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Science lesson plan · 9th grade (Year 10 in England) · 45 minutes

Genetic crosses and Punnett squares

Cells, genetics and evolution

CCSS NGSS.HS-LS3-2CCSS NGSS.HS-LS3-3England: KS4 Combined Science: Evolution, inheritance and variation

Objective and success criteria

Students will: Genetic crosses and Punnett squares.

  • 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: DNA, genes and chromosomes

Leads to: Sex chromosomes and the chance of a boy or a girl; Two-gene (dihybrid) crosses and 9 : 3 : 3 : 1 ratios; Allele frequencies with the Hardy–Weinberg equations

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 (DNA, genes and chromosomes). Students answer on mini whiteboards.
  • Show answers together and note who needs support in the main task.
  1. Where is DNA found in an animal cell? (outside the cell / in the cell wall / mainly in the nucleus / in the chloroplasts) → mainly in the nucleusPrompt: It is the control centre of the cell.
  2. Human body cells have 23 pairs of chromosomes. How many chromosomes are in a skin cell? → 46Prompt: Is this a gamete (sex cell) or a body cell?
  3. What shape is a DNA molecule? (a double helix / a ring of six atoms / a single straight line / a cube) → a double helixPrompt: It looks like a twisted ladder.

Hook · 5 min

  • Show a microscope image of cells and ask what students think they are looking at.
  • Share the objective: Genetic crosses and Punnett squares. 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: 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 white fur?
  • Step 1: Offspring: Bb, Bb, bb, bb
  • Step 2: 2 out of 4 = 50%
  • Step 3: So the answer is: 50%
  • Answer: 50 %
  • 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.
  1. 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? → 75 %Prompt: Draw a Punnett square with one parent's alleles along the top and the other's down the side.
  2. A recessive characteristic only shows when… (there is one recessive allele / there are two recessive alleles / the organism is old / there is a dominant allele) → there are two recessive allelesPrompt: A dominant allele hides it.

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.
  1. Two Pp parents have 40 offspring. Purple flowers is dominant. About how many offspring would you expect to have white flowers? → 10Prompt: What fraction of a Pp × Pp cross is pp?
  2. 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 white fur? → 0 %Prompt: Draw a Punnett square with one parent's alleles along the top and the other's down the side.

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: DNA, genes and chromosomes.
  • 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

  • Move on to: Sex chromosomes and the chance of a boy or a girl; Two-gene (dihybrid) crosses and 9 : 3 : 3 : 1 ratios; Allele frequencies with the Hardy–Weinberg equations.
  • 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-9.

Free lesson plan from eurekami.com/lesson-plan-builder

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Genetic crosses and Punnett squares

Name

Date

Score/ 8

  1. Question 1

    Two Tt parents have 200 offspring. Tall stems is dominant. About how many offspring would you expect to have short stems?

    Answer

  2. Question 2

    What does "heterozygous" mean?

    • A. having two different alleles of a gene, such as Bb
    • B. having two identical alleles, such as BB
    • C. having four alleles
    • D. having no alleles

  3. Question 3

    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 round seeds?

    Answer%

  4. Question 4

    Two Pp parents have 80 offspring. Purple flowers is dominant. About how many offspring would you expect to have white flowers?

    Answer

  5. Question 5

    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?

    Answer%

  6. Question 6

    Two Bb parents have 400 offspring. Brown fur is dominant. About how many offspring would you expect to have white fur?

    Answer

  7. Question 7

    In humans, which pair of sex chromosomes does a typical male have?

    • A. YY
    • B. XXY only
    • C. XX
    • D. XY

  8. Question 8

    Tall stems (T) is dominant over short stems (t). Two parents with genotypes TT and tt are crossed. What percentage of the offspring are expected to have tall stems?

    Answer%

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Answer key: Genetic crosses and Punnett squares

  1. 1. 50

  2. 2. A. having two different alleles of a gene, such as Bb

  3. 3. 50 %

  4. 4. 20

  5. 5. 50 %

  6. 6. 100

  7. 7. D. XY

  8. 8. 100 %