Alleles and Inheritance

Overview

This is lesson 2 of 6 in the unit Variation, Inheritance and Selection. Students build on prior learning about genes, chromosomes and inherited characteristics by using allele notation and Punnett squares to predict possible offspring genotypes and phenotypes. They practise communicating evidence-based genetic conclusions clearly and independently.

Learning intentions

Students will:

• Explain how alleles contribute to inherited characteristics.
• Interpret genetic notation, including dominant and recessive alleles.
• Construct and use Punnett squares for single-gene crosses.
• Justify predicted offspring genotypes and phenotypes using scientific language.

Success criteria

I can:

• Define allele, genotype, phenotype, homozygous, heterozygous, dominant and recessive.
• Complete a Punnett square accurately for a given parental cross.
• State the possible genotypes and phenotypes of offspring.
• Justify my conclusion by linking allele combinations to observable characteristics.

Curriculum links

• Genetic variation in relation to an identified characteristic: sources and nature of variation, and tracking genetic relationships.
• Science capability: interpreting representations, using evidence and communicating explanations.
• NZC Science, Level 6: understanding that genes influence inherited characteristics and that variation can arise through different combinations of inherited genetic information.
• Managing self and participating and contributing through collaborative modelling and independent checking.

Lesson structure (60 minutes)

1. 0–7 min · Hook and retrieval. Teacher opens the hook and retrieval slides with the question, “If two parents both show the same characteristic, must all their children show it?” Students complete a silent think, then write one remembered fact about genes, chromosomes or inherited characteristics before sharing with a partner. Teacher collects common misconceptions for later attention.

2. 7–18 min · Direct teaching: alleles and notation. Teacher uses the allele notation slides to model that alleles are alternative forms of a gene and introduces a consistent example: B represents a dominant allele for brown eyes in a simplified model and b represents a recessive allele. Clarify that this is a classroom model, not a complete description of human eye colour. Students annotate definitions and classify BB, Bb and bb as homozygous dominant, heterozygous and homozygous recessive, respectively; they identify each corresponding phenotype.

3. 18–28 min · Teacher modelling: Punnett squares. Teacher models the cross Bb × Bb step by step using the worked Punnett square slides: place one parent’s alleles across the top, the other parent’s down the side, combine alleles in each box, then count genotype and phenotype possibilities. Students copy the square and respond to hinge questions: “How many possible offspring combinations are shown?” and “Why can two offspring have different genotypes even with the same parents?”

4. 28–43 min · Paired practice and explanation. Teacher distributes the alleles and Punnett squares worksheet and assigns pairs three crosses: BB × bb, Bb × bb and Bb × Bb. Students complete each square, record genotype and phenotype ratios or percentages, and write one justification using the frame: “The offspring may have ___ because ___.” Partners compare answers, explain disagreements and use the checklist to self-correct. Teacher circulates, checking allele placement and the distinction between genotype and phenotype.

5. 43–53 min · Independent application. Teacher displays a new scenario from the application and discussion slides: two heterozygous parents have a recessive characteristic represented by r. Students independently complete Rr × Rr, then answer: “What is the probability that an offspring will show the recessive phenotype?” Students write a complete conclusion using the words allele, genotype, phenotype and probability. Invite two students to explain their reasoning, not just state the answer.

6. 53–60 min · Plenary and exit check. Teacher returns to the opening question using the plenary slides and asks students to revise their original response. Students complete the final worksheet prompt: “Explain why two parents showing a dominant phenotype can have an offspring showing a recessive phenotype.” Collect responses to identify readiness for the next lesson on inheritance patterns and variation.

Resources

• the inheritance teaching and modelling slide deck
• the alleles and Punnett squares worksheet
• Whiteboard and markers
• Student books or devices
• Calculators, if students use percentages
• Coloured pens or highlighters for allele tracking

Assessment

• During retrieval and hinge questions, listen for confusion between a gene, allele, genotype and phenotype.
• Circulate during paired practice to check that students place parental alleles correctly and justify predictions rather than merely list answers.
• Use the independent application and exit response as formative evidence of whether students can connect genotype to phenotype and communicate a conclusion clearly.

Differentiation

• Provide a partially completed Punnett square, a word bank and sentence starters for students who need support. Keep dominant alleles in one colour and recessive alleles in another.
• Read instructions aloud, chunk the worksheet into one cross at a time, and pair students strategically. Permit oral explanation before written recording for students with literacy or processing needs.
• For EAL learners, display labelled examples and explicitly rehearse dominant, recessive, same, different, possible and probability.
• Extend confident students by asking them to compare BB × Bb with Bb × Bb, explain why Punnett squares predict probabilities rather than guaranteed outcomes, and critique the simplified eye-colour model.