AQA A Level Biology

Revision Notes

7.2.2 Gene Pools & Allele Frequency

Gene Pools & Allele Frequency

Gene pools

  • The phenotype of an organism is dependent on its genotype and the environmental influence on the organism
  • Members of the same species will have the same genes, of which there may exist different alleles (alternate versions of genes)
  • A gene pool is the collection of genes within an interbreeding population at a particular time
    • As these genes can have different alleles, a gene pool can also be thought of as the sum of all the alleles of the genes of a population (of a single species) at a particular time

Allele frequency

  • How often different alleles occur in the gene pool of a population is known as the allele frequency
  • The gene pool (or allele frequencies) in a species population can change over time due to processes such as natural selection
  • When the gene pool (or allele frequencies) within a species population changes sufficiently over time, the characteristics of the species population will also change
  • Over time, these changes can become so great that a new species forms

Apparatus & Techniques: Collecting Data about the Frequency of a Phenotype

  • The frequency of a phenotype is simply the number of individuals in a population that have a specific, observable trait (a particular phenotypic characteristic)
    • Many organisms have traits that show more than one phenotype (e.g. shell colour in banded snails can be pink or yellow and flower colour in pea plants can be purple or white)
  • Phenotype frequencies can be calculated by counting the number of times a particular phenotype appears in a population (or sample of a population) and dividing this by the total number of individuals in the population (or the sample)
    • Phenotype frequencies are normally given as a percentage of the total population

Phenotype frequency = (total individuals with phenotype ÷ total individuals in population) × 100

Worked Example

In a population of 9 pea plants, 7 of the plants have purple flowers, whilst 2 have white flowers. Calculate the phenotype frequencies of purple and white flowers. Give your answers as percentages.

Two phenotypes of pea plant flowers

Pea plants phenotype frequency, downloadable AS & A Level Biology revision notes

Step 1: Calculate the phenotype frequency of purple flowers

Phenotype frequency = (total individuals with phenotype ÷ total individuals in population) × 100

= (7 ÷ 9) × 100

= 0.78 × 100

= 78%

Step 2: Calculate the phenotype frequency of white flowers

Phenotype frequency = (total individuals with phenotype ÷ total individuals in population) × 100

= (2 ÷ 9) × 100

= 0.22 × 100

= 22%

Worked Example

In banded snails (Cepaea nemoralis), shell colour is controlled by two alleles of a single gene. The pink allele (Cᴾ) is dominant over the yellow allele (Cʸ). Out of a sample of individuals collected, 275 snails have the genotype CᴾCᴾ, 150 have the genotype CᴾCʸ, and 75 have the genotype CʸCʸ. Calculate the relative phenotype frequencies of the banded snails collected.

Two phenotypes of banded snails

Banded snails phenotype frequency, downloadable AS & A Level Biology revision notes

Step 1: Calculate the total number of snails in the sample

= 275 + 150 + 75

= 500

Step 2: Calculate the total number of pink-shelled snails (genotypes: CᴾCᴾ and CᴾCʸ)

= 275 + 150

425

Step 3: Calculate the phenotype frequency of pink-shelled snails

Phenotype frequency = (total individuals with phenotype ÷ total individuals in population) × 100

= (425 ÷ 500) × 100

= 0.85 × 100

= 85%

Step 4: Calculate the phenotype frequency of yellow-shelled snails

Phenotype frequency = (total individuals with phenotype ÷ total individuals in population) × 100

= (75 ÷ 500) × 100

= 0.15 × 100

= 15%

 

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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