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Grade 7 · Science · 35 minutes

Unit 1 · Living Connections · Lesson 2 of 2

What a Punnett Model Predicts

Use a simple inheritance model while distinguishing probabilities from guaranteed family outcomes.

Mr. S Teaches · Original starter mini-lesson · MS7-SCI-02

Learning objective

Build a four-cell model for a one-gene cross and explain one assumption and one limit.

Before you begin

  • Use fractions and distinguish a model from an observation.

Gather your materials

  • Paper or notebook
  • Pencil and the passage, model directions or data printed in this lesson

Language to know

allele
A version of a gene in this simplified model.
genotype
The combination of alleles represented for an organism.
probability
A measure of how likely an outcome is, not a promise.

Understand the concept

Model instructions: For an invented plant trait controlled by one gene, P is dominant to p. Each parent in a Pp × Pp cross can pass P or p with equal probability. Make a two-by-two table with P and p on each side, then combine one letter from each parent in each cell. PP and Pp express the modeled dominant trait; pp expresses the recessive trait. Many real traits involve several genes and environmental influences, so this one-gene model is deliberately limited.

Teach it step by step

  1. Follow the written table instructions, labeling possible contributions from each parent.
  2. Fill the four cells without treating a cell as one specific future child.
  3. Count genotype outcomes and connect those counts to the model’s probability statements.
  4. Read the final sentence and write why a real trait may not follow the simple pattern.

Make the thinking visible

Worked example or model response

The cells are PP, Pp, Pp and pp. Under the assumptions, genotype probabilities are 1/4, 1/2 and 1/4. The dominant modeled appearance has probability 3/4, but a family of four offspring is not guaranteed to contain exactly three with that appearance.

Guided practice

Together make a Pp × pp table and explain why the second parent contributes only p.

Independent practice

Independently build a PP × pp table. Compare its predicted genotypes with Pp × Pp, and write one sentence explaining why “dominant” does not mean common or better.

Quick assessment

For Pp × Pp, does a first pp offspring change the modeled probability for the next independent offspring?

Answer key & assessment criteria

Answers and teaching guidance

  1. Guided: Pp × pp gives Pp, pp, Pp, pp, so each modeled appearance has probability 1/2.
  2. Independent: PP × pp gives all Pp in the simple model. “Dominant” describes expression in a heterozygote, not frequency or value.
  3. Quick check: No. Assuming independent inheritance under the same model, the next offspring still has probability 1/4 of pp.

Look for: Look for a claim that answers this lesson’s question, accurate use of the supplied evidence and an explanation of a limit or alternative. Ask the learner to point to the detail supporting each inference.

Make it work for your learner

More support

Use two differently colored allele cards and complete one row before the other.

Extension challenge

Compare predicted and simulated proportions after 4, 20 and 100 paper-card trials; discuss sample variation without claiming larger samples force exact ratios.

Selected contributions · Official sources

Standards connections

Selected NGSS band contribution; not complete performance expectation coverage.

Contributes to NGSS performance expectation

MS-LS3-2

NGSS · Grades 6–8 band

Model how reproduction affects inherited similarities and differences.

Read the official performance expectation ↗

This selected sexual-reproduction model practices allele transmission and variation. Asexual reproduction and the wider genetic complexity of real traits require additional lessons.

Source and mapping checked 2026-11-04. This mini-lesson contributes selected skills; it does not claim full attainment.

Supporting Common Core literacy connection

RST.6-8.3

Common Core · Grades 6–8

Follow a written sequence for a technical task.

Read the official standard ↗

The provided scientific passage/model is explicitly analyzed in the activity. This selected literacy practice does not establish subject-content alignment or complete coverage of the standard.

Source and mapping checked 2026-11-04. This mini-lesson contributes selected skills; it does not claim full attainment.