Beacon Lesson Plan Library

Dare to be [Punnett] Square

Dale Peterson


This lesson familiarizes the students with Punnett squares, specifically: purpose, application and interpretation. Key terms from previous lessons (included below) are reviewed/reinforced before data is applied to a Punnett square and interpreted.


The student uses a Punnett square to predict the results of crosses between pure and hybrid organisms.


- Science Textbook
- Pennies
- Dry Erase Board with dry erase markers
- Chart paper (approx. 30-x36-), with markers
- Overhead projector with appropriate pens
- Key terms flash cards for traits, alleles, dominant, homozygous, heterozygous, phenotype, genotype


1. Ensure each student has a textbook.
2. Make flash cards for key terms.
3. Ensure dry erase markers and pens are working.
4. Ensure chart paper is available.
5. Gather at least one penny for every two students.


1. Ask eight volunteer students to hold “key term” flashcards and eight students to hold “definition” flashcards. One or two volunteer students will match the definitions with the correct key terms.

2. Ask the students to identify genetic traits they see among their classmates. A volunteer student should list the traits on the dry erase board, or on chart paper. Inform the students that before this lesson is finished, they will be able to predict the occurrence of various traits.

3. Define “randomness”. Ask one student to find the term “random” or “randomness” in the textbook and another student to find the same terms in the dictionary. Read the definition(s) and write a synopsis definition on chart paper.

4. Reinforce the meaning of “random.”

a) Divide the students into groups of two.
b) Provide each group with a penny.
c) Instruct each team, not each student to toss the coin 50 times, record each result as “heads” or “tails” using hash marks.
d) Write the team results on the dry erase board.
e) Remind the students that random events occur by chance; point out that results of the coin toss exercise will approach 50:50 as the numbers of tosses increase.

5. Pose the question, “Do traits occur randomly?”

6. Review the previous listing of student traits; record the number of students having the trait as a ratio of the total number of students in the classroom.

7. Define and apply the tool, Punnett square.

a) Read, from the textbook, the paragraphs about Punnett squares.
b) Call the students’ attention to the examples which show Punnett squares.
c) Reinforce key terms with respect to Punnett squares.
d) Practice using Punnett squares using simple examples … keep the students in pairs.
TT x tt
Tt x Tt
Tt x tt

e) Ask volunteers to write their solutions on the board.
f) Provide feedback, applying appropriate key terms.
g) Discuss possible phenotypes with the resulting genotypes.
h) Write the genotypes and possible phenotypes as ratios.

8. Formatively assess student understanding by providing a data set of genotypes, and instruct the students (still working in pairs) to determine the resultant offsprings' genotypes and possible phenotypes. Report the genotypes using ratios.

a) pod color green(G) yellow(g)
b) plant height tall(T) short(t)
c) fruit color brown(B) white(b)

1. Cross a homozygous green plant with a heterozygous green plant
2. Cross a homozygous tall plant with a short plant
3. Cross a white seed plant with a heterozygous brown seed plant


Formatively assess Punnett square applications, as they are completed during class. Student volunteers provide solutions by writing them on the board , we discuss the mechanics of filling in a Punnett square, evaluate the solutions - the correct gene pairs and phenotype labels. Include many -Why- questions to help reinforce learning.

Summative assessment will occur during the chapter test; summative assessment may include matching (key terms), compare and contrast, multiple choice questions as well as completing a Punnett square and interpreting the results using the terms genotype and phenotype

Web Links

Web supplement for Dare to be [Punnett] Square
An opportunity to practice filling in Punnett squares

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