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Weekly plan 16 Interaction between gene loci Student book links Specification links 2.1.13 2.1.14 2.1.15 2.1.16 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 1. 2. 3. 4. Epistasis Types of interaction between loci Examples of epistatic problems and prediction of phenotypic ratios 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 Epistasis Epistatic Locus How Science Works Hypostatic Expected result Loci 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 Kinaesthetic Activity S1 Activity M2 Activity P2 Interpersonal Activities S1–3 Activities M1–3 Activities P1–3 Auditory Activities S1–3 Activities M1–3 Activities P1–3 Visual 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 presentation. 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 1 Weekly plan 16 Suggested starter activities Equipment 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 Equipment 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) Equipment 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 2 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. Notes © Pearson Education Ltd 2009 This document may have been altered from the original 3