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.
⏱ 7 min read · Complete guide below
| 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.
Genotype vs Phenotype, Dominant vs Recessive
Two pairs of terms unlock the whole subject. An organism's genotype is its genetic makeup — the actual alleles it carries, such as Aa — while its phenotype is the observable trait those alleles produce, such as purple flowers. The link between them comes from dominance: a dominant allele (written uppercase) masks a recessive one (lowercase), so both AA and Aa show the dominant trait, and only aa shows the recessive one. This is why a 1:2:1 genotype ratio produces a 3:1 phenotype ratio in a classic cross — three of the four genotype combinations display the dominant appearance. The colour coding in the grid makes this visible at a glance: homozygous dominant, heterozygous, and homozygous recessive each get their own colour.
Mendel's Laws Behind the Square
The Punnett square is a visual shortcut for the principles Gregor Mendel deduced from breeding pea plants. His law of segregation says that the two alleles for a trait separate during the formation of gametes, so each gamete carries just one — which is why a heterozygous Aa parent produces equal numbers of A and a gametes along the edge of the square. His law of independent assortment says that alleles for different traits are distributed independently, which is why a dihybrid cross multiplies out to the 9:3:3:1 ratio rather than something simpler. The square simply lays out every equally likely combination of parental gametes so you can count the results directly.
Using Punnett Squares in Practice
Beyond the classroom, this kind of reasoning answers real questions. A test cross — pairing an individual showing the dominant trait with a homozygous recessive partner — reveals a hidden genotype: if any offspring show the recessive trait, the unknown parent must have been heterozygous. The same probabilities underlie genetic-counselling estimates, such as the chance that two carriers of a recessive condition have an affected child (one in four for a simple single-gene trait). Remember, though, that a Punnett square gives probabilities for each offspring, not guarantees — real families are small samples, so actual ratios vary around the predicted ones, exactly as coin flips scatter around fifty-fifty.
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.
What is the difference between genotype and phenotype?
Genotype is the genetic makeup — the specific alleles an organism carries, such as Aa or AaBb. Phenotype is the observable trait those alleles produce, such as purple flowers or round seeds. Because a dominant allele masks a recessive one, different genotypes can share the same phenotype: both AA and Aa look dominant, while only aa shows the recessive trait. This tool reports both the genotype ratio and the phenotype ratio for exactly this reason.
What is a test cross and how do I do one?
A test cross is used to determine whether an individual showing a dominant trait is homozygous (AA) or heterozygous (Aa), which you cannot tell by looking. You cross it with a homozygous recessive partner (aa). Enter that in the generator: if all offspring show the dominant trait, the unknown parent is AA; if about half show the recessive trait, it is Aa. It is the classic way geneticists reveal a hidden genotype.
Why did Mendel get a 9:3:3:1 ratio in dihybrid crosses?
Because of his law of independent assortment: the alleles for two different traits are inherited independently of each other. Each trait on its own follows a 3:1 dominant-to-recessive ratio, and when two independent traits are tracked together, those ratios multiply out to 9:3:3:1 across the sixteen boxes of the 4×4 square. The single 1/16 double-recessive offspring is the visible signature of both recessive alleles landing together.
Does a Punnett square guarantee the exact offspring ratios?
No — it gives probabilities, not certainties. A 3:1 ratio means each offspring has a 75% chance of the dominant phenotype and 25% chance of the recessive one, but any real litter or plant batch is a small sample and will scatter around those figures, just as flipping a coin ten times rarely gives exactly five heads. The larger the number of offspring, the closer the observed ratios tend to match the predicted ones.
Can this handle traits that are not simple dominant/recessive?
This generator models standard Mendelian inheritance with complete dominance, which covers most introductory genetics problems and the classic mono- and dihybrid crosses. More complex patterns — such as incomplete dominance, codominance, multiple alleles, sex-linked traits, or gene linkage — follow different rules and ratios that a basic Punnett square does not fully capture. For those, the square is still a useful starting point but must be adapted to the specific inheritance pattern.