Student #1 & Student #2
1st Period Biology
Mr. Teacher
Student #1 – blue eyes
Father – blue eyes
Paternal Grandparents
Grandfather –
green eyes
Grandmother –
blue eyes
Mother – green eyes
Maternal Grandparents
Grandfather –
brown eyes
Grandmother –
green eyes
Punnett Square for Student #1
Student #1’s Mother has allele combination bbGG and Father has bbgg.
Brown/Blue
b
b
b
bb
bb
b
bb
bb
Green/Hazel
G
G
g
Gg
Gg
g
Gg
Gg
So the offspring (Student #1) is bbGg, which should result in hazel eyes.
However, the green/hazel gene has incomplete dominance. This means
that the expressed phenotype can be anywhere from pure green to dark
hazel depending on the genes given by the parents.
Student #2 – brown eyes
Father – brown eyes
Paternal Grandparents
Grandfather –
brown eyes
Grandmother –
brown eyes
Mother – blue eyes
Maternal Grandparents
Grandfather –
blue eyes
Grandmother –
hazel eyes
Punnett Square for Student #2
Student #2’s Mother has allele combination bbgg and Father has BBGG.
Brown/Blue
b
b
B
Bb
Bb
B
Bb
Bb
Green/Hazel
g
g
G
Gg
Gg
G
Gg
Gg
So the offspring (Student #2) is BbGg, which results in brown eyes. The
dominant phenotype here is brown eyes, as every possible combination
results in brown
eyes.
Punnett Square for Potential Offspring
Student #1’s allele combination is bbgg, and Student #2’s allele combination is BbGg.
Here is the Punnett Square, and the various combinations follow:
Brown/Blue
B
b
B
BB
Bb
b
Bb
bb
Green/Hazel
g
g
G
Gg
Gg
g
gg
gg
Student #1 is homozygous in their gene makeup. Both alleles for each genes
are the same (bb and gg). Student #2 is heterozygous in their gene makeup.
Student #2 posseses both alleles for each gene (Bb and Gg).
Punnett Square for Potential Offspring
See the possible combinations of the two genes (brown/blue, green/hazel):
Brown/Blue
B
b
B
BB
Bb
b
Bb
bb
Green/Hazel
g
g
G
Gg
Gg
g
gg
gg
Each brown/blue gene can be paired up with 4 different green/hazel genes.
This is repeated with each brown/blue gene. Therefore there are 16
different gene pairs. These are listed on the next slide.
Possible Genotypes
 The 4 brown/blue genes we paired are BB, Bb, Bb, and bb. These
were taken from the Punnett Square.
Brown/Blue Gene
Possible Pairs with
Green/Hazel
BB pair
BBGg, BBGg, BBgg, BBgg
Bb pair (first)
BbGg, BbGg, Bbgg, Bbgg
Bb pair (second)
BbGg, BbGg, Bbgg, Bbgg
bb pair
bbGg, bbGg, bbgg, bbgg
 Number of Genotypes = 16
 BBGg = 2, BBgg = 2, BbGg = 4, Bbgg = 4, bbGg = 2, bbgg = 2
Genotype Probabilities
 Number of Genotypes = 16
 BBGg = 2, BBgg = 2, BbGg = 4, Bbgg = 4, bbGg = 2, bbgg = 2
 Probability of BBGg offspring =
 Probability of BBgg offspring =
 Probability of BbGg offspring =
 Probability of Bbgg offspring
=
 Probability of bbGg offsrping =
 Probatility of bbgg offspring =
Total
2/16 * 100 = 12.5%
2/16 * 100 = 12.5%
4/16 * 100 = 25%
4/16 * 100 = 25%
2/16 * 100 = 12.5%
2/16 * 100 = 12.5%
100%
Brown Phenotype Probability
There are 4 genotypes that produce a brown
eyed offspring:
• BBGg, BBgg, BbGg, Bbgg
Since brown is dominant over all other colors
the presence of a big B allele will automatically
give the child brown eyes, regardless of the
green/blue gene.
Adding the probabilities for each of the
genotypes we get the total probability for the
phenotype
• 12.5% + 12.5% + 25% + 25% = 75%
probability of having a brown eyed
offspring
Hazel Phenotype Probability
For the green/hazel gene to be shown in a phenotype
the brown/blue gene must be bb.
There is one genotype that will give you hazel eyes.
Because the gene for green/hazel eyes has incomplete
dominance you need a heterozygous genotype for
green/hazel to have hazel eyes.
• bbGg
Since there is only one genotype for hazel eyes, the
probability for the phenotype is the same as the
probability for the genotype.
• bbGg = 12.5% probability of having a hazel
eyed offspring
Green Phenotype Probability
The green phenotype requires a genotype of bbGG. Since this genotype is not
represented in our hypothetical offspring, the probability of green phenotype is
0%.
Blue Phenotype Probability
The only genotype that produces a blue
phenotype is bbgg.
Because of this the probability of the genotype
is the same as the probability of the
phenotype.
12.5% probability child will have blue eyes.
Conclusion!
It is most probable that if Student #1 and Student #2
were to create an offspring, that offspring would
have the phenotype of brown eyes. It is impossible
for their offspring to have green eyes.