Punnett Square Generator

2026-10-06

Enter the genotypes of two parents — one, two or three genes — to draw the Punnett square and get the gametes, every offspring genotype and phenotype with its fraction and percentage, and ratios such as 3 : 1 and 9 : 3 : 3 : 1. Works with complete dominance, incomplete dominance and codominance.

How to use the Punnett square generator

1. Type each parent's genotype, one letter per gene: Aa, AaBb or AaBbCc. A capital letter is one allele, the small letter the other. Both parents must list the same genes in the same order.
2. Or tap an example to load a classic cross.
3. Open Gene settings to switch a gene to incomplete dominance or codominance, or to name its trait and phenotypes (such as “yellow”).
4. Choose a grid that lists every gamete (the classic square) or one that merges identical gametes; the ratios are the same.
5. Read the square, the ratio lines and the tables. Probability of… gives the chance of one outcome; Copy ratios and Print square export the result.

How it works

A Punnett square is a grid that shows every way the gametes (sex cells) of two parents can combine. One parent's gametes head the columns, the other's head the rows, and each cell holds the genotype of the offspring formed by that pair. It is named after the British geneticist Reginald Punnett, who devised it in the early 1900s.

• Allele — one version of a gene. Diploid organisms such as peas and people carry two alleles of most genes, one from each parent.
• Genotype — the pair of alleles: AA, Aa or aa. Two identical alleles are homozygous, two different ones heterozygous.
• Phenotype — the trait actually observed. With complete dominance AA and Aa both show the dominant phenotype; only aa shows the recessive one.

To fill one in by hand, as the tool does:

1. Find the gametes. Mendel's law of segregation: the two alleles of a gene separate when gametes form, so each gamete receives one of them, either one with probability 1/2. His law of independent assortment: the alleles of different genes are distributed independently of each other. A parent with n genes is therefore listed with 2n equally likely gametes: AaBb gives AB, Ab, aB and ab.
2. Draw the grid with one parent's gametes across the top and the other's down the side.
3. Fill each cell with the alleles from its column and row, capital letter first: Aa, not aA.
4. Count. All cells are equally likely, so probability = matching cells ÷ total cells. The full grid has 2n × 2n cells: 4, 16 or 64.

Fractions are in lowest terms, ratios are divided by their greatest common divisor and percentages are exact, not rounded (1/64 = 1.5625%). Merging identical gametes — an AA parent only makes A — shrinks the grid without changing any ratio.

Examples

Example 1 — monohybrid cross Aa × Aa. Each parent makes the gametes A and a. The four cells are AA, Aa, Aa and aa: genotype ratio 1 : 2 : 1 (25%, 50%, 25%) and phenotype ratio 3 : 1 — 3/4 = 75% dominant, 1/4 = 25% recessive.

Example 2 — test cross Aa × aa. Two cells are Aa and two are aa: 1 : 1, 50% each. A test cross reveals a hidden genotype — if an individual with the dominant phenotype has any recessive offspring, it must be heterozygous.

Example 3 — dihybrid cross AaBb × AaBb. 16 cells, 9 genotypes and a phenotype ratio of 9 : 3 : 3 : 1 (56.25%, 18.75%, 18.75%, 6.25%). AaBb fills 4 of 16 cells = 1/4 = 25%; aabb fills 1 of 16 = 6.25%. In Mendel's peas, with A for yellow seeds over green and B for round over wrinkled, that is 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled.

Example 4 — incomplete dominance Rr × Rr. The last example button sets gene R to incomplete dominance. Phenotypes now follow genotypes: 1 red : 2 pink : 1 white (25%, 50%, 25%).

Where the classic ratios come from

• 3 : 1 — Aa × Aa with complete dominance: one AA and two Aa cells look dominant, one aa cell looks recessive.
• 1 : 2 : 1 — the genotype ratio of that cross, and its phenotype ratio whenever the heterozygote looks different from both homozygotes.
• 9 : 3 : 3 : 1 — AaBb × AaBb. Each gene alone gives 3 : 1 and independent genes multiply: 3/4 × 3/4 = 9/16, 3/4 × 1/4 = 3/16 (twice) and 1/4 × 1/4 = 1/16.
• 1 : 1 : 1 : 1 — the dihybrid test cross AaBb × aabb: the four gametes of the first parent each meet ab.

Incomplete dominance and codominance

In incomplete dominance the heterozygote's phenotype lies between those of the two homozygotes. The textbook case is the snapdragon: a red-flowered plant crossed with a white-flowered one gives pink offspring, and pink × pink gives 1 red : 2 pink : 1 white. The alleles themselves do not blend — red and white reappear unchanged in the next generation.

In codominance both alleles are fully expressed side by side. Roan shorthorn cattle are the classic example: a calf with one red-coat and one white-coat allele has a coat of mixed red hairs and white hairs, and roan × roan gives 1 red : 2 roan : 1 white. In the human MN blood group, heterozygotes likewise carry both the M and the N antigen on their red blood cells.

In both cases every genotype has its own phenotype, so the phenotype ratio equals the genotype ratio.

Limits of a Punnett square

• Ratios are probabilities, not guarantees. Each offspring is an independent event, so four offspring of Aa × Aa are often not exactly 3 : 1. Even Mendel's large counts were only close: 5,474 round and 1,850 wrinkled seeds is 2.96 : 1.
• Linked genes. Genes close together on the same chromosome tend to be inherited together, so they do not assort independently and 9 : 3 : 3 : 1 fails. This tool always assumes independent assortment.
• Complex traits. Many traits, such as human height or skin colour, are polygenic (controlled by many genes) and also shaped by the environment.
• Not covered: sex-linked traits (such as red-green colour blindness, carried on the X chromosome), genes with more than two alleles (such as the ABO blood groups, which have three) and interactions between genes.

Frequently asked questions

What is the difference between genotype and phenotype?

The genotype is the set of alleles an organism carries (Aa); the phenotype is the trait it shows (yellow seeds). Under complete dominance AA and Aa look alike, which is why the two ratios differ.

How many boxes does a Punnett square have?

Four for one gene (2 × 2), 16 for two genes (4 × 4) and 64 for three (8 × 8): each extra gene doubles the gametes of both parents.

How do I find the gametes for a dihybrid cross?

Take one allele of each gene in every possible combination. For AaBb: A with B, A with b, a with B and a with b — the “first, outer, inner, last” pattern used to multiply two brackets in algebra.

Does a 3 : 1 ratio mean every fourth child is recessive?

No. Each child has a 1/4 chance, independently of its brothers and sisters. Small families often differ from the ratio; only large numbers of offspring come close to it.

Is anything I enter uploaded?

No. The square is calculated by JavaScript in your browser. Your genotypes and names are saved only in this browser's local storage; nothing is sent to a server.

Related tools

• Percentage Calculator — turn a fraction of offspring into a percentage.
• Galton Board — watch chance build a bell curve.
• Genetic Algorithm — selection, crossover and mutation at work in a simulated population.
• Molar Mass Calculator — the molar mass of any chemical formula.