Complete Monohybrid Problems Using a Punnett Square
Punnett square diagrams are used to predict the genotypes of offspring resulting from a cross
breeding between two individuals. (Refer to figure 7 on page 188 for another example).
Example problem: A brown eyed man whose genotype is Bb marries a brown eyed woman
whose genotype is Bb. What are the chances of this couple producing a blue eyed baby?
Assign the trait a letter, and write let statements
“Let B represent the dominant brown allele and b represent the recessive blue allele”
Step 1: Write the genotypes of the two individuals that are being crossed and their
phenotypes if they are known
Bb
x
Brown
Bb
Brown
Step 2: Complete a Punnett Square (you can draw a tic tac toe grid)
Each box
represents a
female sex
cells. Sex cells
carry only
one allele.
Each box represents a male sex
cells. Sex cells carry only one
allele.
B
b
B
BB
Bb
b
Bb
bb
Step 3: Analyze the Punnett Square to determine the genotypic and phenotypic frequencies
and ratios.




Genotypic frequencies (the probability of each genotype occurring)
o ¼ BB
½ Bb
¼ bb
Genotypic ratio
o 1 BB :
2 Bb : 1 bb
Phenotypic frequencies (the probability of each phenotype occurring)
o ¾ Brown ¼ Blue
Phenotypic ratio
o 3 brown : 1blue
Step 4: Answer the question in a complete sentence!
The chances of this couple producing a blue eyed baby is ¼ or 25%.
Practice with Monohybrid Crosses
1. Set up the Punnett square for each of the crosses listed below. Round seeds are dominant
to wrinkled seeds.
Rr x rr
What percentage of the offspring
will be round?
___________
Rr x Rr
What percentage of the offspring
will be round?
___________
RR x Rr
What percentage of the offspring
will be round?
___________
Practice with Crosses.
2. A TT (tall) plant is crossed with a tt (short plant).
What percentage of the offspring will be tall? ___________
3. A Tt plant is crossed with a Tt plant.
What percentage of the offspring will be short? ______
4. A heterozygous round seeded plant (Rr) is crossed with a homozygous round seeded plant
(RR).
What percentage of the offspring will be homozygous (RR)? ____________
5. A homozygous round seeded plant is crossed with a homozygous wrinkled seeded plant.
What are the genotypes of the parents? __________ x __________
What percentage of the offspring will also be homozygous? ______________
6. In pea plants purple flowers are dominant to white flowers.
If two white flowered plants are cross, what percentage of their offspring will be white
flowered? ______________
7. A white flowered plant is crossed with a plant that is heterozygous for the trait.
What is the phenotypic ratio of the offspring plants? _____________
8. Two plants, both heterozygous for the gene that controls flower color are crossed.
What is the genotypic ratio of the offspring plants? ______________
What is the phenotypic ratio of the offspring plants? ___________
9. In guinea pigs, the allele for short hair is dominant.
What genotype would a heterozygous short haired guinea pig have? _______
What genotype would a purebreeding short haired guinea pig have? _______
What genotype would a long haired guinea pig have? ________
10. Show the cross for a pure breeding short haired guinea pig and a long haired guinea pig.
What percentage of the offspring will have short hair? __________
11. Show the cross for two heterozygous guinea pigs.
What percentage of the offspring will have short hair? ________
What percentage of the offspring will have long hair? _______
More Genetics Problems: Monohybrid Crosses
1. You are given the genetic cross of pea plants: TT x tt.
a) What is the genotype of each of the parents?
b) What is the phenotype of each of the parents?
c) What are the possible gametes that can be produced by each parent?
d) Show the genotypes of the F1 generation. What are the phenotypes of the F1
generation?
2. If G represents the dominant green trait, and the recessive allele produces a yellow trait,
show the genotypes of the F1 generation of the P1 cross GG x gg. Give the phenotypes
of the parents and the offspring.
3. Show the genotypes and phenotypes of the F1 generation if a heterozygous green pea
plant is crossed with a homozygous yellow plant (given the information in #2).
4. Show the phenotypic and genotypic ratios of the F1 generation if a heterozygous tall pea
plant is crossed with a dwarf pea plant. (Note: tall is dominant)
5. Mendel found that the allele for inflated pods in peas (I) is dominant to its allele for
narrowed pods (i). What offspring phenotypes would be expected from the following
crosses and in what ratio?
a) heterozygous self-fertilized
b) homozygous inflated x heterozygote
c) heterozygote x homozygous narrowed
6. A number of plant species have a recessive allele for albinism; homozygous albino
(white) individuals are unable to synthesize chlorophyll. If a tobacco plant that is
heterozygous for albinism is allowed to self-pollinate and 500 of its seeds germinate:
a) how many of these offspring will be expected to have the same genotype as the
parent plant?
b) how many seedlings will be expected to be white?
7. Sniffles, a male mouse with a coloured coat, was mated with Esmeralda, an alluring
albino. The resulting litter of six young mice all had coloured fur. The next time around,
Esmeralda was mated with Whiskers, who was the same colour as Sniffles. Some of
Esmeralda’s second litter were white (albino).
a) What are the probable genotypes of Sniffles, Whiskers, and Esmeralda?
b) If a male of the first litter were mated with a coloured female of the second litter, what
phenotypic ratio might be expected among the offspring?
c) What would the expected results be if a male from the first litter were mated with an
albino female from the second litter?
8. What are the most likely genotypes of the parents if a cross of two pea plants produces
60 round seeds and 20 wrinkled seeds? (Note: round is dominant)