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Punnett Square Calculator

Draw the Punnett square for any cross of one to four genes and get the exact genotype and phenotype ratios. Choose your own allele symbols and set each gene to complete dominance, incomplete dominance or codominance. Select a genotype or phenotype to highlight its cells in the square.

Biology · InheritanceUpdated Sources cited

Genes

Give each gene two allele symbols. With complete dominance, the first symbol is the dominant allele. Symbols are case-sensitive.

Gene 1
Examples

Result

Aa × Aa

Genotype ratio 1:2:1

Phenotype ratio 3:1

Aa
AAAAa
aAaaa

Select a genotype or phenotype below to highlight its cells in the square.

Genotypes
GenotypeCellsProbabilityPercent
1 / 41/425%
2 / 41/250%
1 / 41/425%
Phenotypes
PhenotypeCellsProbabilityPercent
dominant A trait3 / 43/475%
recessive a trait1 / 41/425%
Assumptions behind this result
  • Each parent passes on one allele of each gene, chosen at random (law of segregation).
  • The genes are autosomal (not sex-linked), and every genotype is equally likely to survive.
  • Each gene has exactly two alleles, and there is no epistasis (genes do not affect each other’s phenotypes).

Step-by-step solution

  1. List the gametes each parent can make

    Each parent passes on one of its two alleles, each with probability ½ (law of segregation).

    Aa→A, aAa→A, a\text{Aa} \to \text{A},\ \text{a} \qquad \text{Aa} \to \text{A},\ \text{a}

  2. Combine every gamete from parent 1 with every gamete from parent 2

    The square has 2 × 2 = 4 cells. Each cell is one equally likely fertilisation, so each has probability 1/4.

  3. Count the genotypes

    There are 3 different genotypes. Dividing each count by 4 gives its probability.

    AA:14, Aa:24, aa:14⇒1:2:1\text{AA}: \tfrac{1}{4},\ \text{Aa}: \tfrac{2}{4},\ \text{aa}: \tfrac{1}{4}\quad\Rightarrow\quad 1 : 2 : 1

  4. Group the genotypes into phenotypes

    A is dominant over a, so AA and Aa look the same. This gives 2 phenotypes.

    A_:34, aa:14⇒3:1\text{A\_}: \tfrac{3}{4},\ \text{aa}: \tfrac{1}{4}\quad\Rightarrow\quad 3 : 1

What the calculator does

  • Draws the full Punnett square for one to four genes: 4, 16, 64 or 256 cells.
  • Lists every genotype and phenotype with its count, exact fraction and percentage.
  • Handles complete dominance, incomplete dominance and codominance, set separately for each gene.
  • Shows each step and, for two or more genes, checks the result with the product rule.

How a Punnett square works

Each parent makes gametes carrying one allele of every gene. For a parent that is heterozygous for n genes, there are 2ⁿ different gametes, all equally likely. The square pairs every gamete of one parent with every gamete of the other, so each cell is one equally likely outcome:

P(genotype)=number of cells with that genotypetotal number of cellsP(\text{genotype}) = \frac{\text{number of cells with that genotype}}{\text{total number of cells}}

For independent genes the same answer comes from multiplying the single-gene probabilities, which is how the calculator checks the square:

P(aabb)=P(aa)×P(bb)=14×14=116P(\text{aabb}) = P(\text{aa}) \times P(\text{bb}) = \tfrac{1}{4} \times \tfrac{1}{4} = \tfrac{1}{16}

Worked example 1: Mendel’s purple flowers (Pp × Pp)

Purple (P) is dominant over white (p). Each parent makes P and p gametes in equal numbers.

  1. The square has 4 cells: PP, Pp, Pp and pp.
  2. Genotype ratio: 1:2:1 (PP : Pp : pp).
  3. Phenotype ratio: 3:1, purple to white, because PP and Pp both look purple.

Mendel observed 705 purple and 224 white plants in this cross, a ratio of 3.15:1, close to the expected 3:1.

Worked example 2: a dihybrid cross (YyRr × YyRr)

Mendel’s pea seeds: yellow (Y) is dominant over green (y), and round (R) over wrinkled (r). Each parent makes four gametes: YR, Yr, yR and yr.

  1. The square has 16 cells and 9 different genotypes.
  2. The most common genotype is YyRr, in 4 of the 16 cells.
  3. Phenotype ratio: 9:3:3:1: yellow round, yellow wrinkled, green round and green wrinkled.

Worked example 3: incomplete dominance in snapdragons

In snapdragons, red (RR) and white (WW) flowers produce pink (RW) offspring. Crossing two pink plants gives a genotype ratio of 1:2:1, and because each genotype has its own colour, the phenotype ratio is also 1:2:1: red, pink and white.

Common mistakes

  • Writing gametes with two alleles of the same gene. A gamete carries one allele per gene: YR, not YY.
  • Missing gamete combinations in dihybrid crosses. A YyRr parent makes four gametes, not two.
  • Confusing genotype and phenotype ratios. Aa × Aa gives 1:2:1 genotypes but 3:1 phenotypes.
  • Assuming independent assortment for linked genes. Genes close together on the same chromosome are inherited together more often than the square predicts.

Practical applications

  • Genetics coursework. Punnett squares are central to GCSE, A-level, AP and introductory university biology.
  • Plant and animal breeding. Predicting the outcome of crosses.
  • Genetic counselling. Estimating the chance that a child inherits a recessive condition when both parents are carriers (1 in 4 for each child).

Assumptions and limitations

The calculator assumes Mendelian inheritance: each gene has two alleles, the genes are autosomal and assort independently, every genotype is equally likely to survive, and genes do not affect each other’s phenotypes (no epistasis). It does not model linkage, sex-linked inheritance, multiple alleles such as the ABO blood group, or polygenic traits. All results are exact fractions computed with whole-number arithmetic. For allele frequencies across a whole population, use the Hardy–Weinberg calculator.

Frequently asked questions

What is the difference between genotype and phenotype?

The genotype is the pair of alleles an organism carries, such as Aa. The phenotype is the trait you can observe, such as purple flowers. With complete dominance, AA and Aa have the same phenotype, so there are fewer phenotypes than genotypes.

Why is the dihybrid phenotype ratio 9:3:3:1?

Each gene on its own gives a 3:1 ratio. Because the genes assort independently, the probabilities multiply. (3:1) × (3:1) expands to 9:3:3:1, which is exactly what the 16 cells of the square show.

How do I enter alleles that aren't a capital and a lower-case letter?

Type your own symbols, up to three letters or digits each, for example R and W for red and white flowers. Then enter genotypes using those symbols, such as RW. Symbols are case-sensitive.

Do the ratios tell me exactly how many offspring will have each trait?

No. They are probabilities for each offspring. In a small family or litter the actual numbers often differ from the ratio, just as tossing a coin four times does not always give two heads.

Can I use this for blood groups or sex-linked traits?

Not fully yet. The ABO blood group has three alleles, and sex-linked genes are inherited differently by males and females. This calculator handles autosomal genes with two alleles each.

References

  1. Mendel, G. (1866). Versuche über Pflanzen-Hybriden. Verhandlungen des naturforschenden Vereines in Brünn, 4, 3–47. English translation by the Electronic Scholarly Publishing Project.
  2. Griffiths, A. J. F., Doebley, J., Peichel, C., & Wassarman, D. A. (2020). Introduction to Genetic Analysis (12th ed.), chapters 2–3. W. H. Freeman.
  3. Punnett, R. C. (1905). Mendelism. Macmillan and Bowes, Cambridge.

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