Punnett Square Generator
Generate Punnett squares for monohybrid and dihybrid genetic crosses. Enter parent genotypes, optionally name the dominant and recessive phenotypes, and instantly see the full grid with genotype and phenotype ratios. No signup, runs entirely in your browser.
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
Genotype ratio
Phenotype ratio
How to Use the Punnett Square Generator
- 1Enter both parent genotypes — two letters for a monohybrid cross (Aa), four for a dihybrid (AaBb).
- 2Optionally name the phenotypes (e.g. “Purple” / “White”) so the ratio table uses real trait labels.
- 3Read the colour-coded grid — green homozygous dominant, yellow heterozygous, red homozygous recessive.
- 4Check the genotype and phenotype ratios summarised below the square.
Worked Example: A Dihybrid Cross and the 9:3:3:1 Ratio
Cross two pea plants that are both heterozygous for seed shape and colour: AaBb × AaBb, where A = round (dominant), a = wrinkled, B = yellow (dominant), b = green. Each parent makes four gamete types — AB, Ab, aB, ab — so the generator builds a 4×4 grid with 16 boxes. Counting phenotypes gives the classic 9 : 3 : 3 : 1 ratio: 9 round-yellow, 3 round-green, 3 wrinkled-yellow, and 1 wrinkled-green.
The single wrinkled-green plant in 16 is the visible signature of two recessive alleles landing together (aabb) — a 1/16 = 6.25% chance. This is the exact experiment Mendel ran to discover the law of independent assortment: the two traits are inherited separately, so the 3:1 ratio for each trait alone multiplies out to 9:3:3:1 when tracked together. Switch the inputs to a monohybrid Aa × Aa to see that same 3:1 collapse to a simple 2×2 grid — the generator scales the square to match whichever cross you enter.
Genetics Tips
Monohybrid cross (2×2)
Enter Aa × Aa for the classic 1:2:1 genotype ratio and 3:1 phenotype ratio. Try AA × aa to see why F1 offspring are all heterozygous (Aa).
Dihybrid cross (4×4)
Enter AaBb × AaBb for the 9:3:3:1 phenotype ratio. This demonstrates Mendel's law of independent assortment when two genes are on separate chromosomes.
Name your phenotypes
Use the optional trait name fields to label the dominant and recessive phenotypes (e.g. "Purple" / "White" for flower colour). The phenotype ratio table will use your labels.
Test crosses
Enter an unknown genotype × aa to perform a test cross. If all offspring show the dominant phenotype, the parent is homozygous dominant (AA). If half show recessive, the parent is heterozygous (Aa).
Frequently Asked Questions
What genotype format should I use?
For a monohybrid cross, enter a two-letter genotype like Aa, AA, or aa — uppercase for dominant, lowercase for recessive. For a dihybrid cross, enter four letters like AaBb or AAbb. Both parents must have the same number of genes.
What is a Punnett square?
A Punnett square is a grid used to predict the probability of offspring genotypes from a genetic cross. Each cell represents one possible offspring, and the frequency of each genotype gives the probability of that offspring type.
What is the 3:1 ratio?
In a monohybrid cross between two heterozygous parents (Aa × Aa), 3 out of 4 offspring have the dominant phenotype (AA or Aa) and 1 out of 4 has the recessive phenotype (aa). This 3:1 phenotype ratio is Mendel's first law in action.
How does a dihybrid cross work?
A dihybrid cross (e.g. AaBb × AaBb) considers two genes simultaneously. Each parent produces 4 types of gametes (AB, Ab, aB, ab), giving a 4×4 grid with 16 possible offspring combinations. The expected phenotype ratio is 9:3:3:1.
What do the colours mean?
Green cells show homozygous dominant offspring (e.g. AA), yellow cells show heterozygous offspring (e.g. Aa), and red cells show homozygous recessive offspring (e.g. aa).
Is my data stored?
No. All calculations run entirely in your browser. No data is sent to any server.