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Science lesson plan · 12th grade (Year 13 in England) · 60 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 · 6 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.
  1. Round seeds (R) is dominant over wrinkled seeds (r). Two parents with genotypes Rr and Rr are crossed. What percentage of the offspring are expected to have wrinkled seeds? → 25 %Prompt: Draw a Punnett square with one parent's alleles along the top and the other's down the side.
  2. What does "heterozygous" mean? (having no alleles / having two different alleles of a gene, such as Bb / having two identical alleles, such as BB / having four alleles) → having two different alleles of a gene, such as BbPrompt: "Hetero" means different.
  3. 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? → 100 %Prompt: Draw a Punnett square with one parent's alleles along the top and the other's down the side.

Hook · 6 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 · 12 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 · 12 min

  • Do these together. Students try each step on whiteboards before you reveal it.
  • Use the hints as prompts rather than giving the answer.
  1. In a population, 25 % of individuals show a recessive phenotype (aa). Assuming Hardy–Weinberg equilibrium, what is the frequency of the recessive allele, q? → 0.5Prompt: The aa frequency equals q².
  2. 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 · 17 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 · 7 min

  • Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.
  1. In a population, 49 % of individuals show a recessive phenotype (aa). Assuming Hardy–Weinberg equilibrium, what is the frequency of the recessive allele, q? → 0.7Prompt: The aa frequency equals q².
  2. The frequency of a recessive allele in a population is q = 0.4. What percentage of the population are expected to be heterozygous carriers (2pq)? → 48 %Prompt: p + q = 1

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

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Allele frequencies with the Hardy–Weinberg equations

Name

Date

Score/ 8

  1. Question 1

    In a population, 9 % of individuals show a recessive phenotype (aa). Assuming Hardy–Weinberg equilibrium, what is the frequency of the recessive allele, q?

    Answer

  2. Question 2

    The frequency of a recessive allele in a population is q = 0.2. What percentage of the population are expected to be heterozygous carriers (2pq)?

    Answer%

  3. Question 3

    Allele frequencies in a population change over generations. What does this show?

    • A. The population is extinct
    • B. No mutations ever happen
    • C. The population is evolving
    • D. Hardy–Weinberg equilibrium

  4. Question 4

    In a population, 1 % of individuals show a recessive phenotype (aa). Assuming Hardy–Weinberg equilibrium, what is the frequency of the recessive allele, q?

    Answer

  5. Question 5

    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)?

    Answer%

  6. Question 6

    Why can recessive alleles for harmful conditions stay in a population for many generations?

    • A. Recessive alleles cannot be inherited
    • B. Most copies are hidden in healthy heterozygous carriers
    • C. Natural selection always favours them
    • D. They mutate back every generation

  7. Question 7

    In a population, 25 % of individuals show a recessive phenotype (aa). Assuming Hardy–Weinberg equilibrium, what is the frequency of the recessive allele, q?

    Answer

  8. Question 8

    The frequency of a recessive allele in a population is q = 0.3. What percentage of the population are expected to be heterozygous carriers (2pq)?

    Answer%

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Answer key: Allele frequencies with the Hardy–Weinberg equations

  1. 1. 0.3

  2. 2. 32 %

  3. 3. C. The population is evolving

  4. 4. 0.1

  5. 5. 18 %

  6. 6. B. Most copies are hidden in healthy heterozygous carriers

  7. 7. 0.5

  8. 8. 42 %