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InstructorTerry Wiseth

GeneticsGenetics 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 genesgenes 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 phenotypesphenotypes. 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 genotypegenotype. 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 dominantdominant and the masked genes are called recessiverecessive.

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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 non-barkers.

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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

D d

D D

d

D d

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.

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D

d

D d

BB bb BB bbXBb

==

BarkersMute

A a A aXConsider the following cross of two individuals with the genotypes as shown to the left.

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Aa

==

Dark furLight fur

Click on each of the gametes (letters) listed to the left to set up the Punnet Square

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Aa

==

Dark furLight fur

A a A aX

AA

a

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

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Aa

==

Dark furLight fur

A a A aX

A A

a

A a

List the possible genotypes for all of the possible offspring by clicking on the Punnet Square

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A AaA

aa

121

=== This is your GENOTYPE RATIO

for this cross (1:2:1)

Aa

==

Dark furLight fur

Click on each of the genotypes listed in the Punnet square to find the phenotype of the possible combinations

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Dark fur 3=

This is your PHENOTYPE RATIO for this cross (3:1)

Light fur 1=

Light fur

Dark fur Dark fur

Dark fur

A A A a

aA a a

There are two dominant genes which results in the dominant characteristic of Dark fur

There is one dominant and one recessive gene which results in the dominant characteristic of Dark fur

There are two recessive genes which results in the recessive characteristic of Light furA

a==

Dark furLight fur

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 d D dXDd

==

Long earsShort ears

D

d

D d

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

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D d D dXDd

==

Long earsShort ears

D D

d

D d

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.

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D

d

D d

D d D dXDd

==

Long earsShort ears

D D

d

D d

Your Genotype ratioGenotype ratio would be:DD = 1, Dd = 2, dd = 1 (1:2:1)

Your Phenotype ratioPhenotype ratio would be:Long ears = 3, Short ears = 1 (3:1)

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D

d

D d

D d D dXDd

==

Long earsShort 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?

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D

D

d d

DD d dXDd

==

Long earsShort ears

D D

d

d d

The genotype ratio for this cross would be all Dd

The phenotype ratio for this cross would be all Long ears

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D

D

d d

DD d dXDd

==

Long earsShort ears

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.

AADDAADD X aaddaadd

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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dark fur light furlong ears short ears

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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.

Another of Mendel’s discoveries is that during the formation of the AD a dXA D a d

Possible sperm or egg cell

genotypes

Possible sperm or egg cell

genotypes

AAAADDDD aaaadddd

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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.

AADD

aa dd

AADD

aaddPossible sperm or egg cell

genotypes

Possible sperm or egg cell

genotypes

AA

AADD

DD

aa aadd dd

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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.

AD

a d

AD

adPossible sperm or egg cell

genotypes

Possible sperm or egg cell

genotypes

AA

DD

a ad dAaDd

AaDd

AaDd

AaDd

AaDd AaDd AaDd

AaDd

AaDd

AaDd

AaDd

AaDd

AaDd

AaDd

AaDd

AaDd

Genotype ration = All AaDd

Phenotype ratio = All Dark fur, Long ears

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.

AaDdAaDd X AaDdAaDddark fur dark furlong ears 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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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.

X

Possible sperm or egg cell

genotypes

Possible sperm or egg cell

genotypes

AAaaDDdd AAaaDDdd

AA

AAaa

aa

DD

DD

dddd

AA AAaa aaDD DD dd dd

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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.

X

Possible sperm or egg cell

genotypes

Possible sperm or egg cell

genotypes

AAaaDDdd AAaaDDdd

AA

AAaa

aa

DD

DD

dddd

AA AAaa aaDD DD dd ddAAAADDDD

AAaa DDDD

AAAADDdd

AAaa DDdd aa aa DDdd AAaa dd dd aa aa dd dd

AAaa dd dd

aa aa DDdd

AAaa DDddAAAADDddAAaa DDDD

aa aa DDDD

AAaa DDdd AAAAdd dd

AAaa DDdd

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Genotype ratioAADD=1 aaDD=1AaDD=2 aaDd=2AADd=2 Aadd=1AaDd=4 Aadd=2aadd=1

X

Possible sperm or egg cell

genotypes

Possible sperm or egg cell

genotypes

AAaaDDdd AAaaDDdd

AA

AAaa

aa

DD

DD

dddd

AA AAaa aaDD DD dd ddAAAADDDD

AAaa DDDD

AAAADDdd

AAaa DDdd aa aa DDdd AAaa dd dd aa aa dd dd

AAaa dd dd

aa aa DDdd

AAaa DDddAAAADDddAAaa DDDD

aa aa DDDD

AAaa DDdd AAAAdd dd

AAaa DDdd

Phenotype ratioDark fur Long ears=9Dark fur Short ears=3Light fur Long ears=3Light fur Short ears=1

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