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Punnett square calculator

Enter the parents' genotypes for a monohybrid or dihybrid cross, and this tool builds the Punnett square and works out the genotype and phenotype ratios.

Type each parent's alleles (for example Tt, or RrYy). The tool builds the grid, lists the offspring genotypes and gives the phenotype ratio, check your own working against it.

How to use it

  1. Type the first parent's genotype into the first box, using two letters per gene, such as Tt for a single gene or RrYy for two genes.
  2. Type the second parent's genotype into the second box in the same letter format.
  3. Press "Compute" to build the grid; the tool lists every possible gamete each parent can produce along the top and side.
  4. Read the offspring genotype in each cell of the grid, then check the genotype list below it against your own working.
  5. Change either genotype and press "Compute" again to try a different cross without starting over.
  6. Double-check that both boxes contain the same two letters (just arranged differently for each parent) before pressing Compute, since a typo in one letter changes every result in the grid.

How to read the result

Each cell in the grid is one equally likely combination of a gamete from the first parent with a gamete from the second parent, so the genotype count listed below the grid comes directly from how many cells produced each combination out of the total number of cells.

For a monohybrid cross with two heterozygous parents, this normally gives the familiar 3:1 phenotype ratio once dominant and recessive alleles are grouped together; for a dihybrid cross with two heterozygous parents, it gives 9:3:3:1. Use the calculator to confirm your own hand-drawn grid rather than as a substitute for drawing one in the exam, since Paper 2 questions ask you to show the grid itself.

The order the gametes are listed in along the top and side does not affect the ratio, only which cell shows which combination, so do not worry if your own hand-drawn grid lists the gametes in a different order from the tool's.

What it does not do

The calculator only handles genotypes written as pairs of letters with independent assortment; it does not model linked genes, sex-linked inheritance, incomplete dominance or codominance, and it does not replace showing full working in a structured answer. It also assumes both alleles for a gene are written correctly as one uppercase and one lowercase letter where the cross is heterozygous, so an unusual genotype format may not be read the way you intended.

Frequently asked questions

What format should I type the genotype in?
Use two letters per gene with no spaces, for example Tt for one gene or RrYy for two genes, keeping the same letter (in upper and lower case) for each gene across both parents.
Can I use it for a dihybrid cross?
Yes. Enter four letters for each parent, such as RrYy for both, and the tool builds the full sixteen-cell grid with every combination of gametes.
Will this tool draw the grid for me in the exam?
No, it is only for checking your own working while you revise. In Paper 2, you are expected to draw and label the Punnett square yourself as part of the answer.
What if I only know one parent's genotype and need to work out the other?
This tool builds a grid from two known genotypes; if one parent is unknown, you will need to reason it out from the offspring ratio first, then use the calculator to confirm your answer once you have a genotype to test.
Does the tool show phenotype as well as genotype?
It lists genotype counts directly; work out the phenotype ratio yourself by grouping genotypes that share the same dominant expression, which is also the working examiners expect to see.

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Written by the spmbiology.com.my editorial team · Updated 2026-09-09

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