Science lesson plan · 12th grade (Year 13 in England) · 45 minutes
Allele frequencies with the Hardy–Weinberg equations
Cells, genetics and evolution
CCSS NGSS.HS-LS4-3CCSS NGSS.SEP 5England: A level Biology: Populations and evolution (extension)
Objective and success criteria
Students will: Allele frequencies with the Hardy–Weinberg equations.
- I can answer questions on today's skill on my own, without help.
- I can explain each step and say why it works.
- I can check my answer and spot a mistake.
Before and after
Needs first: Genetic crosses and Punnett squares; Natural selection and resistance
Leads to: the next skill in this area.
Materials: Microscope images or slides, Cell diagrams, Mini whiteboards, A diagram or short video clip of the idea, Printed student sheet (below).
Warm-up · 5 min
- Quick retrieval from earlier learning (Genetic crosses and Punnett squares; Natural selection and resistance). Students answer on mini whiteboards.
- Show answers together and note who needs support in the main task.
- Purple flowers (P) is dominant over white flowers (p). Two parents with genotypes Pp and pp are crossed. What percentage of the offspring are expected to have purple flowers? → 50 %Prompt: Draw a Punnett square with one parent's alleles along the top and the other's down the side.
- What does "heterozygous" mean? (having two identical alleles, such as BB / having no alleles / having four alleles / having two different alleles of a gene, such as Bb) → having two different alleles of a gene, such as BbPrompt: "Hetero" means different.
- Brown fur (B) is dominant over white fur (b). Two parents with genotypes Bb and bb are crossed. What percentage of the offspring are expected to have brown fur? → 50 %Prompt: Draw a Punnett square with one parent's alleles along the top and the other's down the side.
Hook · 5 min
- Show a microscope image of cells and ask what students think they are looking at.
- Share the objective: Allele frequencies with the Hardy–Weinberg equations. Read the success criteria together.
I do: model it · 10 min
- Work through the example out loud, saying why you do each step, not just what you do.
- Example: In a population, 49 % of individuals show a recessive phenotype (aa). Assuming Hardy–Weinberg equilibrium, what is the frequency of the recessive allele, q?
- Step 1: q² = 0.49
- Step 2: q = √0.49 = 0.7
- Step 3: So the answer is: 0.7
- Answer: 0.7
- Pause after each step and ask a student to say what comes next.
We do: guided practice · 10 min
- Do these together. Students try each step on whiteboards before you reveal it.
- Use the hints as prompts rather than giving the answer.
- In a population, 36 % of individuals show a recessive phenotype (aa). Assuming Hardy–Weinberg equilibrium, what is the frequency of the recessive allele, q? → 0.6Prompt: The aa frequency equals q².
- The frequency of a recessive allele in a population is q = 0.1. What percentage of the population are expected to be heterozygous carriers (2pq)? → 18 %Prompt: p + q = 1
You do: independent practice · 10 min
- Students work on the student sheet. Circulate and check the first two answers of every student early.
- Students who finish: write a question of their own for a partner, with the answer.
The 8 questions are on the student sheet below, with answers in the key.
Exit ticket · 5 min
- Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
- In a population, 16 % of individuals show a recessive phenotype (aa). Assuming Hardy–Weinberg equilibrium, what is the frequency of the recessive allele, q? → 0.4Prompt: The aa frequency equals q².
- Which condition is NOT needed for Hardy–Weinberg equilibrium? (random mating / strong natural selection on the gene / a very large population / no migration in or out) → strong natural selection on the genePrompt: The model describes a population that is NOT evolving.
Watch out for
- Everyday meanings of words getting in the way of the science meaning: define key words explicitly.
- Remembering facts without the reason: ask "why?" and "what is the evidence?"
Support and stretch
Support
- Review first: Genetic crosses and Punnett squares.
- Use concrete materials: microscope images or slides.
- Give the first step of each question, or a partly worked example to finish.
- Pair with a confident partner for the guided practice.
Stretch
- Explain a common mistake in writing and how to fix it.
- Write a word problem that uses today's skill, then solve a partner's.
Homework and more practice
Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.
Free lesson plan from eurekami.com/lesson-plan-builder