Module 7. Populations
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What is a population?
A group of organisms of the same species in one area at one time that can interbreed
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 frequencieswill 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 This provides a mathematical model.
What is the Hardy-Weinberg equation?
p² + 2pq + q² = 1 This can be used simultaneously with: p + q = 1 ● **p = frequency **of one (usually dominant) alleleof the gene ● q = frequency of the other (usually recessive) **allele **of the gene ● p² = frequency of homozygous (usually dominant) genotype ● 2pq= frequency of heterozygous genotype ● q² = 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 q² → q² = 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 Cʳ (red) and Cʷ (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 → q² = 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.
• Find q² → p² + 2pq = 64% = 0.64 so q² = 1 - 0.64 = 0.36 • Find q → q = √0.36 = 0.6 • Find p → p = 1 - 0.6 = 0.4 • Find 2pq → 2 x 0.6 x 0.4 = 0.48 • Convert 48%