Biology 8.3 Studying Heredity: Punnett Squares
Studying
Heredity
Punnett Squares
Animal breeders try to breed
animals with specific
characteristics such as speed,
endurance or strength.
To do this, breeders must be able
to predict how a trait will
appear when two animals are
crossbred.
One way to predict expected
results of a single trait is with a
diagram called a Punnett
Square
Punnett Square

A Punnett Square is
a diagram that
predicts the
outcome of a
genetic cross by
considering all
possible
permutations of the
gene combinations
for a trait when
crossed.
Punnett Square

The possible gametes that
one parent can produce
are written along one side;
B and b

The possible gametes that
the other parent can
donate are written across
the other side, B and b.

Remember, capital B
represents a dominant trait
while lowercase b
represents a recessive
trait.
Remember: when a dominant trait is
present (B) it will overpower the
recessive trait (b) and control the
appearance (purple flowers). When only
the recessive is present (bb), than the white
will appear.
Punnett Square

Each box is filled in with the
letter combinations that show
the result when an allele
from one genotype is
combined with the allele from
the another genotype.

Bb represents the genotype
of each parent for the trait.

B or b represents the allele
Punnett Square

When allele B is combined
with the other allele B, the
result is a genotype BB in the
box on the Punnett square.

Allele B and allele b combine
to create genotype Bb in two
of the grid boxes.

Last, alleles b and b combine
to create a genotype bb in
the last grid box.
Punnett Square

The figure at right shows the results
of a hybrid cross between two pea
plants that are both heterozygous
(Bb)

Since the B represents the dominant
trait, any of the combinations with a
B in it will result in a purple flower.

3 out of the 4 flowers will be purple.
We can also say that the genotype
ratio is 1 BB : 2 Bb : 1 bb

The phenotype ratio is 3 purple : 1
white
Remember: the genotype is
the pair of alleles (BB,Bb,bb)
The phenotype is the physical
appearance that results
purple or white flower
Punnett Square

Animal and plant breeders often
need to determine if an organism
with a dominant trait they are
breeding is heterozygous (Bb) or
homozygous (BB or bb)

Horticulturists could perform a test
cross to determine the genotype.

In a test cross, an individual whose
phenotype is dominant (purple
flowers) but whose genotype is NOT
KNOWN, is crossed with a
homozygous recessive individual
Remember: the genotype is
the pair of alleles (BB,Bb,bb)
The phenotype is the physical
appearance that results
purple or white flower
Punnett Square

In the diagram at right, the
genotype tt represents a
recessive genotype.

What are the possible
combinations that will result?

What is the probability that the
recessive trait (t) will become
the visible phenotype?

What is the probability that the
dominant trait (T) will become
the visible phenotype?
Punnett Square

In the diagram at right, the genotype
tt represents a recessive genotype
that would ordinarily be
overshadowed by the dominant T.

What are the possible combinations
that will result?

The 4 squares should read TT, Tt,
tt, and tt.

Two of the squares will carry the
dominant T trait while the other two
squares will allow the recessive trait
to come forward.
Ratios and Probabilities

Probability is the likelihood that a
specific event will occur.

In the chart at right, the B is a
dominant trait. Whenever the B is
present in the crossbred genotype,
the phenotype will be a purple
flower.


The ratio is 3 : 1 in favor of purple
flowers. (3 purple for every 1 white)
The probability is 3 out of 4 will be
purple or 75% probability of purple.
Remember: the genotype is
the pair of alleles (BB,Bb,bb)
The phenotype is the physical
appearance that results
purple or white flower
Ratios and Probabilities

In the table at right showing a
crossbreed, the probability of the
flower being purple (phenotype) is 3
out of 4 or 75%

The probability of the genotype
being homozygous (BB or bb) are 2
out of 4 or 50%

The probability of the phenotype
being heterozygous (Bb) is 2 out of
4 or 50%
Remember: the genotype is
the pair of alleles (BB,Bb,bb)
The phenotype is the physical
appearance that results
purple or white flower
Ratios and Probabilities

The Punnett square at right shows the
possible combinations for dogs with a
particular color.

The mother (dam) has the dominant G
and a recessive g making up her
genotype.

The father (sire) has two recessive g’s
(gg) making up his genotype.

When crossbred, the result is 4
possibilities; 2 blue phenotypes (Gg) and
2 black phenotypes (gg); a 50-50 split.

The black only appear because the
recessive g in the father combine with
both recessive traits in the mother to
allow 2 gg combinations where a
dominant trait does not overpower.
This Punnett square shows
a 50% probability for the pups
to be either color
Pedigrees

Suppose you want to learn about an inherited trait in your
family that is passed down through generations.

Geneticists create a pedigree to do this.

A pedigree is a chart of a family history that shows how a trait
is present over generations in a family
Pedigrees

In the pedigree below, we see a key to it’s right that explains
what each symbol means in the pedigree. Like a map, the
key is important to reading the pedigree.

The pedigree shows us who is male, female, who is married
to whom else, and each couples direct children both male
and female. In this pedigree we follow the path of a genetic
disease passed through three generations.
1st generation
Key
2nd generation
3rd generation
Pedigrees

In this pedigree we can also see which children in which
generations have been affected by a disease that was
genetically passed down.

We see the original carrier was Sue but in following
generations the disease only showed up in male children of
the family.
1st generation
Key
2nd generation
3rd generation
Pedigrees

A gene or trait may not appear in the genders equally.

If a gene is autosomal, it appears in both sexes equally.

If a gene is sex-linked however, it always appears only in one
gender, usually males.

In the pedigree below we see that the genetic disorder has
affected only the males in the following generations. This indicates
it is a sex-linked genetic disorder
1st generation
Key
2nd generation
3rd generation
Pedigrees

Here we see an example of a pedigree. The key at the
bottom shows us the males and females that are the carriers
of a genetic condition over three generations.

The disease appears in both males and females, therefore it
is autosomal.
Key Concepts:

The results of genetic crosses can be predicted
with the use of Punnett Squares

A test cross can be used to determine whether an
individual expressing a dominant trait is
heterozygous or homozygous

A trait’s pattern of inheritance within a family can
be determined by analyzing a pedigree.