High school · Science · 20 minutes
Use a Simple Inheritance Model
Use a Punnett square to describe probabilities and explain the model’s limits.
High School Biology Foundations · Lesson 2 · Starter collectionLearning objective
Use a Punnett square to describe probabilities and explain the model’s limits.
Gather your materials
- Paper or a shared note-taking document
- The original passage, example or data provided in this lesson
The lesson time covers the core activity. Allow extra time for video viewing and extended discussion.
Supporting Common Core connection
SL.9-10.1Standard level: Grades 9–10
Prepare evidence, share roles, ask questions and explain how a discussion changes or strengthens your conclusion.
Read the official standard ↗This connection applies to the structured discussion below. It does not establish alignment with science or social studies content standards. Code and level checked 2026-11-04; selected skill coverage.
Teach it step by step
- Ask what the learner already knows. Introduce the objective and read the explanation together.
- Work through the example aloud. Pause for the learner to explain why each step or piece of evidence matters.
- Choose a video if useful, then connect it to the example. Let the learner complete the independent practice before checking the explained answers.
- Discuss the question below and finish with the exit check. Revisit the example if the reasoning is unclear.
Watch, notice, discuss
Video choices for this lesson
Preview a video, then choose what fits your learners. “Show to class” opens a larger view; use the player’s play and full-screen controls when you’re ready.
Teacher preview needed. Check the language, level and length before showing. YouTube may show ads or related videos.
Monohybrids and the Punnett Square Guinea Pigs
By Amoeba Sisters
Use this explanation as an introduction, then pause to connect it to the worked example and discussion question.
Open on YouTube: Monohybrids and the Punnett Square Guinea Pigs (opens a new tab) ↗Mega Genetics Review: Mendelian and non-Mendelian Genetics
By Amoeba Sisters
Extension beyond simple monohybrid crosses. Preview and select only the sections appropriate to your learner.
Open on YouTube: Mega Genetics Review: Mendelian and non-Mendelian Genetics (opens a new tab) ↗What does a Punnett square predict, and what does it not guarantee for a small family?
An external player loads only after you choose Preview or Show to class. Playback connects to YouTube. Videos belong to their creators; availability and embedding can change.
Understand the idea
An allele is a version of a gene. In a simplified single-gene model with complete dominance, an organism with genotype Aa shows the dominant trait, while aa shows the recessive trait. A Punnett square lists possible allele combinations. Its ratios express probabilities for each offspring, not a guaranteed order or exact family outcome. Many real traits involve multiple genes, environmental influences or other inheritance patterns.
Worked example
In a model cross Aa × Aa, each parent can contribute A or a. Combining possibilities gives AA, Aa, aA and aa. The model predicts genotype probabilities of 1/4 AA, 1/2 Aa and 1/4 aa. With complete dominance, the dominant phenotype has probability 3/4. This does not guarantee that exactly three out of every four offspring show it.
Try it yourself
- Find genotype probabilities for Aa × aa.
- What is the recessive phenotype probability in the original Aa × Aa model?
- Why should this model not be used to make simple predictions about a complex human trait?
Check your reasoning
- Two possible genotypes, Aa and aa, each with probability 1/2 under the model.
- 1/4, or 25%, for each offspring under the stated assumptions.
- Complex traits may involve many genes and environmental influences, so the single-gene complete-dominance assumptions may not apply.
Make it work for your learner
For support, model the example one step at a time and ask the learner to explain each choice in their own words. Use a sketch, organizer or physical model where helpful. For a challenge, create a new example, test an alternative explanation or describe when the model would not apply.
Exit check
Use a Punnett square to describe probabilities and explain the model’s limits. Explain your reasoning with a specific example before moving on.
Prepare · Discuss · Reflect
Use evidence in a conversation
Discussion question: What does a Punnett square predict, and what does it not guarantee for a small family?
- Before discussing, write a tentative answer with two details from the lesson’s passage, example or data. For a longer discussion, the teacher can add an age-appropriate source.
- In pairs, a group or a tutor–learner conversation, agree on roles: facilitator, evidence finder and note taker. Combine roles when needed and allow equal time to respond.
- Share evidence, ask a clarifying question and respond to a different interpretation. Explain which details support or challenge a claim.
- Paraphrase another person’s point. Write a final conclusion explaining what changed, what remains uncertain and what evidence you would seek next.
Teacher look-for: Accurate use of the example, reasons linked to evidence, thoughtful questions and a response to another viewpoint. A well-supported alternative answer is welcome.