AS Science In Society 1.9
Teacher Notes
Introduction
In this activity, students simulate natural selection of the sickle cell
alleles, using beans and a flipped coin. They should find that the sickle
cell allele rapidly becomes more common where there is a high incidence
of malarial infection giving an example of natural selection in practice.
The context draws on knowledge of infectious diseases and inheritance
from earlier in the course.
Requirements for each group
75 red* beans, 25 white* beans, 5 pots (paper cups)
Coin or heads/tails flipper.
*any contrasting colour will do
Class results can be pooled on the OHT provided.
Science explanations
Gb The mechanism which
explains evolution is natural
selection. There is always
variation between individuals in
the same species. Some
individuals will have
characteristics which give them
a better chance of surviving in a
particular environment. Those
individuals that survive will
reproduce and pass on their
characteristics to the next
generation. The genes for the
advantageous characteristic will
become more common.
Answers to questions
1. The heterozygote AS genotype is favoured in areas where there is a high incidence of malarial
infection. There would be a conflicting selective pressure in areas with no malarial infection. Carriers
of the disease have no symptoms normally, but may get ‘sickling ‘ at times of high oxygen demand.
2. The A allele becomes less frequent, and the S allele more frequent.
3. The S allele only survives in areas with malaria, where it survives due to the ‘heterozygous
advantage’.
4. The S allele would become more frequent because SS homozygotes would pass on their alleles as
frequently as AA people pass on theirs. The S allele may become more common than the A allele in
areas with malaria. In these areas the advantage in being SS or AS would not be countered by the
disadvantages of sickle cell disease.
Page 1
©The Nuffield Foundation, 2008
Copies may be made for UK in schools and colleges
AS Science In Society 1.9
Teacher Notes
November 2008
Page 2
©The Nuffield Foundation, 2008
Copies may be made for UK in schools and colleges
AS Science In Society 1.9
Student sheets
Introduction
Make sure you know something about sickle cell anaemia before starting the activity. You may have
read the article ‘Genetics of sickle cell anaemia’ in a previous class. This activity simulates, or models,
the selective pressures that exist in nature, which can change the frequency of a particular allele in a
population. Selective pressures are factors which change the probability of a particular allele being
passed on to the next generation. You will know that sickle cell is a very painful, and in some cases
lethal mutation. So why, if it is so deadly, is the allele still very common in the population? Surely it
should have “died out”?
You will need:
75 red* beans, 25 white* beans,
5 pots (paper cups)
Coin or heads/tails flipper.
*any contrasting colour will do
The beans represent the different alleles (forms) of the gene controlling the formation of the haemoglobin
molecule.
RED
= the sex cells carrying the A (unaffected) allele
WHITE = the sex cells carrying the S (sickle cell) allele
Procedure
You will be simulating the breeding patterns of humans by combining at random, the gametes that carry
either the A allele or the S allele.
The gene pool in this simulation represents the gene pool in parts of Africa that are infested with
mosquitoes transmitting malaria. You will see the effects of a high incidence of malaria on the in this
population. Try to predict what will happen, you may be surprised with the result.
1. Write down what you think will happen to the frequency of the sickle cell S allele as you ‘breed’
several generations of people in malaria-infested Africa.
2. Work in pairs or small groups. Label the four containers with the following;
Page 1
©The Nuffield Foundation, 2008
Copies may be made for UK in schools and colleges
AS Science In Society 1.9
Student sheets
3. Place all the red and white beans into the Gene pool container. Mix them up thoroughly.
4. Without looking, take out TWO beans from the gene pool. This is simulating the act of fertilisation to
produce a new individual.
5. For every two beans chosen, your partner will flip a coin (or flipper) to determine whether the new
individual is infected with malaria or not (grey box of table below).
Using the table below, the flip of the coin will determine in which container to put the two beans.
Essentially you are deciding whether the individual lives or dies.
Genotype –
genetic make up
AA (red-red)
Phenotype –
sickle cell or not
reaction to malaria
No sickle cell disease
Susceptible to malaria
AS (red-white)
No sickle cell disease
Resistant to malaria
SS (white-white)
Sickle cell disease
DIE
Tails –
not infected with
malaria
LIVE
place in non-survivors
LIVE
place in AA
LIVE
place in AS
DIE
place in AS
DIE
place in non-survivors
place in non-survivors
Heads with malaria
6. Continue pairing up your beans in the same way until the gene pool is empty.
7. At the end record all your results carefully. Draw up a table and use a tally system to record the
numbers of alleles left.
8. Now add these survivors back into the gene pool to produce the next generation; put all the beads
from the survivors containers into the gene pool container. Don’t touch the beads in the nonsurvivors container.
9. Repeat the process for a second generation.
Page 2
©The Nuffield Foundation, 2008
Copies may be made for UK in schools and colleges
AS Science In Society 1.9
Student sheets
Class results
Your small sample is not enough to see a clear pattern, so we need to put together all the group’s
results.
Questions
You will need to discuss each of these in your group, and then be prepared to put your points over to
others.
1. What are the ‘selective pressures’ that seem to change the allele frequency?
2. What was the trend for the frequency of allele A over three generations? What was the trend for S
over the same time?
3. The SS trait is lethal and it is unlikely that anyone with this combination will survive to have their own
children. Why has the mutant allele S not been eliminated from the population as a whole?
4. If it was possible to treat sickle cell disease effectively, what would happen to the frequencies of the
A and S alleles in the population?
Page 3
©The Nuffield Foundation, 2008
Copies may be made for UK in schools and colleges