Gene Pools & Allele Frequency (AQA A Level Biology): Revision Note
Exam code: 7402
Gene pools & allele frequency
Gene pools
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
Allele frequencies 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 lead to evolution or can become so great that a new species forms
Collecting data about the frequency of a phenotype
Phenotype frequency is the number of individuals in a population showing a particular observable trait
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)
We calculate the phenotype frequency by:
Counting individuals showing a specific phenotype
Dividing by the total number of individuals in the population (or sample)
The answer is usually expressed as a percentage
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
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%
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