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Weekly plan 16
Interaction between gene loci
Student book links
Specification links
5.1.2 (f)–(h)
Link to GCSE/AS specification
GCSE Genetic crosses
Cell division
DNA, genes and chromosomes
Genetic disease
Selective breeding
AS 1.1.1 Cell structure
1.1.3 Cell division
2.1.2 Nucleic acids
2.1.3 Enzymes
2.3.3 Evolution
Suggested time allowed (includes contact and non-contact time):
4 hours
Suggested teaching order
Types of interaction between loci
Examples of epistatic problems and prediction of phenotypic
Chi-squared test
Weekly learning outcomes
Students should be able to:
 Describe the interactions between loci (epistasis).
 Predict phenotypic ratios in problems involving epistasis.
 Use the chi-squared test to test the significance of the difference between observed and
expected results – the formula for 2 will be provided.
Key words
How Science Works
Expected result
Observed result
Chi-squared test
Phenotypic ratio
Recessive epistasis
Dominant epistasis
HSW 5 Present, analyse and evaluate data using the chisquared test.
Learning styles (S = Starter activities, M = Main activities, P = Plenary activities)
ICT activities
Activity S1
Activity M2
Activity P2
Activities S1–3
Activities M1–3
Activities P1–3
Activities S1–3
Activities M1–3
Activities P1–3
Activities S1–3
Activities M1–3
Activities P1–3
See Activity S3 below – anagram maker on the Internet.
See Activities M1and M3 below – use of spreadsheet to
undertake a chi-squared test.
See Activity M2 below – word processor for poster making.
See Homework suggestions below – using PowerPoint for a
The web links referred to here are some that the author has found personally helpful but are not intended to be a comprehensive list, many other
good resources exist.
© Pearson Education Ltd 2009
This document may have been altered from the original
Weekly plan 16
Suggested starter activities
1. In groups, students have to rearrange cards to depict a
genetic diagram for a dihybrid cross.
Cards with aspects of a genetic diagram
for a dihybrid cross on them
Teacher notes
2. Invent a number of scenarios in which students must judge
whether certain sets of numbers (preferably counts) really do
reflect a difference between two groups – e.g. number of
children gaining five A–C grades at GCSE in different schools.
Ask each group to devise their own rules for establishing
whether two sets of numbers really reflect an underlying
difference – use their ideas to lead into a discussion on
significance testing.
3. Choose a selection of key words from Weekly plan 15 and
mix them up to make anagrams.
In pairs, students unscramble the anagrams and write down
the corresponding key words – use their responses to recap
the meanings of the words.
Suggested main activities
1. Set some questions that involve epistatic crosses.
2. In groups, students make a poster to describe inheritance in
an imaginary animal.
Students work in pairs to answer these questions, which
should involve them working through a cross, making
phenotypic predictions according to the information given
about the epistasis and testing expectation using a chisquared test.
Poster-making materials
3. In pairs, students write a flowchart to describe how to
undertake a chi-squared test.
Suggested plenary activities
3. Draw a concept cartoon on the board with the prompt
question: ‘Why do you need to do a chi-squared test?’
Make up two scenarios: (i) where one locus works
antagonistically to another; and (ii) where one locus works
complementarily to another.
Download a plug-in for Microsoft Excel which helps you to
carry out statistical tests. (See second ICT activity)
1. In pairs, students describe the difference between certain
pairs of words.
2. Play Pairs game.
Teacher notes
Teacher notes
Word pairs could include: dominant and recessive epistasis;
dihybrid and monohybrid; dominant and codominant; and
epistatic and hypostatic.
Prepare cards which have: (i) a genetic
diagram and phenotypic ratios on one
card; and (ii) the explanation for those
ratios on another
Students pick up two cards at a time. If they match, they
keep them. If they don’t match, they put them back and try
to remember where they are for their next turn, etc.
Use the cartoon as a stimulus for class discussion. Explore
students’ reasons in order to review the purpose and
mechanism of the test.
© Pearson Education Ltd 2009
This document may have been altered from the original
Weekly plan 16
Homework suggestions
Practise some questions on epistasis and significance testing.
Imagine you are an agricultural consultant working with a poultry farmer. Produce a handbook to explain how to choose different chickens for selective breeding.
Write an interactive PowerPoint presentation to summarise the patterns of inheritance and epistasis you’ve encountered. The presentation should use internal
hyperlinks to aid navigation and should provide an example in each case.
Devise a set of questions on: (a) a dominant epistasis; and (b) a recessive epistasis.
Cross-curriculum links
Mathematics – ratios and statistical tests
Stretch and Challenge
The links to the AS specification stated on page 1 are a good opportunity to develop Stretch and Challenge skills.
Practise some questions on epistasis which require you to suggest explanations for given phenotypic ratios.
Potential misconceptions
Some students may confuse the terms: genotype/phenotype; and homozygous/heterozygous.
Students frequently find it difficult to interpret phenotypic ratios based on epistatic gene expression.
Many students will have a poorly-formed conception of dominant and recessive, which is based on one overpowering the other.
Some students find it difficult to distinguish between the terms: locus/gene/allele; and chromosome/chromatid.
Students often do not explicitly think about how each stage equates to gametes, potential zygotes, etc.
Some students will intuitively think that each possible zygote will be produced if two parents have multiple children, rather than there being an equal chance of each
zygote being produced at each fertilisation event.
Understanding probability can present many students with difficulties.
© Pearson Education Ltd 2009
This document may have been altered from the original