Punnett Square Calculator
Select both parents' genotypes (AA, Aa, or aa) to instantly compute offspring genotype ratios (AA, Aa, aa) and dominant/recessive phenotype ratios, with a reference table of classic Mendelian ratios.
Input Data
Results
At a glance:The Punnett square is a diagram tool invented by British geneticist Reginald Punnett to predict offspring genotype and phenotype ratios of a cross. List the possible gametes of each parent along the rows and columns of the square; the cells show the paired offspring genotype, from which the probability of each genotype is counted. For a single locus with two alleles (dominant A, recessive a), the square is 2 × 2.
Formula
Each parent produces gametes by genotype: AA gives only A, aa only a, Aa gives A or a with equal chance.
The 2 × 2 square has 4 cells; count AA, Aa, aa occurrences and divide by 4 for the ratio.
Dominant phenotype ratio = AA ratio + Aa ratio; recessive phenotype ratio = aa ratio.
$$Aa \times Aa \rightarrow \tfrac{1}{4}AA : \tfrac{1}{2}Aa : \tfrac{1}{4}aa$$$$\text{Dominant : Recessive} = 3:1$$How to Use
- Select each parent's genotype (AA, Aa, or aa).
- The tool auto-enumerates gamete combinations and fills the Punnett square.
- The right panel instantly shows the three offspring genotype ratios and the dominant/recessive phenotype ratios.
Reference: offspring ratios of common monohybrid crosses
| Parental cross | AA | Aa | aa | Dominant:Recessive phenotype |
|---|---|---|---|---|
| AA × AA | 1 | 0 | 0 | all dominant |
| AA × Aa | 0.5 | 0.5 | 0 | all dominant |
| AA × aa | 0 | 1 | 0 | all dominant (all carriers) |
| Aa × Aa | 0.25 | 0.5 | 0.25 | 3 : 1 |
| Aa × aa | 0 | 0.5 | 0.5 | 1 : 1 |
| aa × aa | 0 | 0 | 1 | all recessive |
Aa × Aa gives the classic Mendelian 3:1 ratio; Aa × aa (test cross) gives 1:1, commonly used to determine whether an unknown individual is a carrier.
Case Studies
Recessive disease risk for two carrier parents
If both parents are carriers of a recessive disease (genotype Aa × Aa), select both as Aa.
Offspring ratios are AA 0.25, Aa 0.5, aa 0.25 — each pregnancy has about a 25% chance of an affected (aa) child, 50% chance of being a carrier, 25% completely normal.
This is the most common scenario in genetic counseling, helping carrier parents understand the next child's disease and carrier probabilities.
Test cross to determine a dominant individual's genotype
A dominant phenotype could be AA or Aa, indistinguishable by eye. A breeder crosses it with a homozygous recessive aa ('test cross').
If all offspring are dominant, the unknown is AA (AA × aa gives all Aa); if recessive appears (about 1:1), the unknown is Aa.
Selecting Aa × aa here gives Aa 0.5, aa 0.5, the 1:1 result that is exactly the test-cross discriminant.
FAQ
What is a Punnett square?
A Punnett square is a diagram predicting offspring genotype ratios: gametes of the parents are listed around the square, and cells combine into offspring genotypes. For one locus with two alleles it is a 2×2 square with 4 combinations.
Why does Aa × Aa give 3:1?
Aa × Aa offspring genotypes are 1 AA : 2 Aa : 1 aa. Because A is completely dominant over a, both AA and Aa show the dominant trait, totaling 3 parts, while aa shows the recessive trait at 1 part — hence the phenotype ratio 3:1.
Can this calculator do two-gene (dihybrid) crosses?
This calculator targets single-locus monohybrid crosses. Two traits (dihybrid, giving 9:3:3:1) need a dihybrid Punnett square, a different calculation that could be added as an extension.
How is the dominant phenotype ratio computed?
The dominant phenotype includes all offspring carrying at least one dominant allele, i.e. the AA and Aa ratios summed; the recessive phenotype is only aa. This calculator sums and displays them automatically.
Is the Punnett square suitable for DSE Biology?
Very much so. Mendelian genetics and the Punnett square are core content of the Hong Kong DSE Biology 'Genetics' unit. This calculator instantly verifies offspring ratios, helping study and check answers.
Related Tools
References
Content review: Calculatorism Science Team. Results are for reference only; please refer to the relevant authorities for the official figures.