Punnett Squares
Be ready to answer!

Which is an example of heterozygous
alleles?



Tt
MM
mm
Be ready to answer!

Which is an example of heterozygous
alleles?



Tt
MM
mm
Review

Which is an example of homozygous
dominant alleles?
Tt
 MM
 mm

Review

Which is an example of homozygous
dominant alleles?
Tt
 MM
 mm

Review
Which is an example of a purebred?

Tt
 MM
 mm

Review
Which is an example of a purebred?

Tt
 MM
 mm

Review
Which is an example of a phenotype?

Tt
 tt
 Can roll tongue
 None of these are examples of
phenotypes

Review
Which is an example of a phenotype?

Tt
 tt
 Can roll tongue
 None of these are examples of
phenotypes

Probability and
Punnett Squares
{

The likelihood that a specific event will
occur


If you flip a coin 100 times, you should get
about 50 heads, 50 tails
http://www.betweenwaters.com/probab/flip/coinmainD.html
Probability
Probability in Genetics
Fertilization: endless
possibilities depending on
which sperm fertilizes the
egg
 Figuring out the
probability of specific
traits can be HARD! But
one tool makes it easier…



The value of studying genetics is in
understanding how we can predict the
likelihood of inheriting particular traits. This
can help plant and animal breeders in
developing varieties that have more desirable
qualities. It can also help people explain and
predict patterns of inheritance in family lines.
One of the easiest ways to calculate the
mathematical probability of inheriting a
specific trait was invented by an early 20th
century English geneticist named Reginald
Punnett
Read Only
PUNNETT SQUARES
charts showing the possible
combinations of alleles in a cross
 show the probability of getting a
certain type of offspring

Example with flies:
Phenotypes:
Genotypes:
Monohybrid Cross: a cross that involves
only one trait
Each square shows the chance
of getting a baby with that
genotype
If there are
4 squares,
what
percent is
each
square
worth?
THE PARENTS
GENOTYPES
DAD = Tt (heterozygous)
MOM = Tt (heterozygous)
PHENOTYPES
DAD = Tall
MOM = Tall
One parent’s genes go across the
top
(one per box)
The other
parent’s
genes go
down the
side (one
per box)
Each letter goes into each box below
it
Each letter
goes into
each box
next to it
Now we can predict the possibility that
the parents will have a baby with that
kind of phenotype
What is the possibility (%) that the
parents will have a tall baby?
TT = tall
25%
Tt = tall
50%
So 75%
chance
What is the possibility that the parents
will have a short baby?
tt = short
25%
So 25%
chance
THE OFFSPRING
GENOTYPES
1TT:2Tt:1tt (1:2:1)—ratio
25% TT, 50% Tt, 25% tt




TT (homozygous dominant)
Tt (heterozygous)
Tt (heterozygous)
tt (homozygous recessive)
THE OFFSPRING
PHENOTYPES
3 Tall :1 Short (3:1)
75% Tall, 25% Short
TT (tall)
Tt (tall)
Tt (tall)
tt (short)

Cross a homozygous
dominant with a recessive
(for height where T is
dominant and tall).
What are my genotypes?
Cross a homozygous
dominant with a recessive
(for height where T is
dominant and tall).
What are my genotypes?
TT and tt
Cross a homozygous
dominant with a recessive
(for height where T is
dominant and tall).
Create a Punnett square.
Cross a homozygous dominant with
a recessive (for height where T is
dominant and tall).
Create a Punnett square.
Cross a homozygous dominant with
a recessive (for height where T is
dominant and tall).
Create a Punnett square.
Cross a homozygous dominant with
a recessive (for height where T is
dominant and tall).
Create a Punnett square.
Find the percent chance of
each genotype and each
phenotype
Find the percent chance of
each genotype and each
phenotype
-Tt: 100%
-Tall: 100%
Cross a heterozygote with a
homozygous recessive (for
height where T is
dominant and tall).
What are the genotypes to
cross?
Cross a heterozygote with a
homozygous recessive (for
height where T is
dominant and tall).
What are the genotypes to
cross?
-Tt and tt
Find the percent chance of
each genotype and each
phenotype
Find the percent chance of each
genotype and each phenotype
-Tt:
50%
-tt:
50%
-tall:
50%
-short:
50%
What is the ratio of dominant: recessive?
Find the percent chance of each
genotype and each phenotype
-Tt:
50%
-tt:
50%
-tall:
50%
-short:
50%
What is the ratio of dominant: recessive?
2:2

Red is dominant to white. Using the letter R
to represent red flower color, make a cross
between:
 homozygous dominant x heterozygous
 Find the percent chance of each genotype
and each phenotype
 Write the ratio of dominant:recessive
genotypes
Punnett Squares: Practice

Red is dominant to white. Using the letter R to
represent red flower color, make a cross
between:
 homozygous dominant x heterozygous—RR
and Rr
 Find the percent chance of each genotype and
each phenotype—RR: 50%, Rr: 50%, Red:
100%
 Write the ratio of dominant:recessive
genotypes—4:0
Punnett Squares: Practice

Brown is dominant to blue eyes. Using
B to represent brown, make a cross
between:
Heterozygous x heterozygous
 Find the percent chance of each genotype
and each phenotype
 Write the ratio of dominant:recessive
genotypes

Punnett Squares: Practice

Brown is dominant to blue eyes. Using B
to represent brown, make a cross
between:
Heterozygous x heterozygous—Bb and Bb
 Find the percent chance of each genotype and
each phenotype—BB: 25%, Bb: 50%, bb: 25%,
brown: 75%, blue: 25%
 Write the ratio of dominant:recessive
genotypes—3:1

Punnett Squares: Practice

http://anthro.palomar.edu/mendel/mendel_2.ht
m
Additional Resources