Lesson Plan Outline Punnett Squares and Probability
I. Students will be able to understand how to use Punnett Squares to determine the
probability of a specific trait in the offspring.
II. Grade 8
III. Biology - Genetics
IV. 42 minutes
Science Living Environment Standard 4 Students will understand and apply scientific
concepts, principles, and theories pertaining to the physical setting and living
environment and recognize the historical development of ideas in science.
Key Idea 2: Organisms inherit genetic information in a variety of ways that result in
continuity of structure and function between parents and offspring.
Math Standards: 8.PS Problem Solving; 8.N Number Sense and Operations
8.R Representation; 8.CM Communication
Teacher reviews that dominant trait is represented by the upper case letter and the
recessive trait is represented by the lower case letter. Review of terms genotype and
R: Round r: wrinkled
If a heterozygous round pea plant was crossed with a wrinkled plant what is the
probability that the offspring will be wrinkled? Use a Punnett Square to show your
Heterozygous means “different alleles” so Rr is the genotype for Round peas
The wrinkled peas have a genotype: rr
It doesn’t matter which space you use for the genotype as long as the alleles of that
parent stay together. For example
Now we fill in the Punnett Square. Upper Case always comes first, just like the
beginning of a sentence.
Write in the phenotypes (what we look like) in the space in each box:
R Round Round
Rr: 2 possibilities out of a total of 4
rr: 2 possibilities out of a total of 4
Now we know that the offspring have a 2 out of 4 chance of being wrinkled. In
other words, a 50% chance that the offspring will be wrinkled.
We also know that there is a 2 out of 4 chance that the offspring will be round; in
other words a 50% chance the offspring will be round.
There is a __50%___chance that the offspring will be round.
There is a __50%____chance that the offspring will be wrinkled.
VI. Guided Practice
If a homozygous round pea plant was crossed with a wrinkled pea plant what is the
probability that the offspring will be wrinkled?
1. Complete this by first writing the genotypes of the parents.
2. Then complete the genotypes for each possible combination.
3. Then write the phenotypes.
4. Then complete the Percent chance statements.
Completing the Genotypes for the offspring.
Now let’s fill in the box.
R Rr Rr
Completing the Phenotypes for the offspring
R Rr Rr
R Round Round
We found out that there is 0 out of 4 possible combinations that the offspring will
There is a ___100%_ chance that the offspring will be round.
There is a ____0%_chance that the offspring will be wrinkled.
VII. Independent Practice
A heterozygous round pea plant was crossed with a heterozygous round pea plant.
What is the probability that the offspring will be wrinkled peas?
There is a _______ chance that the offspring will be round.
There is a _______ chance that the offspring will be wrinkled.
Share your response with your table partner. Discuss any other possible
combinations for which you would like to create a Punnett square.
A man with curly hair (C ) married a woman with straight hair (c). One of their
children had straight hair. What do you know about the genotype of the man with
curly hair. Show your work using a Punnett Square.
There is a _______ percent chance that the offspring will have curly hair.
There is a _______ percent chance that the offspring will have straight hair.
In your notebook, write down one paragraph about what you learned during today’s
lesson. I learned that_______________. I am having difficulty in
understanding_____________. I want to learn more about__________________.
Include more details as you write.
Show the first generation and the second generation of the following pea plants:
Homozygous round x heterozygous round
Write down percent chance statements for round and wrinkled peas.
Now cross two of the offspring’s genotypes and show the next generation.
Write down percent chance statements for round and wrinkled peas for this Punnett
Use Punnett Squares to show your work and write down all genotypes, phenotypes
and corresponding percentages.