Punnett square maker with genotype and phenotype ratios

How to make a Punnett square

A Punnett square lists the gametes one parent can make along the top, the gametes of the other parent down the side, and fills each cell with the offspring genotype formed when those two gametes meet. Under Mendelian assumptions every gamete combination is equally likely, so each cell carries the same probability. You can do it by hand in five steps, or let the Punnett square maker above do the bookkeeping.

• Write each parent genotype with one letter per gene: uppercase for the dominant allele and lowercase for the recessive allele. AaBb is heterozygous at two genes.
• List the gametes. Each gamete receives one allele from every gene, so a parent heterozygous at n genes makes 2^n gamete types: Aa gives A and a, while AaBb gives AB, Ab, aB, and ab.
• Draw a grid with one column per gamete from the first parent and one row per gamete from the second. One gene gives a 2×2 grid, two genes a 4×4 grid, and three genes an 8×8 grid of 64 cells.
• Fill each cell by combining the row and column alleles, then count identical genotypes and group them by phenotype.
• Divide each count by the number of cells to get the expected probability. This Punnett square calculator does the counting, simplifies the fractions, and lists genotypes in a fixed order with dominant alleles first.

Genotype vs phenotype

The genotype is the pair of alleles an individual carries, such as AA, Aa, or aa. The phenotype is the trait you observe, such as purple or white flowers. Different genotypes can share one phenotype: with complete dominance AA and Aa look the same, which is why a monohybrid cross has three genotypes but only two phenotypes. Dominance decides how genotypes map to phenotypes, and you can set it separately for every gene in the tool and give each phenotype its own name.

• Complete dominance: one copy of the dominant allele is enough, so heterozygotes look like the dominant homozygote.
• Incomplete dominance: heterozygotes show an intermediate phenotype, like pink flowers from a cross between red and white snapdragons.
• Codominance: heterozygotes express both alleles. In the ABO blood group, the IA and IB alleles are codominant and i is recessive, so IAIB is type AB and ii is type O.
• Sex-linked (X-linked) genes: males carry a single X chromosome, so one recessive allele on the X is expressed. Use X^A, X^a, and Y notation for a sex-linked Punnett square and read the phenotype table separately for daughters and sons.

Classic Punnett square ratios

These are the expected ratios for unlinked genes. Load a preset above to see the full grid behind each one, and use the genetics cross calculator to compare them with the counts you actually observed.

• 3:1 phenotypes and 1:2:1 genotypes: the monohybrid cross Aa × Aa with complete dominance, the ratio Mendel reported for pea traits such as flower color and seed shape.
• 1:2:1 phenotypes: the same Aa × Aa cross with incomplete dominance or codominance, because each genotype now has its own phenotype.
• 9:3:3:1 phenotypes: the dihybrid cross AaBb × AaBb with complete dominance at both genes, spread over 16 cells and nine genotypes.
• 1:1:1:1 phenotypes: the dihybrid test cross AaBb × aabb, or the ABO cross IAi × IBi, which gives types A, B, AB, and O in equal proportions.
• 1:1 phenotypes: the monohybrid test cross Aa × aa, used to find out whether an individual with the dominant phenotype is heterozygous.
• 27:9:9:9:3:3:3:1 phenotypes: the trihybrid cross AaBbCc × AaBbCc across 64 cells.

Assumptions and limits

A Punnett square gives expected proportions, not guaranteed outcomes. It rests on assumptions that real inheritance often breaks, so treat the ratios as a model to compare with observed counts, for example with a chi-square goodness-of-fit test.

• Independent assortment: every gene is assumed to sort independently of the others. Genes close together on the same chromosome are linked and are inherited together more often than the grid predicts, unless crossing over separates them.
• Equal survival: every genotype is assumed to be equally viable. Lethal genotypes remove cells from the real ratio.
• Large numbers: the ratios are probabilities. A family with four children can easily differ from 3:1 by chance, and only large populations approach the expected proportions.
• One gene, one trait: epistasis, where one gene masks the effect of another, turns 9:3:3:1 into ratios such as 9:3:4 or 9:7. Polygenic traits such as height depend on many genes plus the environment.
• Not modeled: incomplete penetrance, variable expressivity, mitochondrial inheritance, and genomic imprinting.

Using the grid in slides, worksheets, and papers

The SVG export keeps every label as text, so you can restyle it in a vector editor such as Inkscape or Illustrator. The PNG export is rendered at twice the on-screen size for slides and handouts, and the CSV contains the genotype table, the phenotype table, and the full grid for spreadsheets. Cell colors come from Paul Tol's light qualitative scheme, which was designed for cells with dark labels on top. Every cell also carries a phenotype code (P1, P2, and so on) that matches the key, with the HEX value printed next to each swatch, so the square still reads in grayscale and for readers with color vision deficiency.