Introduction of Punnett Squares Lesson
Grade Level: 8th
Grade Life Science Class
Topic: Introduction of how genotypes and phenotypes can be determined through the use of
Objectives: 1. Students should become familiar with using a Punnett Square to determine
offspring genotypes and phenotypes.
2. Students should understand the differences between genotypes and phenotypes
3. Students should be able to calculate percents of different genotype combinations
from the use of a Punnett Square.
4. Students should recognize the terms of dominance and recessive as they are
concerned with the use of a Punnett Square.
PA Standard: 3.3.7 C. Know that every organism has a set of genetic instructions that
• Identify and explain inheritable
• Identify that the gene is the basic
unit of inheritance.
• Identify basic patterns of inheritance
(e.g., dominance, recessive, codominance).
Body of Lessons:
1. Characteristics of Genetics (5-8 minutes)
• The teacher will go over the “Principle of Genetics” part on the Learning Guide. (This
can be seen at the end of this lesson plan.
• Teacher will prompt students about past terms that were used today to see if they
understood terms such as dominant, recessive, incomplete dominance, etc.
2. History of Punnett Squares (5-10 minutes)
• The teacher will give background history on to who first invented the Punnett Square and
where it originated from.
• The teacher will also explain why we use the Punnett Square and why the students are
doing this lesson and this chapter.
• Students may ask questions about what they have learned so far before moving on to next
part of the lesson.
3. Introduction of Punnett Squares (30-35 minutes)
• Students will be introduced to how to use a Punnett Square through a series of small steps
that build upon each other.
• First, Students will be asked to describe how a dominant gene is written and how a
recessive gene is written (i.e. B and b). They will describe this through a prompt of
brown eyes being dominant over blue eyes.
• Then students will be asked how many different combinations of two letters can be made
through the two letters of B and b. This will show them the homozygous dominant
genotype BB, the heterozygous genotype Bb, and the homozygous recessive genotype of
• Students will then be prompted as to as to which eye color will be seen with each
• Next, an example of a Punnett square will be given to students. This will not be a word
problem but a cross between BB and bb genotypes. From the Punnett Square, students
will have to determine the number of different offspring genotype combinations. Students
will also then calculate the percent of the genotype offspring combinations. This entire
example will be gone over with the teacher prompting the students along the way on how
to fill in the Punnett Square and how to calculate the percents. This example will be
written down on the back of the Learning Guide for this chapter. (See example of
Learning Guide at end of this lesson plan).
• A second example will then be gone over in class very similar to the first example. If a
variety of students do well on the first example this may be given as independent work
then gone over in class. However, if the first example did not go well this will also be
gone over slowly and efficiently so students understand how to use a Punnett Square.
This example will also be placed on the Learning Guide.
• A third example will be given and students will work on this independently and will then
be gone over in class. Terms such as homozygous dominant, heterozygous, and
homozygous recessive may be introduced if the class as a whole seems to understand
material. If this is not the case, these terms will be introduced next class. The terms
genotype and phenotype will be introduced as terminology used in a Punnett Square.
This example will be given to the students in a form of a worksheet if students understand
material well. It will be collected at the end of this class, looked at by the teacher, and
handed back. No grade will be associated with this worksheet. (See this at the end of the
• The teacher will conclude the lesson by telling students that tomorrow they will be using
more terminology associated with the Punnett Square and will be introduced to ratios as
TEST CROSS for Example 3
Bb X Bb
How many BB are there? __________
How many Bb are there? __________
How many bb are there? ___________
What percent of BB are there? ___________
What percent of Bb are there? ____________
What percent of bb are there? ____________
Genotypes of Parents _________________
Genotypes of Offspring _________________
Phenotypes of Parents __________________
Phenotypes of Offspring ________________