A Punnett square is the standard way to work out the possible genetic outcomes of a cross. This Punnett square calculator takes the genotype of both parents and returns the full grid with the probability of each combination, plus the resulting phenotype ratio in the familiar 3:1 or 2:1 form that textbooks use.
What Is the Punnett Square Calculator?
A Punnett square lays out every possible combination of alleles that two parents can pass on. Each parent contributes one allele per offspring, and the square lists every pairing of those alleles with the probability of each outcome.
The calculator takes the genotype of both parents and returns the grid, the four possible offspring genotypes with their percentages, and the phenotype ratio that follows from those genotypes.
How the Cross Works
Each parent contributes one allele from each of its two gametes. A homozygous parent, AA or aa, contributes only one allele type, so half the grid is identical. A heterozygous parent, Aa, contributes either A or a, which is what produces the 1:2:1 genotype ratio.
When you look at phenotypes rather than genotypes, the three genotypes that include at least one dominant allele all show the dominant trait, which is why the classic monohybrid cross of two heterozygotes gives a phenotype ratio of 3:1.
The Four Common Crosses
AA crossed with AA gives all homozygous dominant offspring, and the phenotype ratio is 4:0. AA crossed with aa gives all heterozygous offspring with a 4:0 phenotype ratio. AA crossed with Aa gives a 1:1 genotype and phenotype ratio, and Aa crossed with Aa gives the classic 1:2:1 genotype ratio and 3:1 phenotype ratio.
A cross of Aa with aa gives a 1:1 ratio and is known as a test cross. It is used to determine the genotype of an organism that shows the dominant phenotype, because the ratio of offspring immediately reveals whether the unknown parent is heterozygous or homozygous.
What the Square Does Not Show
The ratios are probabilities, not promises. A 3:1 ratio in a cross of two heterozygotes is the expectation over a large number of offspring, and a small family of four will often show something quite different.
The square also assumes a single gene with two alleles, simple dominance and independent assortment. Real traits are usually more complicated, because they are often polygenic, because the environment affects them, or because the gene has more than two alleles in the population.
Frequently Asked Questions
What does a Punnett square show?
It shows every possible allele combination a cross between two parents can produce, with the probability of each outcome, which is written as a 1:2:1 or 3:1 ratio.
What is the difference between genotype and phenotype ratio?
The genotype ratio counts the different allele combinations, so Aa crossed with Aa gives 1 AA, 2 Aa and 1 aa. The phenotype ratio counts how those look, which gives 3 dominant to 1 recessive because AA and Aa look the same.
Why is the ratio 3:1 and not 1:2:1?
Both are correct at different levels. The genotype ratio is 1:2:1 because there are three distinct genotypes. The phenotype ratio is 3:1 because AA and Aa are indistinguishable by appearance and both show the dominant trait.
What is a test cross?
It is a cross between an organism showing the dominant trait and a homozygous recessive individual. The ratio of offspring then reveals whether the dominant parent is heterozygous, so a 1:1 ratio means heterozygous and an all dominant result means homozygous.
Does a 3:1 ratio always appear?
No. It is the expected ratio over a large number of offspring. With a small number of offspring, chance can produce results that look very different, which is why small family traits often appear to skip a generation.