Problems for Grade 11 Biology (SBI3U)
Problem #1 (Non-numerical)
Overall Expectations
By the end of this course, students will:
D2. investigate genetic processes, including those that occur during meiosis, and
analyse data to solve basic genetics problems involving monohybrid and dihybrid
crosses;
Specific Expectations
By the end of this course, students will:
D2.3 use the Punnett square method to solve basic genetics problems involving
monohybrid crosses, incomplete dominance, codominance, dihybrid crosses, and sexlinked genes [PR, AI, C]
Problem
Consider a cross between a pea plant that is heterozygous for round seeds and a pea
plant that has wrinkled seeds. The allele for round seeds is dominant over that for
wrinkled seeds. R can be used to indicate the round dominant allele and r can be used
to represent the wrinkled recessive allele. Determine the phenotypes of the offspring.
Solution
Given:
Round seed plant is heterozygous
Other plant has wrinkled seeds
Round seed allele is dominant
Round dominant allele = R
Wrinkled recessive allele = r
Required:
Genotype of offspring
Analysis:
Genotype of round seed heterozygous plant = Rr (since it is heterozygous
and R allele is dominant)
Genotype of wrinkled seed plant = rr (since r allele is recessive)
Solution:
1) Draw a Punnett square to show the cross between the two plants.
Wrinkled Plant, rr
r
r
Round Plant, Rr
R
Rr
Rr
r
rr
rr
The table tells us that ½ of the offspring are round (Rr) and half are
wrinkled (rr)
Paraphrase: Therefore, the phenotype of half of the offspring will be round and half will
be wrinkled.
Problem # 2 (Numerical)
Overall Expectations
By the end of this course, students will:
D2. investigate genetic processes, including those that occur during meiosis, and
analyse data to solve basic genetics problems involving monohybrid and dihybrid
crosses;
Specific Expectations
By the end of this course, students will:
D2.3 use the Punnett square method to solve basic genetics problems involving
monohybrid crosses, incomplete dominance, codominance, dihybrid crosses, and sexlinked genes [PR, AI, C]
Problem
A pea plant can have round seeds and wrinkled seeds. The allele for round seeds is
dominant over that for wrinkled seeds. R can be used to indicate the round dominant
allele and r can be used to represent the wrinkled recessive allele. Consider a cross
where only the offspring are observable (See table). It is still possible to determine the
genotypes of the parents in many of the cases even if the parents are unknown.
Offspring Phenotype
Numbers
Round-seed peas
5472
Wrinkled-seed peas
1850
Solution
Given:
The number of round-seed and wrinkled seed plants
Required:
The genotype of the parents
Analysis:
The ratio of the round plants and the wrinkled plants can be determined
and used to create Punnett squares to infer the genotype of the parents.
Solution:
Ratio = Number of round plants/Number of wrinkled plants
= 5472/1850
= 3/1 (rounded)
Now list the possible genotypes for each phenotype
- Round seed plant = RR, Rr
- Wrinkled seed plant =rr
The only known genotype of the offspring is that of the wrinkled peas.
Place rr in one of the boxes for the Punnett square.
r
r
rr
We know that the rest of the offspring are round-seed plants. This
indicates that the other alleles of the parents must be round. Complete the
Punnett square using this information.
Round Seed Plant,
Rr
R
r
Round Seed plant, Rr
R
r
RR
Rr
Rr
rr
Paraphrase: Therefore, it is possible to determine the genotype of the parents using the
number of type of offspring. Here, the genotype for both parents is Rr.