What a Punnett Square Can
Do for You
1.
WHAT ARE THE FACTORS THAT DETERMINE THE
EXPRESSION OF TRAITS?
2.
HOW CAN INHERITANCE BE MODELED?
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Punnett Square Article Scenario
 You
are a gardener and you were given two pea
plants. The person who gave them to you said that
one plant has the genotype Gg and the other plant has
a genotype of gg. You decide to plant them close
together to make sure that they will breed with each
other and produce offspring. So . . .
 How
can inheritance be modeled?
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 1.
The trait being studied in this scenario is pea
color.
 It
takes two alleles to determine some traits,
like pea color.
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 An
allele is a different form of the same gene.
A
dominant allele is a hereditary unit that
blocks the expression of another allele.
 2a.
The dominant allele expresses for the color
green and is represented by the capital letter
G.
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A
recessive allele is a hereditary unit that is
blocked by a dominant allele.
 2b.
The recessive allele expresses for the color
yellow and is represented by the lower case
letter g.
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Your Punnett
Square Should
Look Like This
•
•
The alleles of one parent’s
genes (mother) are
separated into gametes
during meiosis. The
possible gametes are
written on the top side.
The alleles of the other
parent’s genes (father) are
separated into gametes
during meiosis. The
possible gametes are
written along the left side.
G
g
g
g
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Your Punnett
Square Should
Look Like This
•
A genotype is the
alleles that contain the
DNA that expresses for a
particular trait.
•
3. The mother’s
genotype is Gg.
•
4. The father’s genotype
is gg.
G
g
g
g
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Your Punnett
Square Should
Look Like This
•
•
When an organism has
two different alleles for
a trait, it is called
heterozygous.
3. In this case, the
genotype Gg is said to
be heterozygous.
G
g
g
g
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Your Punnett
Square Should
Look Like This
•
•
When an organism has
two of the same allele,
it is called
homozygous.
4. In this case, the
genotype gg is
homozygous recessive.
G
g
g
g
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Your Punnett
Square Should
Look Like This
•
A phenotype is the
observable expression of
a trait.
•
The genotype controls the
phenotype.
•
5. The mother’s
phenotype is green peas.
•
6. The father’s phenotype
is yellow peas.
G
g
g
g
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Your Punnett
Square Should
Look Like This
•
•
Each offspring receives
one allele from the mother
and one allele from the
father.
When using a Punnett
Square NEVER move
gametes in diagonal lines.
Always move gametes
vertically or horizontally.
G
g
g Gg
g
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Your Punnett
Square Should
Look Like This
•
•
The first offspring received
a dominant allele from
mom and a recessive
allele from dad.
This offspring’s genotype
is Gg; therefore, its
phenotype is green
colored peas.
G
g
g Gg
g
©Copyright 2014 by www.cpalms.org
Your Punnett
Square Should
Look Like This
•
•
The second offspring
received a recessive
allele from mom and a
recessive allele from dad.
This offspring’s genotype
is gg; therefore, its
phenotype is yellow
colored peas.
G
g
g Gg gg
g
©Copyright 2014 by www.cpalms.org
Your Punnett
Square Should
Look Like This
•
•
The third offspring
received a dominant
allele from mom and a
recessive allele from dad.
This offspring’s genotype
is Gg; therefore, its
phenotype is green
colored peas.
G
g Gg
g
gg
g Gg
©Copyright 2014 by www.cpalms.org
Your Punnett
Square Should
Look Like This
•
•
The fourth offspring
received a recessive
allele from mom and a
recessive allele from dad.
This offspring’s genotype
is gg; therefore, its
phenotype is yellow
colored peas.
G
g
g Gg gg
g Gg gg
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Your Punnett
Square Should
Look Like This
7.
G
g
g Gg gg
g Gg gg
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Figuring
Probability
8. The possible
genotypes for the
offspring in this cross
are:
Gg = heterozygous
gg = homozygous
recessive
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Figuring
Probability
•
There is a 50%, or one
in two, chance that the
mother will pass on the
first allele G.
•
There is a 50%, or one
in two, chance that the
mother will pass on the
second allele g.
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G g
G 1/2
g
1/2
g
Gg
gg
g Gg gg
1/2
g
Gg
gg
g Gg gg
1/2
Figuring
Probability
•
There is a 50%, or one
in two chance, that
the father will pass on
the first allele g.
•
There is a 50%, or one
in two chance, that
the father will pass on
the second allele g.
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G g
G 1/2
g
1/2
g
Gg
gg
g Gg gg
1/2
g
Gg
gg
g Gg gg
1/2
Figuring
Probability
•
•
This means that each
offspring has a 25%,
or one in four, chance
to inherit a particular
allele combination
from mom and dad.
½x½=¼
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g
1/2
g
1/2
G
1/2
Gg
1/4
Gg
1/4
g
1/2
gg
1/4
gg
1/4
Figuring
Probability
•
•
9. The probability
that the offspring
will inherit the
genotype Gg is
50%, or two in four,
chance.
¼+¼=½
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g
1/2
g
1/2
G
1/2
Gg
1/4
Gg
1/4
g
1/2
gg
1/4
gg
1/4
Figuring
Probability
•
•
10. The probability
that the offspring
will inherit the
genotype gg is
50%, or two in four,
chance.
¼+¼=½
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g
1/2
g
1/2
G
1/2
Gg
1/4
Gg
1/4
g
1/2
gg
1/4
gg
1/4
Figuring
Probability
11. The possible
phenotypes for the
offspring in this cross are:
•
Green Peas = (controlled
by the genotype Gg)
•
Yellow Peas =
(controlled by the
genotyhpe gg)
©Copyright 2014 by www.cpalms.org
Figuring
Probability
•
•
12. The probability
that the offspring
will inherit the
phenotype of green
pea color is 50%, or
two in four, chance.
¼+¼=½
©Copyright 2014 by www.cpalms.org
g
1/2
g
1/2
G
1/2
Gg
1/4
Gg
1/4
g
1/2
gg
1/4
gg
1/4
Figuring
Probability
13. The probability
that the offspring
will inherit the
phenotype of
yellow pea color is
50%, or two in four,
chance.
• ¼+¼=½
•
©Copyright 2014 by www.cpalms.org
g
1/2
g
1/2
G
1/2
Gg
1/4
Gg
1/4
g
1/2
gg
1/4
gg
1/4
Ratios
•
•
The ratio of genotypes is
shown like this (Gg : gg)
14. So, the genotype
ratio would be 2 : 2,
which is reduced to 1 : 1.
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g
1/2
g
1/2
G
1/2
Gg
1/4
Gg
1/4
g
1/2
gg
1/4
gg
1/4
Ratios
•
•
The ratio of
phenotypes is
shown like this
(Green : yellow)
15. So, the
phenotype ratio
would be also 2 : 2,
which is reduced to
1 : 1.
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g
1/2
g
1/2
G
1/2
Gg
1/4
Gg
1/4
g
1/2
gg
1/4
gg
1/4
Exit Ticket
1.
How do you use a Punnett Square?
©Copyright 2014 by www.cpalms.org
Guided Practice Problem #1
 Tongue rolling is a genetic trait. The ability
to roll the tongue is expressed by a
dominant trait. Suppose you have a friend
whose parents are heterozygous for rolling
their tongues. Answer the following
questions about the parents and possible
offspring.
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1. What is the trait being studied in
this scenario?
 The
ability to roll the tongue is the trait
being studied in this scenario.
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2. Indicate the dominant and recessive
alleles and assign an appropriate letter
to each allele:
 A.
The dominant allele is a rolling tongue.
We will use the letter R.
 B.
The recessive allele is not a rolling
tongue. We will use the letter r.
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3. What is the genotype of Parent #1
(mother)?
 Genotype
shown as letters: Rr
 Scientific
term for this genotype:
Heterozygous
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4. What is the genotype of Parent #2
(father)?
 Genotype
shown as letters: Rr
 Scientific
term for this genotype:
Heterozygous
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5. What is the phenotype of Parent #1
(mother)?
 The
phenotype of the mother is a rolling
tongue.
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6. What is the phenotype of Parent #2
(father)?
 The
phenotype of the father is a rolling
tongue.
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7. Draw a Punnett Square to represent
the offspring the parents above are likely
to have.
R
r
R
RR
Rr
r
Rr
rr
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8. What are the possible genotypes of
the offspring?
 The
possible genotypes of the offspring are:
RR
Rr
rr
= Homozygous dominant
= Heterozygous
= Homozygous recessive
©Copyright 2014 by www.cpalms.org
9. What are the possible phenotypes of
the offspring?
 The
possible phenotypes of the
offspring are:
Rolling
No
Tongue (controlled by the genotype RR or Rr)
Rolling Tongue (controlled by the genotype rr)
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10. What is the probability that the offspring
will have a homozygous dominant genotype?
 The
probability of having an offspring with
a homozygous dominant genotype is ¼ or
25%.
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11. What is the probability that the offspring
will have a heterozygous genotype?
 The
probability of an offspring inheriting a
heterozygous genotype is ½ or 50%.
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12. What is the probability that the offspring
will have a homozygous recessive genotype?
 The
probability that an offspring will have a
homozygous recessive genotype will be ¼
or 25%.
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13. What is the genotype ratio for the
possible offspring?
 The
genotype ratio set-up is (RR : Rr : rr).
 The
genotype ratio would be 1 : 2 : 1.
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14. What is the probability that the offspring
will have a phenotype of a rolling tongue?
 The
probability of having an offspring with
a rolling tongue phenotype is ¾ or 75%.
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15. What is the probability that the offspring
will have a phenotype of a non-rolling tongue?
 The
probability of an offspring inheriting a
non-rolling tongue phenotype is ¼ or 25%.
©Copyright 2014 by www.cpalms.org
16. What is the phenotype ratio for the
possible offspring?
 The
phenotype ratio set-up is
(rolling tongue : non-rolling tongue).
 The
phenotype ratio would be 3 : 1.
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17. What is the probability that your friend
will be able to roll his/her tongue?
 There
is a ¾ or 75% chance that my friend
will be able to roll his/her tongue.
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Guided Practice Problem #2
 Complete
the following questions if
you bought a pair of pea plants on
sale at the local nursery. The clerk at
the store told you that the tall
stemmed pea plant’s genotype is
homozygous dominant. The other
plant’s genotype is homozygous
recessive and has a short stems.
©Copyright 2014 by www.cpalms.org
1. What is the trait being studied in
this scenario?
 The
height of the plant (stem length) is
the trait being studied in this scenario.
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2. Indicate the dominant and recessive
genes and assign an appropriate letter
to each allele:
 A.
The dominant gene is a tall height. We
will use the letter T.
 B.
The recessive allele is a short height.
We will use the letter t.
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3. What is the genotype of Parent #1
(mother)?
 Genotype
shown as letters: TT
 Scientific
term for this genotype:
Homozygous dominant
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4. What is the genotype of Parent #2
(father)?
 Genotype
shown as letters: tt
 Scientific
term for this genotype:
Homozygous recessive
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5. What is the phenotype of Parent #1
(mother)?
 The
phenotype of the mother is a tall
height.
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6. What is the phenotype of Parent #2
(father)?
 The
phenotype of the father is a short
height.
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7. Draw a Punnett Square to represent
the offspring the parents above are likely
to have.
T
T
t
Tt
Tt
t
Tt
Tt
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8. What are the possible genotypes of
the offspring?
 The
is:
Tt
only possible genotype of the offspring
= Heterozygous
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9. What are the possible phenotypes of
the offspring?
 The
possible phenotypes of the
offspring are:
Tall
Height (controlled by the genotype Rr)
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10. What is the probability that the offspring
will have a homozygous dominant genotype?
 The
probability of having an offspring with
a homozygous dominant genotype is 0/4
or 0%.
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11. What is the probability that the offspring
will have a heterozygous genotype?
 The
probability of an offspring inheriting a
heterozygous genotype is 4/4 or 100%.
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12. What is the probability that the offspring
will have a homozygous recessive genotype?
 The
probability that an offspring will have a
homozygous recessive genotype will be
0/4 or 0%.
©Copyright 2014 by www.cpalms.org
13. What is the genotype ratio for the
possible offspring?
 The
genotype ratio set-up is (Tt).
 The
genotype ratio would be 1 for the
heterozygous genotype.
 In
other words, all of them.
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14. What is the probability that the offspring
will have a phenotype of a tall height?
 The
probability of having an offspring with
a tall height phenotype is 4/4 or 100%.
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15. What is the probability that the offspring
will have a phenotype of a short height?
 The
probability of an offspring inheriting a
short height phenotype is 0/4 or 0%.
©Copyright 2014 by www.cpalms.org
16. What is the phenotype ratio for the
possible offspring?
 The
phenotype ratio set-up is
(tall height : short height).
 The
 In
phenotype ratio would be 1 for tall height.
other words, all of them.
©Copyright 2014 by www.cpalms.org
17. If you wanted to produce more short pea plants
rather than tall pea plants, would you be able to
with these two plants? Explain why on the back of
this sheet of paper.
 Answer
1—If you answered no.
I
would not be able to produce short pea plants with this
pair of plants because one parent is homozygous
dominant and can only pass on the dominant get to their
offspring. This means that every offspring produced will
have at least one dominant allele. So, all offspring will by
tall.
©Copyright 2014 by www.cpalms.org
17. If you wanted to produce more short pea plants
rather than tall pea plants, would you be able to
with these two plants? Explain why on the back of
this sheet of paper.
 Answer
2—If you answered yes.
I
would not be able to produce short pea plants in this
generation; however, if I crossed two of the heterozygous
offspring, I would be able to produce some short pea
plants. If two heterozygous tall pea plants crossed about 1
out of every 4 plants will be short.
©Copyright 2014 by www.cpalms.org
Exit Ticket
1.
What kinds of information can you find
while using a Punnett Square?
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Guided Practice Problem #3
A
white flowered plant is crossed with
a plant that is heterozygous for purple
flowers. Purple flowered plants have
at least one dominant allele, while
white flowered plants have no
dominant alleles. Answer the
following questions for this cross.
©Copyright 2014 by www.cpalms.org
1. What is the trait being studied in
this scenario?
 Flower
color is the trait being studied
in this scenario.
©Copyright 2014 by www.cpalms.org
2. Indicate the dominant and recessive
genes and assign an appropriate letter
to each allele:
 A.
The dominant gene is the color purple.
We will use the letter P.
 B.
The recessive allele is the color white.
We will use the letter p.
©Copyright 2014 by www.cpalms.org
3. What is the genotype of Parent #1
(mother)?
 Genotype
shown as letters: pp
 Scientific
term for this genotype:
Homozygous recessive
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4. What is the genotype of Parent #2
(father)?
 Genotype
shown as letters: Pp
 Scientific
term for this genotype:
Heterozygous
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5. What is the phenotype of Parent #1
(mother)?
 The
phenotype of the mother is white
flowers.
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6. What is the phenotype of Parent #2
(father)?
 The
phenotype of the father is purple
flowers.
©Copyright 2014 by www.cpalms.org
7. Draw a Punnett Square to represent
the offspring the parents above are likely
to have.
p
p
P Pp Pp
p pp
pp
©Copyright 2014 by www.cpalms.org
8. What are the possible genotypes of
the offspring?
 The
possible genotypes of the offspring are:
Pp = Heterozygous
pp
= Homozygous recessive
©Copyright 2014 by www.cpalms.org
9. What are the possible phenotypes of
the offspring?
 The
possible phenotypes of the
offspring are:
Purple
While
Flowers (controlled by the genotype Pp)
Flowers (controlled by the genotype pp)
©Copyright 2014 by www.cpalms.org
10. What is the probability that the offspring
will have a homozygous dominant genotype?
 The
probability of having an offspring with
a homozygous dominant genotype is 0/4
or 0%.
©Copyright 2014 by www.cpalms.org
11. What is the probability that the offspring
will have a heterozygous genotype?
 The
probability of an offspring inheriting a
heterozygous genotype is ½ or 50%.
©Copyright 2014 by www.cpalms.org
12. What is the probability that the offspring
will have a homozygous recessive genotype?
 The
probability that an offspring will have a
homozygous recessive genotype will be ½
or 50%.
©Copyright 2014 by www.cpalms.org
13. What is the genotype ratio for the
possible offspring?
 The
genotype ratio set-up is (Pp : pp).
 The
genotype ratio would be 2 : 2.
 Reduce
the ratio to 1 : 1.
©Copyright 2014 by www.cpalms.org
14. What is the probability that the offspring
will have a phenotype of purple flowers?
 The
probability of having an offspring with
a purple flower phenotype is ½ or 50%.
©Copyright 2014 by www.cpalms.org
15. What is the probability that the offspring
will have a phenotype of white flowers?
 The
probability of an offspring inheriting a
non-rolling tongue phenotype is ½ or 50%.
©Copyright 2014 by www.cpalms.org
16. What is the phenotype ratio for the
possible offspring?
 The
phenotype ratio set-up is
(purple flowers : white flowers).
 The
phenotype ratio would be 2 : 2.
 Reduce
the ratio to 1 : 1.
©Copyright 2014 by www.cpalms.org
17. You want to have a pea plant garden with about fifty
percent white and fifty percent purple flowers. Which
offspring would you cross to ensure that you would keep
producing about half of each flower color every
generation?

Create more Punnett Squares to help you answer this question.
P
p
P PP Pp
p
Pp
pp
p
p
p
p
P
Pp Pp
p
pp
pp
p
pp
p
pp
pp
pp
©Copyright 2014 by www.cpalms.org
17. You want to have a pea plant garden with about fifty percent
white and fifty percent purple flowers. Which offspring would you
cross to ensure that you would keep producing about half of each
flower color every generation?

Create more Punnett Squares to help you answer this question.
P
p
P PP Pp
p
Pp
pp
p
p
p
p
P
Pp Pp
p
pp
pp
p
pp
p
pp
pp
pp
©Copyright 2014 by www.cpalms.org
17. You want to have a pea plant garden with about fifty
percent white and fifty percent purple flowers. Which
offspring would you cross to ensure that you would keep
producing about half of each flower color every
generation?

Explain.
I
would cross a heterozygous offspring with a homozygous recessive
offspring to make sure that I had half purple and half white flowers.
The first cross (Pp x Pp) would create too many purple flowers and the
third cross (pp x pp) would create nothing but white flowers. The
second cross (Pp x pp) would create the right amount because only
half of the offspring would most likely inherit the dominant allele from
one parent.
©Copyright 2014 by www.cpalms.org
Wrap Up
 What
are some advantages of using a Punnett
Square mathematical model?
 What
are some disadvantages of using a Punnett
Square mathematical model?
©Copyright 2014 by www.cpalms.org
Advantages and Disadvantages of a
Punnett Square


Advantages

Punnett squares are a mathematical model of what could happen in the real world.

Punnett squares show you the possible genetic combinations for all offspring.

You can calculate the probability of

Genotypes

Phenotypes

Genotype ratios

Phenotype ratios
Disadvantages

Punnett Squares will only predict what could happen, NOT what will happen exactly in the
real world.
©Copyright 2014 by www.cpalms.org