How to:
Punnett Squares
Biology 103
Punnett Squares:
- determine the
probability of
the possible
genotypes and
phenotypes of
two organisms
offspring
Words to Know
• Homozygous: 2 of the same alleles for
the same trait
• Heterozygous: 2 different alleles for
the same trait
• Dominant: the gene that is expressed;
capital letter
• Recessive: the gene that is masked;
lower-case letter
Example: In humans brown eyes
are dominant to blue eyes.
1. Homozygous Dominant
BB (same capital letter)
2. Heterozygous Dominant
Bb (one capital and one lowercase letter [different])
3. Homozygous Recessive
bb (same lower-case letter)
YOU DO IT!!
Q. If black fur is dominant to white fur,
what are the possible genotypes for a
black furred rabbit?
A. BB for
homozygous dominant
and Bb for
heterozygous
dominant
YOU DO IT!!
Q. If long tails are dominant to short tails
in squirrels the what would the genotype
for a short tailed squirrel be?
A. tt for
homozygous
recessive
Monohybrid Cross
A monohybrid cross is a
cross of only one trait.
It is the simplest and
fastest.
Example 1:
If brown eyes are dominant
to blue eyes and a
homozygous brown eyed male
mates with a blue eyed
female, what are the possible
genotypes and phenotypes of
the offspring?
Step 1:
Perform the cross of the
parents.
BB X bb
Step 2:
Draw a square with four
boxes in it.
Step 3:
Figure and
place the
Parents
genotypes.
Other
Parent
Mom or Dad
B
b
b
B
Step 4:
Perform the
cross and write
down the
possible
phenotypes of
the offspring
b
b
B
B
Step 4:
Perform the
cross and write
down the
possible
phenotypes of
the offspring
All offspring are
heterozygous
dominant for
brown eyes
B B
b Bb Bb
b Bb Bb
YOU DO IT!!
Q. In horses, brown fur is
dominant to chestnut color fur.
What would the possible
genotypes and phenotypes be
for the offspring if a two
heterozygous brown furred
horses mated?
A.
1-homozygous brown furred
2-heterozygous brown furred
1-chestnut
B b
B BB Bb
b Bb bb
Dihybrid Cross
A dihybrid cross is a cross
between two organisms that
differ in two traits instead of
just one.
Ex. A homozygous tall/homozygous green
Example 2
If tall is dominant to short and purple is
dominant to white, what would the
phenotypes of the offspring be if a
heterozygous tall/heterozygous purple
plant is crossed with a heterozygous
tall/heterozygous purple plant?
Step 1: Write down all given information
2 Traits
Height~Tall=dominant
~short=recessive
Color~purple=dominant
~white=recessive
So our cross would
be…
TtPp X TtPp
This tells me …
Homozygous tall = TT
Heterozygous tall = Tt
Short (must be homozygous) = tt
Homozygous purple = PP
Heterozygous purple = Pp
White (must be homozygous) = pp
Hints
1. Look for the “with” in the problem.
~this is where the cross separates the two parents
If a heterozygous tall/heterozygous purple
plant is crossed with a heterozygous
tall/heterozygous purple plant?
Parent #1
TtPp X TtPp
Parent #2
Step 2: Separate the Gametes
TtPp X TtPp
TP
~take each parent individually and produce 4 gametes
~Each gamete must contain a “t” or “T” and a “P” or “p”
Hint: Use the “FOIL” method!!!
Step 2: Separate the Gametes
TtPp X TtPp
TP
Tp
~take each parent individually and produce 4 gametes
~Each gamete must contain a “t” or “T” and a “P” or “p”
Hint: Use the “FOIL” method!!!
Step 2: Separate the Gametes
TtPp X TtPp
TP
Tp
tP
~take each parent individually and produce 4 gametes
~Each gamete must contain a “t” or “T” and a “P” or “p”
Hint: Use the “FOIL” method!!!
Step 2: Separate the Gametes
TtPp X TtPp
TP
Tp
tP
tp
~take each parent individually and produce 4 gametes
~Each gamete must contain a “t” or “T” and a “P” or “p”
Hint: Use the “FOIL” method!!!
Step 3: Draw a 16 square punnett square
Step 4: Place gametes on top and side of
square
TP
TP
Tp
tP
tp
Tp
tP
tp
Step 5: Perform the crosses
TP
Tp
tP
tp
TP TTPP TTPp TtPP TtPp
Tp TTPp TTpp TtPp Ttpp
tP TtPP TtPp ttPP ttPp
tp TtPp Ttpp ttPp ttpp
Step 6: Count your offspring
Tall Purple IIIIIIIII
Tall White III
Short Purple III TP
Short White I
Tp
9:3:3:1
tP
tp
TP TTPP TTPp TtPP TtPp
Tp TTPp TTpp TtPp Ttpp
tP TtPP TtPp ttPP ttPp
tp TtPp Ttpp ttPp ttpp
YOU DO IT!!
In pea plants, green pod
color is dominant to
yellow pod color and
tallness is dominant to
shortness.
You cross a short/yellow
pea plant with a
homozygous
tall/heterozygous green
plant.
What are the offspring
phenotype and genotype
possibilities?
Q.
A.
Tall Green IIIIIIII
Tall Yellow IIIIIIII
1:1
or
4:4
or
8:8
tg
Ttgg X TTGg
tg
tg
tg
TG TtGg TtGg TtGg TtGg
Tg Ttgg Ttgg Ttgg Ttgg
TG TtGg TtGg TtGg TtGg
Tg Ttgg Ttgg Ttgg Ttgg