Using Punnett Squares to Predict the Outcomes of Crosses
The possible gene combinations in the offspring that result from a genetic cross can
be determined by drawing a diagram known as a Punnett square. A Punnet square
shows the genes (represented by letters) find the parents' gametes along the top and
left-hand side of a square and the possible gene combinations in the offspring, within
the square. A completed Punnett square gives the probable outcome of a given cross.
However, actual results may vary from the probable results, especially if only a few
organisms are considered. ONE-FACTOR CROSSES -Crosses that involve one trait,
such as seed coat color, are called one-factor crosses. For the one-factor crosses in
this activity, we will use some of the traits Mendel observed in pea plants. The
expressions of the dominant and recessive· alleles for the genes controlling these
traits are described in the following chart The chart also assigns letters to represent
the different alleles.
Trait
Dominant
Allele
Recessive Allele
Pod. shape
Smooth (N)
Constricted (n)
Pod color
Yellow (g)
Flower position
Green (G)
Axial (A)
Terminal (a)
Plant height
Tall (T)
Short (t)
Sample Problem: A plant that is heterozygous for green pods is crossed with a plant
that has yellow pods. What are the probable genotypic and phenotypic ratios in the
offspring resulting from this cross?
Step 1: Choose a letter to represent the alleles in the cross and write the genotypes of
the parents. In this case, the letters have already been selected- G for the dominant
green allele and g for the recessive yellow allele. Since the plant with the green pods
is heterozygous for the trait, its genotype must be Gg. The problem does not state
whether the plant with yellow pods is homozygous or heterozygous. But we know
that yellow pods are a recessive character, and that recessive characters are expressed
only in a homozygous recessive individual. Thus the genotype of this plant must be
gg. The cross: therefore, is Gg X gg.
Step 2: Identify the possible gametes that each plant can form and enter them on the
top and side of the Punnett Square. Complete the Punnett Square by writing the
alleles that would come together during fertilization.
G
g
g
g
Gg
gg
Gg
gg
Step 3: Determine the genotype and phenotype of the offspring. Since green (G) is
dominant to yellow (g) only plants with a G would be green and plants with the
genotype gg would be yellow.
Phenotype: ½ of the offspring would be yellow and ½ would be green
Genotype: ½ would be Gg and ½ would be gg
Complete the problems below using this example of a monohybrid cross or one factor
cross.
Using P
1) A blue-eyed (bb) woman marries a brown-eyed (Bb) man. Draw a punnett square that shows the
possibilities for children. Write the egg on left side of the punnett square(s). Write the sperm on top.
What are the possible genotypes? Phenotypes? Write them as a percent or fraction.
(bb) x (Bb)
Meow
2) A small
(hh) mates with a Huge cat (HH). Draw a punnett square that shows the possibilities
for kittens. What are the possible genotypes and phenotypes? (hh) x (HH)
The possib
be determi
shows the
left-hand s
the square
However,
organisms
such as se
this activit
expression
traits are d
the differe
T
Pod
Pod
Flower
Plan
Sample Pr
that has ye
offspring r
3) Woody Guthrie died of Huntington's chorea, a dominant autosomal disease. His wife's genotype
was “hh” and his was “Hh”. Draw a punnett square. What is the probability that his children may
have the disease. (hh) x (Hh)
Step 1: Ch
the parents
green allel
is heterozy
whether th
that yellow
only in a h
gg. The cr
Step 2: Ide
top and sid
alleles tha
4) Two carriers (Nn) of the nasty disease, Tay-Sachs, marry and have children. To have the disease a
person must have this genotype: “nn”. Use a punnett square to show the possible genetic outcomes of
the children. (Nn) x (Nn)
5) Using the chart on page one for the pea plant characteristics, answer the questions below:
a. What is the genotypic ratio and phenotypic ratio of the offspring when you cross a homozygous
smooth pea plant (NN) with a heterozygous smooth pea plant (Nn)?
b. What is the genotypic and phenotypic ratio for the offspring when you cross a heterozygous axial
flowering plant with a homozygous terminal flowering plant. Hint: find the genotype of the parents
first!
c. Cross two heterozygous pea plants together. What are the phenotypic and genotypic ratios of the
offspring?
d. Cross two plants that are heterozygous for green pods. What are the phenotypic and genotypic
ratios of the offspring?
e. Cross a homozygous tall plant with a short plant. What are the phenotypic and genotypic ratios of
the offspring.
f. When a tall plant is crossed with a short plant, some of the offspring are short. What are the
genotypes of the parent plants?
g. A cross between two plants yields ¾ of the offspring as tall and ¼ as short. What are the
genotypes of the parents?
h. What cross would result in half the offspring being yellow and half green?
6) In roses red is dominant to white. Cross two heterzygous red plants. What are the genotypes and
phenotypes of the offspring?
7) Using the information about roses in problem 6, cross a homozygous red rose with a white rose.
What would be the genotypes and phenotypes of the offspring?
8) Using the information about roses in problem 6, cross two heterozygous roses together. What
would be the phenotype and genotype of the offspring?
9) Using the information about roses in problem 6, cross two white roses together. What are the
phenotypic and genotypic ratios of the offspring?