Yakima WATERS Mini Lesson
Inherited Traits in Corn: Punnett Square and Chi Square
Targets and Assessment
WA Science Standards Addressed:
Science 9-12 INQB Inquiry investigation
Science 9-12 INQC Inquiry explain
Lesson Parameters
Content Area: Genetics
Overview: Students will use a chi-square test to compare
results of expected versus observed corn phenotypes for an
F2 generation. Students will also look for differences
between phenotypes on a cellular level.
Grade Level: 10
Assessments:
 Students will be graded based on completion of
worksheet.
Suggested Time: 110 minutes
Special Materials:
 4 week old Dwarf and Tall Corn Plants
Learning Outcomes:
Knowledge: Students will be able to relate the knowledge they gain about genetic inheritance to the
importance of genetic diversity in natural settings.
Skill: Students will be able to identify different corn phenotypes and compare their observed results to
the expected results they calculated using the Punnett Square. They will then be able to calculate
whether their observed results differ from their expected results using a chi-square goodness of fit
test.
Science Concept Background:
If the parents’ alleles are known for any given gene, a Punnett Square can be used to predict the
probability that their offspring will inherit specific genotypes. In this activity students will look at the F2
generation of corn which originated from the cross of a tall corn plant (TT) with a short corn plant (tt). In this
example, the F1 generation is all tall with the genotype (Tt). When two of these F1 plants are crossed, the
resulting offspring are known as the F2 generation and will have the following genotypes: TT, Tt, Tt, and tt.
Since the gene for a tall plant (T) is dominant, each offspring containing one (T) will be tall, which means
students can predict 75% of the individuals in this population will be tall.
Chi square statistics can be simplified into an easy calculation that students can use to determine if
there is a difference between their expected and observed corn populations. The expected population is
determined using a Punnett Square, while the observed population is determined by counting the plants
provided for this lab. If students follow the steps listed in the table in their worksheet they can compare it to
the critical value (x2=3.841) found by looking at a x2 probability table.
Materials:
 10 Trays of F2 generation Corn Plants (from the cross of Tall and Short Corn)
 Punnett Square and Chi Square Worksheet
 Calculators
Procedure:
Key questions:
What are observed versus expected results?
What is a Punnett Square and how can it be used to determine expected results?
Why is genetic diversity important in nature? How are short versus normal height peas an
example of genetic diversity?
1. Give a quick lecture to set the foundation for why genetics are important and to refresh students on
genetic terminology and how to make a Punnett Square.
2. Handout worksheet and go over basics of lab. Remind students to find expected corn phenotype ratios
for an F2 population (75% tall and 25% short; 3:1) before they write their alternate and null
hypotheses.
3. Lab involves each lab group counting the number of tall and short corn plants in their tray in order to
determine the observed ratio of tall to short corn plants. They will then use these numbers and the
directions on the worksheet to run a chi-square analysis to test their hypotheses.
4. Once students have struggled through the analysis on their own, show example to walk them through
it if they need extra help.
5. Have students make wet mounts of a thin layer of leaf cells to see if they can find any differences by
examining both tall and short plant leaves under the microscope. They can record their observations
on the lab handout or a separate piece of paper to turn in with the lab at the end of the period.
Extension(s):
 Leave time at the end of class for a group discussion about whether the chi-square test is a
good way to determine if observed and expected results are significantly different. This is a
good time to explain the purpose of statistics and see if students found differences between
their expected and observed plant populations.
Teaching Tips:
 Germinate F2 seeds from the original cross of a normal corn plant with a short corn plant in
trays several weeks prior to activity. This should result in trays of short and normal plants
interspersed within each tray.
 Brush up on how to run a chi-square analysis to be able to answer student questions.
Supplements:
 Google “short and tall corn genetics” to get links to lesson plans if more information is
necessary to understand experiment.
Author: Jennifer Lannoye, Yakima WATERS Project, CWU, 2009-2010