Grade 11 Biology
Unit: Genetic Processes
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Fun with Punnett Squares Lesson B6
A monohybrid cross is a cross of two individuals that differ by one trait.
The Punnett square is a diagram that is used to predict the likelihood of inheriting a
particular outcome of a particular cross.
In this activity you will complete a series of monohybrid crosses using the Punnett
square method. An example is provided for you below.
Example:
In pea plants green is dominant over yellow.
A homozygous green pea plant (GG) is being crossed with a heterozygous green pea plant (Gg).
Step 1: Start with a “let” statement to define the letters that you will use to represent the
dominant and the recessive traits. As a general convention, dominant traits are represented
by uppercase and recessive traits are represented by lowercase.
Let G represent the allele for green pea plants and
Let g represent the allele for yellow pea plants.
Step 2: Place the genotype of the parents on the top row and left column of the Punnett
square. Match the letters on the top row with the ones on the left column to determine the
genotype of the offsprings.
G
G
G
GG
GG
g
Gg
Gg
Step 3: Analyze the genotype of the offspring to determine their phenotypes.
Recall that genotype represents the dominant and recessive alleles and phenotype represents
the physical characteristics of the trait. Three ways to present data from a cross are
using the number of each genotype and phenytype, ratios and percentages.
Genotype: 2 GG: 2 Gg ; 0 gg (Ratio 2:2)
Phenotype: 4 green; 0 yellow (Ratio 4:0)
50% GG 50% Gg
100% green 0% yellow
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