particular time
  • That can (potentially) interbreed (to produce fertile offspring)
• What is a gene pool?


  All the alleles of all the genes in a population at any one time
• What is allele frequency?


  Proportion of an allele of a gene in a gene pool (decimal or percentage)
• What does the Hardy-Weinberg principle state and what are the conditions under which the principle applies?


  Allele frequencies will not change from generation to generation, given:

  • Population is large
  • No immigration / emigration (to introduce / remove alleles)
  • No mutations (to create new alleles)
  • No selection for / against particular alleles
  • Mating is random
• What is the Hardy-Weinberg equation? (and what does each stand for)


  $p^2+$$2pq\ +$$q^2=$$1$

  This can be used simultaneously with: p + q = 1

  • p = frequency of one (usually dominant) allele of the gene
  • q = frequency of the other (usually recessive) allele of the gene
  • $p^2$ = frequency of homozygous (usually dominant) genotype
  • 2pq = frequency of heterozygous genotype
  • $q^2$ = frequency of homozygous (usually recessive) genotype

  Note - if alleles are codominant, either can be assigned p and q
• Worked example: using the Hardy-Weinberg equation (basic)

  The gene for the Rhesus blood group has two alleles. The allele for Rhesus positive, R, is dominant to that for Rhesus negative, r.

  16% of the population of Europe is Rhesus negative. Use the Hardy-Weinberg equation to calculate the percentage of this population that you would expect to be heterozygous for the Rhesus gene.

  Find $q^2$= $q^2$ = 16% = 0.16

  Find q = q = √0.16 = 0.4

  Find p = p = 1 - 0.4 = 0.6

  Find 2pq = 2 x 0.4 x 0.6 = 0.48

  Convert 48%
• Worked example: using the Hardy-Weinberg equation with codominance

  In one species of flowering plant, two alleles $C^R$ (red) and $C^W$ (white) code for flower colour. When both are

  present, flowers are pink.

  A population contained 9% red-flowered plants. Use

  the Hardy-Weinberg equation to calculate the

  percentage of pink-flowered plants in this population.

  • Find $q^2$= $q^2$ = 9% = 0.09
  • Find q = q = √0.09 = 0.3
  • Find p = p = 1 - 0.3 = 0.7
  • Find 2pq = 2 x 0.3 x 0.7 = 0.42
  • Convert 42%
• Worked example: using the Hardy-Weinberg equation (more challenging)

  In fruit flies, a gene for body colour has a dominant allele G (grey) for and a recessive allele g (black).

  A population contained 64% grey-bodied flies. Use the Hardy–Weinberg equation to calculate the percentage of flies heterozygous for this gene.

  1. Find $q^2$ = $p^2$ + 2pq = 64% = 0.64 so $q^2$ = 1 - 0.64 = 0.36
  2. Find q = q = √0.36 = 0.6
  3. Find p = p = 1 - 0.6 = 0.4
  4. Find 2pq = 2 x 0.6 x 0.4 = 0.48
  5. Convert 48%