Punnett Square Calculator

Frequently Asked Questions

What does a Punnett square show?

A Punnett square displays all possible genotype combinations in offspring from two parents, given their alleles. Each cell in the grid represents one equally probable offspring genotype. The grid assumes simple dominance and independent assortment.

What is the expected phenotype ratio for Aa × Aa?

Crossing two heterozygotes (Aa × Aa) yields genotypes 1 AA : 2 Aa : 1 aa and phenotypes 3 dominant : 1 recessive, because AA and Aa both express the dominant phenotype.

What is the expected ratio for a dihybrid cross AaBb × AaBb?

The classic 9:3:3:1 phenotype ratio. Nine offspring show both dominant traits (A_B_), three show only the first dominant (A_bb), three show only the second dominant (aaB_), and one shows both recessive traits (aabb).

What is the ratio for a carrier test cross (Aa × aa)?

Crossing a heterozygous carrier with a homozygous recessive partner (Aa × aa) gives a 1:1 ratio: half the offspring are Aa (carriers with the dominant phenotype) and half are aa (recessive phenotype). This test cross is the classic way to reveal whether a dominant-looking parent is AA or Aa, since only an Aa parent can produce recessive offspring.

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How This Calculator Works

A Punnett square is a grid-based tool for predicting the possible genotypes of offspring when two parents with known genotypes are crossed. Each axis of the grid lists the gametes (reproductive cells) that one parent can produce. Each cell in the grid represents one possible offspring genotype formed by combining one gamete from each parent. Because each cell is equally likely, counting the genotypes across the grid gives you the probability of each outcome.

For a monohybrid cross (one gene), the grid is 2x2. For a dihybrid cross (two independent genes), each parent produces four gamete types and the grid is 4x4. This calculator handles both cases and assumes simple dominance: one allele (uppercase) is completely dominant over the other (lowercase).

Genotype vs Phenotype

A genotype is the actual pair of alleles an organism carries for a given gene, such as AA, Aa, or aa. A phenotype is the observable physical trait that results from those alleles. With simple dominance, having at least one dominant allele (A_) produces the dominant phenotype, while having two recessive alleles (aa) produces the recessive phenotype. This is why the phenotype ratio in a standard Aa x Aa cross is 3:1 even though the genotype ratio is 1:2:1 - two of the three genotype categories look identical.

Worked Example: Aa x Aa (Monohybrid)

Consider crossing two heterozygous parents (Aa x Aa). Each parent produces gametes A and a. The 2x2 grid gives four cells: AA, Aa, aA, aa. Combining like terms, the genotype ratio is 1 AA : 2 Aa : 1 aa. Since AA and Aa both express the dominant phenotype, the phenotype ratio is 3 dominant : 1 recessive. In pea plants, this is how Mendel got a 3:1 ratio of tall to short offspring from two heterozygous tall parents.

Dihybrid Ratios: AaBb x AaBb

When two genes are on different chromosomes (or far enough apart on the same chromosome to be effectively unlinked), they assort independently. In the classic AaBb x AaBb cross, each parent makes four gamete types - AB, Ab, aB, ab - in equal proportions. The 4x4 grid produces 16 equally likely cells. Counting the four phenotype classes gives the famous 9:3:3:1 ratio:

9/16 offspring show both dominant traits (A_B_)

3/16 show dominant A only (A_bb)

3/16 show dominant B only (aaB_)

1/16 show both recessive traits (aabb)

This ratio only holds when both genes assort independently. If the genes are linked (physically close on the same chromosome), recombination rates affect the actual proportions, and a Punnett square no longer gives accurate predictions without additional data.

Tips and FAQ

What does A_ mean in the phenotype table?

The underscore is a wildcard standing for any allele - either A or a. A_B_ means the offspring has at least one A allele and at least one B allele, so it expresses both dominant phenotypes regardless of whether it is homozygous (AA) or heterozygous (Aa) at each locus.

Can I use this for more than two genes?

For three genes, use the Trihybrid Cross Calculator on this site, which handles each gene pair independently and multiplies the probabilities using the product rule for independent events. For four or more genes, the same multiplication principle applies but the grid becomes impractically large to draw.

What if dominance is not simple?

This calculator assumes complete dominance. In incomplete dominance (e.g. red x white flowers producing pink offspring), the heterozygote shows an intermediate phenotype, so you would reclassify the Aa cells separately. In codominance (e.g. blood type AB), both alleles are expressed simultaneously. The genotype grid itself is still correct - only the phenotype groupings change based on the inheritance pattern.

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