Punnett square calculator
Enter two parent genotypes and instantly see the Punnett square, genotype ratios, phenotype ratios, and a plain-English explanation of the cross.
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Use the pedigree tool to map multiple generations and find which inheritance pattern fits.
Open the pedigree toolHow the Punnett square calculator works
This tool takes the genotypes of two parents and constructs a Punnett square showing every possible combination of alleles in their offspring. It then counts the results and reports both the genotype ratio (the genetic makeup) and the phenotype ratio (the observable traits).
What a Punnett square shows
A Punnett square is a grid that organizes all possible gametes from each parent along two axes. Each cell in the grid represents one possible genotype for an offspring. For a monohybrid cross (one gene), the grid is 2x2 and produces four cells. For a dihybrid cross (two genes), the grid is 4x4 and produces sixteen cells. The square itself does not predict which offspring will actually be born. It gives you the probabilities.
Genotype vs phenotype ratios
The genotype ratio breaks down the offspring by their genetic combinations. In a monohybrid cross between two heterozygous parents (Aa x Aa), the genotype ratio is 1 AA : 2 Aa : 1 aa. The phenotype ratio collapses those genotypes into observable categories. If A is dominant over a, the phenotype ratio becomes 3 dominant : 1 recessive, because both AA and Aa look the same. The 3:1 ratio is the signature of a monohybrid cross between two heterozygotes.
The 9:3:3:1 dihybrid ratio
For a dihybrid cross between double heterozygotes (AaBb x AaBb), the phenotype ratio is 9:3:3:1. Nine offspring show both dominant traits, three show only the first dominant trait, three show only the second, and one shows both recessive traits. This ratio only appears when both genes assort independently, which is what Mendel's law of independent assortment describes. If the genes are linked on the same chromosome, the actual ratios will deviate from 9:3:3:1.
Limitations
This tool assumes simple dominant and recessive relationships. It does not handle codominance, incomplete dominance, or sex-linked genes (where you would need to track X and Y chromosomes separately). It also assumes independent assortment, so linked genes will produce results that do not match real offspring. For those cases, a pedigree chart is more useful.
Frequently asked questions
What is a Punnett square?
A Punnett square is a diagram used in genetics to predict the possible genotypes of offspring from a particular cross. Parent gametes are listed along the top and left side of a grid, and each cell shows one possible genetic combination for a child. It was developed by Reginald Punnett in 1905.
What is the difference between monohybrid and dihybrid cross?
A monohybrid cross tracks the inheritance of a single gene. A dihybrid cross tracks two genes at the same time. Monohybrid crosses produce a 2x2 Punnett square with four outcomes. Dihybrid crosses produce a 4x4 square with sixteen outcomes and can reveal whether the two genes assort independently.
What does a 3:1 ratio mean?
A 3:1 phenotype ratio means that for every four offspring, you expect three to show the dominant trait and one to show the recessive trait. This ratio appears in a monohybrid cross between two heterozygous parents (Aa x Aa). It is one of the most basic patterns in Mendelian genetics.
What is the phenotype ratio of a dihybrid cross?
A dihybrid cross between two double heterozygotes (AaBb x AaBb) produces a 9:3:3:1 phenotype ratio. Nine show both dominant traits, three show only the first dominant trait, three show only the second dominant trait, and one shows both recessive traits. This assumes the two genes are on different chromosomes or far enough apart to assort independently.
What is the law of segregation?
The law of segregation states that the two alleles a person carries for a given gene separate during gamete formation. Each sperm or egg receives only one of the two alleles. This is why a parent who is Aa can pass either an A or an a to each child, not both. Mendel figured this out without knowing what DNA or chromosomes were.