BYTETOOLS

Punnett Square Calculator

Build monohybrid and dihybrid Punnett squares from two parent genotypes and get the genotype and phenotype ratios with percentages for every outcome.

Monohybrid cross
Cross
4
Offspring squares
1 : 2 : 1
Genotype ratio
3 : 1
Phenotype ratio

Punnett square

Punnett square for Aa crossed with Aa
Aa
AAAAa
aAaaa

Columns are the gametes from parent 1, rows the gametes from parent 2. Each square is one equally likely fertilisation outcome.

Genotypes — ratio 1 : 2 : 1

GenotypeSquaresChance
AA125.0%
Aa250.0%
aa125.0%

Phenotypes — ratio 3 : 1

Phenotype classSquaresChance
A_375.0%
aa125.0%

“A_” means at least one dominant allele, so AA and Aa look the same.

Ratios assume simple Mendelian inheritance: complete dominance, independent assortment of unlinked genes and equal survival of every genotype. Linked genes, incomplete or co-dominance, epistasis, sex linkage and lethal alleles all produce different ratios in real crosses.

What is the Punnett Square Calculator?

A Punnett square is a grid that shows every possible combination of parental gametes. Crossing two Aa heterozygotes gives a 1:2:1 genotype ratio and a 3:1 phenotype ratio in the offspring.

  • Monohybrid, dihybrid and trihybrid crosses
  • Automatic gamete generation from parent genotypes
  • Genotype and phenotype ratios reduced to simplest form
  • Percentage chance for every outcome
  • Helpful validation for mistyped genotypes
  • Runs entirely in your browser — nothing is uploaded

How to use the Punnett Square Calculator

  1. 1

    Enter parent 1's genotype, for example Aa or AaBb.

  2. 2

    Enter parent 2's genotype using the same gene letters in the same order.

  3. 3

    Read the completed Punnett square with each offspring genotype.

  4. 4

    Check the genotype and phenotype ratio tables below it.

  5. 5

    Copy the whole result for your homework or lab book.

About the Punnett Square Calculator

The ByteTools Punnett Square Calculator draws the grid for you. Enter the two parent genotypes — Aa × Aa for a monohybrid cross, AaBb × AaBb for a dihybrid — and it generates every gamete, fills in the square, and tallies the genotype and phenotype outcomes with counts, ratios and percentages.

Gametes are produced as the combination of one allele from each gene, so a two-gene parent yields four gametes and a 4 × 4 square. Uppercase letters are treated as dominant and lowercase as recessive, and the phenotype table groups results into classes such as A_ B_ where any dominant allele masks the recessive one. Crosses of up to three genes are supported.

Everything is generated in your browser with JavaScript — nothing is uploaded. The ratios assume simple Mendelian inheritance, which is exactly what genetics coursework asks for but is a simplification of what happens in real populations.

Frequently asked questions

How do you make a Punnett square?

Write one parent's gametes across the top and the other's down the side, then fill each cell with the combination of the two. Every cell is one equally likely fertilisation, so counting them gives you the probabilities directly.

What is the ratio for a monohybrid cross?

Crossing two heterozygotes, Aa × Aa, gives a 1:2:1 genotype ratio of AA to Aa to aa and a 3:1 phenotype ratio of dominant to recessive. Three of the four squares contain at least one dominant allele.

What is the 9:3:3:1 ratio?

It is the phenotype ratio from a dihybrid cross of two double heterozygotes, AaBb × AaBb. Of the 16 squares, 9 show both dominant traits, 3 show each single dominant trait, and 1 shows both recessives.

What is the difference between genotype and phenotype?

Genotype is the actual pair of alleles an organism carries, like AA or Aa. Phenotype is the observable trait those alleles produce. Because a dominant allele masks a recessive one, AA and Aa share the same phenotype.

Do real crosses always match these ratios?

Not exactly. Punnett squares assume complete dominance, unlinked genes and equal survival of every genotype. Linked genes, incomplete or co-dominance, epistasis, sex linkage and lethal alleles all shift the observed ratios, and small sample sizes vary by chance alone.

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