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D2.3 · Solve monohybrid, dihybrid, and sex-linked inheritance problems
Learn to solve monohybrid, dihybrid, and sex-linked inheritance problems through clear examples and targeted practice.
Ontario Grade 11 Biology
Genetic Processes
A probability model, not a promise about a child
A Punnett square organizes the possible allele combinations from two parents. In a simple hypothetical trait with complete dominance, use A for one allele and a for the other. An Aa parent can contribute A or a to a gamete. The square predicts proportions across many possible outcomes; it cannot tell you the exact genotype of one future child.
What you will learn
- List possible gametes from each parent.
- Complete a monohybrid Punnett square.
- Distinguish genotype probability from guaranteed outcome.
Set up the model
First write parental genotypes and state the inheritance assumptions. For Aa × Aa, each parent has one A and one a allele. During meiosis, a gamete receives one allele from the pair. Put A and a from one parent across the top of a two-by-two grid and A and a from the other down the side.
The symbols alone do not define a real disease or visible trait. Complete dominance is an assumption for this example. Incomplete dominance, codominance, sex linkage, and two-trait crosses need their own careful models.
Combine alleles
Fill each box with one allele from its row and one from its column. The combinations are AA, Aa, Aa, and aa. There are three genotype categories with an expected 1:2:1 ratio. If A is completely dominant, AA and Aa have the dominant phenotype, and aa has the recessive phenotype, giving an expected 3:1 phenotype ratio.
These are probabilities per offspring under the stated assumptions, not a schedule in which every fourth child must be aa. Even if a family has four children, the actual count may differ from the model’s expected ratio.
Check scope and interpretation
The square assumes each parental gamete type has the probability stated by the model, and that their combinations are considered without other complications. Before solving a new problem, check whether it specifies complete dominance, codominance, sex linkage, or a different pattern. Never assume that a capital letter means a trait is common or desirable.
To communicate the answer, give the parental cross, the gametes, the four combinations, and the probability requested. For an aa result in Aa × Aa, one of four boxes qualifies, so the expected probability is 25%. State the genotype separately from the phenotype.
Worked example
Two heterozygous parents
For a hypothetical trait with complete dominance, parents are Aa and Aa. What is the expected chance of an aa child?
- GametesEach parent can contribute A or a.
- CombinationsThe four grid boxes are AA, Aa, Aa, and aa.
- ProbabilityOne of four equally likely combinations is aa.
Answer: The expected probability of aa is 1/4, or 25%.
Check: The result is a probability for each child, not a guarantee for any particular family.
Common mistakes and how to avoid them
Claiming two Aa parents must have one aa child among their next four children.
Correction: The 1/4 result is an expected probability for each child; small family outcomes vary.
Lesson summary
- List parental gametes before filling the square.
- Aa × Aa gives expected genotypes AA, Aa, Aa, aa.
- Explain assumptions and report probability, not certainty.
Check your understanding
Question 1
In an Aa × aa cross, what is the expected proportion of aa offspring?
- 0%
- 50%
- 100%
Show answer and explanation
50%
The Aa parent contributes A or a; the aa parent contributes only a, giving Aa and aa equally often in the model.
Key terms
- Genotype
- An organism’s allele combination for the trait being modelled.
- Gamete
- A reproductive cell carrying one allele from each chromosome pair.
Continue through SBI3U
View the complete SBI3U Ontario Grade 11 Biology curriculum and lessons
- B2.4 · Create and use a dichotomous identification key
- A1.1 · Form research questions, predictions, and testable hypotheses
- A1.2 · Choose suitable instruments, materials, and inquiry procedures
- A1.3 · Locate relevant print and electronic research sources
- A1.4 · Plan investigations using safe laboratory practices
- A1.5 · Conduct safe inquiries while controlling relevant variables
About this lesson and its review
Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Biology (SBI3U), expectation D2.3. It is a study resource, not an official curriculum publication.
Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.