Type the genotypes of two parents and this free online Punnett square calculator draws the whole square for you: every gamete along the edges, every offspring genotype in the cells, and the genotype and phenotype ratios worked out underneath. Use it as a Punnett square maker or generator for homework, a solver to check your own grid, or a simulator to explore what happens when you change the parents. It handles monohybrid, dihybrid, trihybrid, tetrahybrid and pentahybrid crosses, complete, incomplete and codominant inheritance, and it lets you switch between genotype letters, colour-coded phenotypes and picture themes such as Mendel’s peas.
Reginald Punnett drew the first Punnett square in 1905 to work out the results of Mendel’s pea crosses. The idea is simple. Each parent makes gametes that carry one allele of every gene. Write one parent’s gametes across the top of a grid and the other parent’s down the side, and each cell of the grid is one way the gametes can meet. Because every cell is equally likely, counting cells gives the probability of each offspring genotype, and grouping cells that look the same gives the phenotype ratio.
By hand, a Punnett square takes four steps. Work out each parent’s gametes: a parent with genotype Aa makes A and a gametes, and a parent with AaBb makes AB, Ab, aB and ab. Draw a grid with one parent’s gametes across the top and the other’s down the side, a 2 × 2 grid for one gene and a 4 × 4 grid for two. Fill each cell with the allele from its column followed by the allele from its row, dominant letter first. Finally count the cells: the tally of each genotype is the genotype ratio, and grouping cells with the same appearance gives the phenotype ratio and the probability of each outcome.
The calculator does the same thing instantly:
Aa × Aa or AaBb × aabb. The presets fill in a P generation cross, an F1 × F1 cross or a test cross.Crossing two heterozygous parents, Aa × Aa, fills a 2 × 2 Punnett square with AA, Aa, Aa and aa. That is a 1:2:1 genotype ratio. With complete dominance the AA and Aa offspring look the same, so three quarters show the dominant trait and one quarter the recessive trait, the 3:1 ratio Mendel observed in his F2 peas. A test cross, Aa × aa, gives 1:1 and is the classic way to find out whether a dominant-looking parent is homozygous or heterozygous. The monohybrid cross and test cross pages open with these crosses ready, and the pea plant Punnett square draws Mendel’s seed colour and shape as peas.
With two genes each parent makes four kinds of gametes, so the square has 16 cells. AaBb × AaBb produces the 9:3:3:1 phenotype ratio: nine offspring dominant for both traits, three dominant for the first trait only, three for the second only and one recessive for both. This is Mendel’s law of independent assortment in action, and it only holds when the two genes are on different chromosomes or far apart on the same one. Three genes give a 64-cell square and a 27:9:9:9:3:3:3:1 ratio, so the same tool serves as a dihybrid cross calculator and a trihybrid cross calculator; it goes on to four and five genes, where the square has 256 and 1,024 cells. The dedicated dihybrid cross and genotype-phenotype ratio pages open with those crosses ready.
Not every trait follows the dominant and recessive pattern. In incomplete dominance the heterozygote is intermediate, so red × white snapdragons give pink flowers and the phenotype ratio is 1:2:1. In codominance both alleles show at the same time, as in the AB blood group. The calculator has a mode for each, and separate tools cover X-linked inheritance, blood type with its three ABO alleles, and allele frequencies in a population with the Hardy-Weinberg calculator.
A Punnett square is a grid that shows every possible combination of the alleles two parents can pass on. Each parent's gametes are written along the top and the side, and each cell is one possible offspring genotype. Counting the cells gives the genotype and phenotype ratios of the cross.
Write one parent's possible gametes across the top and the other parent's down the side, then fill each cell with the allele from its column and the allele from its row. For a monohybrid cross Aa × Aa the four cells are AA, Aa, Aa and aa, a 1:2:1 genotype ratio and 3:1 phenotype ratio. This calculator fills the square for you for one to five genes.
A dihybrid cross between two heterozygotes (AaBb × AaBb) gives the classic 9:3:3:1 phenotypic ratio: 9 dominant for both traits, 3 dominant for the first only, 3 dominant for the second only and 1 recessive for both. The genotype ratio is 1:2:1:2:4:2:1:2:1 across the 16 cells.
The genotype ratio counts the allele combinations themselves (AA : Aa : aa), while the phenotype ratio groups genotypes that look the same. Under complete dominance AA and Aa share the dominant phenotype, so 1:2:1 becomes 3:1. Under incomplete dominance or codominance the heterozygote looks different, so both ratios are 1:2:1.
Yes. Switch the dominance mode to Incomplete for blended heterozygotes, such as pink flowers from red and white parents, or Codominant for heterozygotes that show both traits, such as AB blood type. The square, colours and ratios update to match.
Up to five genes. A monohybrid cross has 4 cells, a dihybrid 16, a trihybrid 64, a tetrahybrid 256 and a pentahybrid 1,024. The calculator draws all of them and works out the ratios, gametes and probabilities automatically.