Unit Plan 23 (Grade 7 Science): Probability & Punnett Squares
Grade 7 unit using Punnett squares and probability to model inheritance, showing how sexual reproduction creates genetic variation while asexual copying does not.
Focus: Use mathematical models (Punnett squares and probability) to predict inheritance patterns and explain how sexual reproduction generates genetic variation compared with asexual reproduction.
Grade Level: 7
Subject Area: Science (Life Science — Inheritance & Variation; Quantitative Reasoning)
Total Unit Duration: 5 sessions (one week), 50–60 minutes per session
I. Introduction
In this unit, students connect inheritance to mathematical probability. Building on prior work with genetic variation and asexual vs. sexual reproduction, they learn how Punnett squares model the random joining of genes from two parents. Students compare a “copy-only” model of asexual reproduction (no new combinations) with Punnett-square models of sexual reproduction where offspring show different genotypes and phenotypes. Throughout the week, they interpret ratios, fractions, and percentages of predicted offspring and explain how these patterns relate to variation in real populations, aligned with MS-LS3-2.
Essential Questions
- How can probability and Punnett squares help us predict inheritance patterns in sexually reproducing organisms?
- Why does asexual reproduction tend to produce identical offspring, while sexual reproduction leads to variation in traits?
- What is the difference between genotype and phenotype, and how do Punnett squares help us model both?
- Why do predicted ratios from Punnett squares sometimes differ from what we actually see in a small group of offspring?
II. Objectives and Standards
Learning Objectives — Students will be able to:
- Define genotype, phenotype, dominant, recessive, and probability in the context of inheritance.
- Use Punnett squares as models of sexual reproduction to predict possible genotypes and phenotypes from a cross between two parents.
- Calculate and interpret fractions, ratios, and percentages of predicted offspring with certain traits from simple monohybrid crosses.
- Compare a “copy-only” asexual model (identical genetic information) with Punnett square models (mixed genetic information) to explain why sexual reproduction leads to variation while asexual reproduction does not.
- Create a final Inheritance & Probability Model & Explanation that uses Punnett squares to show how sexual reproduction generates multiple possible outcomes, aligned with MS-LS3-2.
Standards Alignment — 7th Grade (NGSS-based custom)
- MS-LS3-2 — Develop and use a model to describe why asexual reproduction results in offspring with identical genetic information while sexual reproduction results in offspring with genetic variation.
- Here, the “copy-only” card model represents asexual reproduction; Punnett squares represent mixing of genes in sexual reproduction and resulting variation.
Success Criteria — Student Language
- I can explain the difference between genotype (gene pair) and phenotype (observed trait).
- I can fill in a Punnett square for a simple cross and list all possible offspring genotypes and phenotypes.
- I can use fractions, ratios, or percentages to describe how likely a trait is to appear in offspring.
- I can explain, using models, why asexual reproduction produces identical genetic information and sexual reproduction leads to variation.
- I can create a final diagram and explanation that clearly connects Punnett squares, probability, and variation in offspring.