Biology · Introductory biology · Worked example
Interpret an Aa × Aa cross
Under complete dominance, what genotype and phenotype probabilities follow from two heterozygous parents?
State the cross and the inheritance rule
Both parents are heterozygous, Aa. We are modeling one autosomal locus with complete dominance of A over a, ordinary segregation and independent gamete contributions.
List each parent’s possible contributions
Each parent makes A and a gametes with equal probability. The parents contribute one allele each; they do not pass their entire Aa genotype to a single offspring.
Combine the gametes
Fill each cell by taking the allele from its row and its column. The four equally likely pairings are AA, Aa, aA and aa. In genotype summaries we write both Aa and aA as Aa.
| Parent 1 ↓ / Parent 2 → | A (½) | a (½) |
|---|---|---|
| A (½) | AA (¼) | Aa (¼) |
| a (½) | Aa (¼) | aa (¼) |
Calculate the genotype probabilities
Each cell has probability ¼. AA occupies one cell, Aa occupies two, and aa occupies one. Add the probabilities as a check: they must total 1.
Group genotypes by phenotype
Complete dominance groups AA and Aa together. The dominant phenotype therefore has probability ¾, while the recessive phenotype has probability ¼. The familiar 3:1 ratio refers to phenotypes; the genotype ratio is 1:2:1.
Explain what the prediction does and does not say
For 100 offspring, the expected counts are 25 AA, 50 Aa and 25 aa. Those are expectations, not a guarantee. The editable Punnett object keeps the parental alleles and inheritance rule visible so you can see which assumption a change affects.
Result
Genotypes: ¼ AA, ½ Aa, ¼ aa. Phenotypes: ¾ dominant and ¼ recessive.
Your turn
Under the same complete-dominance model, cross Aa with aa. Find the genotype and phenotype probabilities. What would change if the inheritance rule were incomplete dominance?
Show the answer and explanation
The genotypes are ½ Aa and ½ aa. Under complete dominance, the phenotypes are ½ dominant and ½ recessive.
The Aa parent contributes A or a with equal probability; the aa parent contributes only a. Under incomplete dominance, the same genotype probabilities remain, but Aa has its own intermediate phenotype instead of the AA phenotype.
Keep exploring
Use the workspace to explore a changed input. Return to the concept if you want to revisit the reasoning.
Return to the concept →Sources and scope
Authored study material. Tool results depend on the stated inputs and model assumptions.