Study guides & original practice
Punnett square practice
A Punnett square organizes possible allele combinations. Its proportions describe probabilities across offspring, not a promise that a small family will contain every listed outcome.
By ExamCat · Original study lesson · Published September 24, 2026
Understand the method
For a single gene with complete dominance, a heterozygote carries two different alleles. List each parent’s possible gametes, combine one from each parent, and distinguish genotype from phenotype before counting.
Worked example, step by step
Cross Aa with Aa, where A is completely dominant over a.
- Each parent can contribute A or a, each with probability 1/2 under simple Mendelian segregation.
- Combine the gametes to obtain AA, Aa, Aa and aa.
- Genotype probabilities are 1/4 AA, 1/2 Aa and 1/4 aa.
- Dominant phenotype probability is 3/4; recessive phenotype probability is 1/4 under the stated inheritance model.
A mistake to watch for
Dominant does not mean common, stronger or beneficial. The model may not apply unchanged to incomplete dominance, sex linkage, multiple genes or environmental effects.
Try it yourself: three questions
Write an answer before opening the explanation. For calculations, keep your units and intermediate steps; for concepts, state the reason for your choice.
1. Cross Aa with aa. What fraction is expected to be aa?
Show answer and explanation
One half. The first parent contributes a half the time, while the second always contributes a.
2. Can two recessive-phenotype aa parents produce Aa in this simple model?
Show answer and explanation
No. Neither parent provides an A allele. This conclusion assumes the specified model without a new mutation.
3. If three children show the dominant phenotype, must the next be recessive?
Show answer and explanation
No. Each new offspring still has the model’s probability; earlier outcomes do not enforce a fixed family ratio.
Build your next practice session
Mark the first step where your reasoning differed from the explanation. Turn that step into a question, answer it without looking, then change one condition in the example and explain what would change. A correct guess is a reason to revisit the method.
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Further reading
OpenStax Biology 2e provides textbook background. The worked scenario and practice questions on this page were written independently for ExamCat.
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