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.
- Where is DNA found in an animal cell? (in the chloroplasts / in the cell wall / mainly in the nucleus / outside the cell) → mainly in the nucleusPrompt: It is the control centre of the cell.
- Human body cells have 23 pairs of chromosomes. How many chromosomes are in an egg cell? → 23Prompt: Is this a gamete (sex cell) or a body cell?
- Rosalind Franklin's X-ray image helped scientists discover… (the structure of DNA / the shape of the Earth / the first vaccine / penicillin) → the structure of DNAPrompt: Watson and Crick used her data.
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.
- 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? → 100 %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 different alleles of a gene, such as Bb / having two identical alleles, such as BB / having no alleles / having four alleles) → having two different alleles of a gene, such as BbPrompt: "Hetero" means different.
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.
- Two Tt parents have 80 offspring. Tall stems is dominant. About how many offspring would you expect to have short stems? → 20Prompt: What fraction of a Tt × Tt cross is tt?
- 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.
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