1. Heredity, Genetics
Punnett Squares & Probability
Mr. Miller

2. Lets find out if the two pigeons on
the left produce offspring that have
the traits of the pigeon on the right

3. To do this we must
understand how to use a
Punnet Square
What is a Punnet Square?
• A chart that allows us to determine
the different genotypes that
offspring can inherit from their
parents

4. Lets see how to use it
• First we must get the genotype of each
parent
Example:
T: allele for tallness
t: allele for shortness
tt – mothers genotype
Tt – fathers genotype

5. Now lets set up the Punnet
Square
T t
t T t t t
t T t t t

6. What are the number of genotypes
for the traits the offspring can
inherit?
•The offspring can
inherit two sets of
genes that will
T t
produce a tall
phenotype
•The offspring can
t Tt tt
inherit two sets of
genes that will t Tt tt
produce a short
phenotype

7. Now lets talk about
probability
What is probability?
Probability is the chance that an event will occur
How do you find the probability of an event?
You must know:
• the number of specific events
• the number of possible events

8. Lets find the probability the
child will be tall
How many T t
genotypes will
produce a tall
offspring? 2 t Tt tt
What are number
of possible
genotypes? 4
t Tt tt

9. Now lets find the probability!
Probability =
Number of specific event 2 1
Number of possible events 4 2
The probability of having a tall
offspring is ½, 1:2, or 50%

10. Now try this one
Determine the types of
genes an offspring can
inherit from parents that
Z z
have the following
genotypes:
Z Z Z Zz
Zz - father’s genotype
Zz - mother’s genotype
z Zz zz
Z: gene for gray feathers
z: allele for white feathers

11. Now lets look at the possible genotypes and
the phenotypes that will be expressed
There is a single
possibility of inheriting
a homozygous (pure) Z z
dominant genotype
which will produce a
phenotype with grey
Z Z Z Zz
feathers
There are two
possibilities of
z Zz zz
inheriting
heterozygous (hybrid)
genotypes which will
produce a phenotype
with grey feathers

12. Now lets look at the possible genotypes and
the phenotypes that will be expressed
There is a single
possibility of inheriting
a homozygous (pure)
Z z
Z Z Z Zz
recessive genotype
which will produce a
phenotype with white
feathers
z Zz zz
So two grey birds can
produce a pigeon with
white feathers!

13. Now lets find the probability
Z z
Lets find the ZZ Zz
Z
probability the
offspring will have z Zz zz
grey feathers.
number of specific events 3
number of possible events
=4

14. Now lets find the probability
Z z
Lets find the ZZ Zz
Z
probability the
offspring will have z Zz zz
white feathers.
number of specific events 1
number of possible events
=4

15. Vocabulary
• Probability: A number that
describes how likely it is that an
event will occur.
• Punnett Square: A chart that
shows all the possible combination
of alleles that can result from a
genetic cross.

16. Vocabulary
• Phenotype: An organism’s physical
appearance, or visible traits.
• Genotype: An organism’s genetic
makeup or allele combination.
• Homozygous: An organism that
has two identical alleles for a trait.

17. Vocabulary
• Heterozygous: An organism that
has different alleles for a trait.
• Codominance: When alleles are
neither dominant nor recessive. As
a result, both alleles are expressed
in the offspring.