Modelling Allele Frequency in Populations
The term “allele frequency” simply describes how common a particular allele is within
the gene pool of a population at a given time.
Outline the meaning of the following words:

allele

population

gene pool
The dreaded
+
=
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Hardy Weinberg equation shows that the frequency of alleles in a
population remains stable over time, providing 7 conditions are met.
Three of their conditions are:
1. the size of the population is infinite.
2. all members of the population reproduce
3. everyone has the same number of offspring.
In the following exercise, you will be modelling other effects on the gene pool of a
population.
Take a beaker and fill it with:
 55 red beads
 35 blue beads
 10 yellow beads
Each bead represents an allele and the beaker represents the gene pool of an entire
population.

What is the population size?

List the possible genotypes in this population.

Express the frequency of each allele as a proportion of 1 where 1 = 100% (i.e.
0.2, 0.4, or whatever).
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Your modelling career is now ready to begin!
Give your beaker a good shake so that the beads are well mixed.
Model 1
Pour out 20 beads into another beaker. This is your new population.

Calculate the allele frequency for your new population.

Describe what has happened to the allele frequency and suggest a real life
example of what could have caused a change like this in a real population.
Put the 20 beads back into the beaker with all the other beads to restore your
original population.
Model 2
Now pull out 30 individuals (how many beads?!?!?) at random. Any individuals that have
a blue bead in their genotype should be put aside in a completely separate pile (ignore
them and do not use them again in the lesson!). The remaining individuals from the
30 removed are another new population.

Calculate the allele frequency for this new population.

Describe what has happened to the allele frequency and suggest a real life
example of what you have modelled.
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Model 3
Say “hello” to another group.
Swap your new small populations (i.e. the survivors of the 30: see Model 2) and add
the alleles they give you to your remaining 20 individuals of the original population
(the ones still in the beaker).

Calculate the allele frequency for your new population.

Describe what has happened to the allele frequency and explain what real life
event you have modelled.
Model 4
Replace half of your blue beads with green beads.

Calculate the allele frequency for your new population.

Describe what has happened to the allele frequency and explain what real life
event you have modelled.
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
From your modelling work, can you suggest other conditions that need to be
met for allele frequency to remain stable over time?

What happens to a population when its allele frequency changes?
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