Lesson Plan

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Lesson Plan
Grade: A-Level Date: 25/02/2026
Subject: Biology
Lesson Topic: interpret and construct genetic diagrams, including Punnett squares, to explain and predict the results of dihybrid crosses that involve autosomal linkage and epistasis (knowledge of the expected ratios for different types of epistasis is not expecte
Learning Objective/s:
  • Describe the relationship between genotype, gene expression, and phenotype.
  • Construct Punnett squares for dihybrid crosses, including cases with autosomal linkage.
  • Apply epistatic rules to predict phenotypic outcomes from combined genotypes.
  • Calculate expected phenotypic percentages using recombination frequencies.
Materials Needed:
  • Projector and screen
  • Whiteboard and markers
  • Printed worksheets with Punnett‑square templates
  • Coloured pens for diagramming
  • Handout summarising linkage and epistasis concepts
  • Calculator (optional)
 Introduction:
Begin with a quick think‑pair‑share: students predict the seed colour and shape of pea plants from a simple dihybrid cross. Review key terms—allele, genotype, phenotype, linkage, epistasis—to activate prior knowledge. Explain that by the end of the lesson they will be able to build and interpret Punnett squares for linked genes and apply epistatic rules.
Lesson Structure:
  1. Do‑now (5’) – short quiz on dominance and independent assortment.
  2. Mini‑lecture (10’) – recap genotype‑to‑phenotype pathway; introduce linkage & recombination.
  3. Guided practice (15’) – demonstrate constructing gametes for a linked dihybrid cross on the board.
  4. Collaborative activity (20’) – groups fill out a 4×4 Punnett square using given recombination frequencies and apply recessive epistasis rule.
  5. Whole‑class check (10’) – groups present results; teacher highlights common errors and correct phenotypic conversion.
  6. Exit ticket (5’) – students write one example of how epistasis alters expected ratios.
Conclusion:
Summarise how linkage reduces gamete diversity and how epistasis can mask expected phenotypes. Students complete an exit ticket stating the phenotypic ratio for the worked example. Assign homework: complete a worksheet with two additional dihybrid crosses, one linked and one independent.