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Instructor
Terry Wiseth
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Genetics is the study of how characteristics are passed
from one generation to the next. Characteristics that
offspring exhibit are the result of combinations of traits
exhibited by their parents and subsequently passed down
to their offspring. Traits are determined by the types of
genes that are carried by an individual. Genes carried by
the chromosomes in the sperm and egg will combine in
various combinations during fertilization.
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Traits observed and exhibited by an individual are
referred to as phenotypes. The coat color exhibited by a
dog for instance would be the phenotype. The actual
combination of genes that produce the phenotype is
called the genotype. Gregor Mendel has shown us that
traits are controlled by pairs of genes, one gene from the
mother and the other gene from the father. Mendel also
proved that some genes have the ability to mask the
effect of other genes. We call these “masking” genes
dominant and the masked genes are called recessive.
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We may consider the capability of a dog to bark as a trait
controlled by a dominant gene. We can use a capital letter
to designate this dominant gene (B). The inability to bark
is a mutation in dogs that results from a recessive gene.
We can use a lower case letter to represent this recessive
gene (b).
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Individuals that possess both dominant genes (BB) or
one dominant gene and one recessive gene (Bb) would
exhibit the trait of having the ability to bark. A genotype of
BB or Bb would result in a phenotype of “barkers”.
Another possible genotype would be two recessive genes
(bb) which would result in a phenotype of “mute” or nonbarkers.
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One of the tools that scientists often use to analyze the
possible combination of genetic traits that we might find
in succeeding generations is the Punnet square. In this
exercise we will learn how to use a Punnet square in
predicting the possible combinations of genes that may
result in a specific cross.
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D
d
D
D D
Dd
d
D d
dd
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B bX B b
B = Barkers
b = Mute
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A completed Punnet square for
a cross between two individuals
each with genotypes of Bb is
shown below. Click on NEXT to
see how a Punnet square is set
up and completed.
D
d
D
D D
Dd
d
D d
dd
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A a
X
A = Dark fur
a = Light fur
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A a
Consider the following cross
of two individuals with the
genotypes as shown to the
left.
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A a
X
A a
Click on each of the gametes
(letters) listed to the left to
set up the Punnet Square
A = Dark fur
a = Light fur
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A a
X
A = Dark fur
a = Light fur
A a
Click on each of the gametes
listed in the Punnet Square
to place them in the possible
combinations of gametes in
the offspring
A
a
A
a
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A A= 1
Aa = 2
aa = 1
List the possible genotypes for
all of the possible offspring by
clicking on the Punnet Square
This is your GENOTYPE RATIO
for this cross (1:2:1)
A = Dark fur
a = Light fur
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A A
Aa
A a
aa
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Dark fur = 3
Light fur = 1
Click on each of the genotypes
listed in the Punnet square to
find the phenotype of the
possible combinations
This is your PHENOTYPE
is one
twodominant
dominantand
recessive
genes
one
RATIO for this cross (3:1) There are
recessive
which
results
gene
inwhich
the recessive
dominant
results in
the dominant characteristic
characteristic
of Light
Dark fur
fur of
A = Dark fur
Dark fur
a = Light fur
Dark fur
A A
Dark fur
Aa
Light fur
A a
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Dark fur
aa
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D dX D d
D = Long ears
d = Short ears
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How would you set this
cross up on the Punnet
square given below? Click
on NEXT when you have
made your decisions
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D dX D d
D = Long ears
d = Short ears
Below is how you might have
set the Punnet square up for
this cross. How would you fill in
the middle squares to show the
possible combination of genes.
Click on NEXT when you have
made your decision
D
d
D
d
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D dX D d
D = Long ears
d = Short ears
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Your completed Punnet square
should look as the table shows
below. What are the genotype
and phenotype ratios for this
cross? Click on NEXT when you
have made your decisions.
D
d
D
D D
Dd
d
D d
dd
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D dX D d
D = Long ears
d = Short ears
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Your Genotype ratio would be:
DD = 1, Dd = 2, dd = 1 (1:2:1)
Your Phenotype ratio would be:
Long ears = 3, Short ears = 1 (3:1)
D
d
D
D D
Dd
d
D d
dd
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DD
Xdd
D = Long ears
d = Short ears
Now let us consider a different type
of cross. If we were to cross a Long
eared dog with a genotype of DD
with a short eared dog with a
genotype of dd, what would the
genotype and phenotype ratios be?
d
d
D
D
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DD
Xdd
D = Long ears
d = Short ears
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The genotype ratio for this cross
would be all Dd
The phenotype ratio for this cross
would be all Long ears
d
d
D
Dd
Dd
D
Dd
Dd
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Genotypes that exhibit both genes as identical are called
homozygous or purebred. Examples of homozygous
genotypes would be BB or bb. BB would represent a
homozygous dominant genotype while bb would
represent a homozygous recessive genotype.
Genotypes that exhibit genes that are not identical are
called heterozygous or hybrid. An example of a
heterozygous genotype would be Bb.
When we consider only a single trait, such as ear length
or coat color by itself it is considered to be a monhybrid
cross. “Mono” in this case refers to considering only a
single trait. If we consider two traits at the same time
such as ear length and coat color together it is
considered to be a dihybrid cross. “Di” refers to
considering two traits at the same time. Our next exercise
will involve performing a dihybrid cross.
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The dihybrid cross we will consider is that of a dog with
dark fur color with long ears mated to a dog with light fur
color with short ears. The genotypes of the two dogs are
listed below.
AADD
dark fur
long ears
X aadd
light fur
short ears
Set up a Punnet square for this cross. You will need to
use a Punnet square that has a total of 16 boxes (4 X 4)
instead of the smaller 4 box square that you used in the
monohybrid crosses. Click on NEXT to see how the
Punnet square will be set up.
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AADD
AADD X aad
a dd
Another of Mendel’s discoveries is
that during the formation of the
sperm and egg cells of an indivdual, the genes will separate
independently. This means that the two genes that determine a
trait will not end up in the same sperm or egg cell. Click on the
individual genes to set up the Punnet square.
Possible
sperm or
egg cell
genotypes
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Possible
sperm or
egg cell
genotypes
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We can now see how the possible combinations of genes can
occur in the offspring. Fill in the squares as you had performed
earlier. It is best to match the homologous chromosomes in each
square. Click on NEXT to see if you have filled in the squares
correctly.
a d ad a d ad
Possible
sperm or
egg cell
genotypes
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Possible
sperm or
egg cell
genotypes
AD
AD
AD
AD
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Note how the distribution of the genes have occurred in each of
the squares. Click on the Punnet square to find the Phenotype
and Genotype ratios.
Genotype ration = All AaDd
Phenotype ratio = All Dark fur, Long ears
a d ad a d ad
Possible
sperm or
egg cell
genotypes
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AD
AD
AD
AD
Possible
sperm or
egg cell
genotypes
AaDd AaDd AaDd AaDd
AaDd AaDd AaDd AaDd
AaDd AaDd AaDd AaDd
AaDd AaDd AaDd AaDd
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The next dihybrid cross we will consider is that of a dog
with dark fur color with long ears mated to a dog with
dark fur color with long ears. The genotypes of the two
dogs are listed below. Both of these dogs are hybrids.
AaDd X AaDd
dark fur
long ears
dark fur
long ears
Set up a Punnet square for this cross. Click on NEXT to
see how the Punnet square will be set up.
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AaDd X AaDd
Now that you have set up the Punnet square you can fill out
each of the inside squares. Click on NEXT to see if you have
filled it out correctly.
A D a D Ad a d
Possible
sperm or
egg cell
genotypes
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Possible
sperm or
egg cell
genotypes
AD
aD
Ad
ad
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AaDd X AaDd
How does your finished Punnet square compare with the one
displayed below.? Now determine the genotype and phenotype
ratios. Click on NEXT when you have made your determination.
A D a D Ad a d
Possible
sperm or
egg cell
genotypes
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AD
aD
Ad
ad
Possible
sperm or
egg cell
genotypes
AADD Aa DD AADd Aa Dd
Aa DD a a DD Aa Dd a a Dd
AADd Aa Dd AAd d Aa d d
Aa Dd a a Dd Aa d d a a d d
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AaDd X AaDd
Genotype ratio
AADD=1
aaDD=1
AaDD=2
aaDd=2
AADd=2
Aadd=1
AaDd=4
Aadd=2
aadd=1
Phenotype ratio
Dark fur Long ears=9
Dark fur Short ears=3
Light fur Long ears=3
Light fur Short ears=1
A D a D Ad a d
Possible
sperm or
egg cell
genotypes
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AD
aD
Ad
ad
Possible
sperm or
egg cell
genotypes
AADD Aa DD AADd Aa Dd
Aa DD a a DD Aa Dd a a Dd
AADd Aa Dd AAd d Aa d d
Aa Dd a a Dd Aa d d a a d d
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Sites of Interest for Punnet Squares
http://curriculum.calstatela.edu/courses/builders/lessons/less/les4/casino/cas1ck
.html
http://www.athro.com/evo/gen/punexam.html