Punnett Square CalculatorMonohybrid, dihybrid, trihybrid, blood type and X-linked crosses
Choose up to three genes, set each parent’s genotype and dominance, and get the full Punnett square with genotype and phenotype ratios.
By Manzoor Ahmad, web development expert · Updated How does a Punnett square work?
Put each parent’s possible gametes along the top and side, then combine one allele from each in every box. A cross of two heterozygotes, Aa × Aa, gives 1 AA : 2 Aa : 1 aa, which is a 3 : 1 phenotype ratio with complete dominance. Two genes (AaBb × AaBb) give the famous 9 : 3 : 3 : 1 ratio across 16 boxes.
Know only the blood type, not the genotype? Type A could be IᴬIᴬ or Iᴬi. Try both to see the range of possibilities.
| Genotype | Boxes | Probability |
|---|
How to use the Punnett square calculator
- Pick a mode: Mendelian genes, ABO blood type, or an X-linked trait.
- For genes, choose 1, 2 or 3 genes. Pick a preset trait or type your own letter and trait names.
- Set the dominance: complete (3 : 1), incomplete (blended heterozygote) or codominant (both shown).
- Choose each parent’s genotype and read the grid, genotype probabilities and phenotype ratio.
Classic Punnett square ratios
Worked example: Mendel’s dihybrid pea cross
Mendel crossed peas heterozygous for seed colour (Yy, yellow dominant) and seed shape (Rr, round dominant).
- Each parent makes four gametes in equal proportions: YR, Yr, yR and yr.
- A 4 × 4 grid gives 16 equally likely boxes.
- Counting phenotypes: 9 yellow round, 3 yellow wrinkled, 3 green round, 1 green wrinkled.
- The probability of a green wrinkled seed (yyrr) is 1/16 = 6.25%.
Mendel’s real counts were 315 : 108 : 101 : 32, very close to 9 : 3 : 3 : 1. Select “Dihybrid” and the pea presets above to reproduce this cross.
Blood type inheritance chart
ABO blood type is controlled by three alleles: Iᴬ and Iᴮ are codominant, and both are dominant over i. Possible child blood types by parent type:
| Parents | Possible child types | Not possible |
|---|---|---|
| O × O | O | A, B, AB |
| A × O | A, O | B, AB |
| A × A | A, O | B, AB |
| B × O | B, O | A, AB |
| A × B | A, B, AB, O | — |
| AB × O | A, B | AB, O |
| AB × AB | A, B, AB | O |
Rare variants such as the Bombay phenotype and cis-AB can break these rules, so blood typing alone is not a reliable test of parentage.
What a Punnett square cannot tell you
- Exact outcomes. It gives probabilities per offspring. A family of four children may not follow 3 : 1.
- Linked genes. Genes close together on one chromosome do not assort independently.
- Polygenic traits. Height, skin colour and most human traits depend on many genes plus environment.
- Human eye colour. It involves several genes, so a simple brown-over-blue square is only a rough teaching model.
To move from individual crosses to whole populations, use the Hardy-Weinberg calculator.
Punnett square FAQs
A Punnett square is a grid that shows every possible combination of alleles from two parents. Each parent's gametes run along one side, and each box shows one equally likely offspring genotype.
Write the possible gametes of one parent across the top and the other parent down the side, then fill each box with one allele from its column and one from its row. Count the boxes to get genotype and phenotype ratios.
Aa × Aa gives a 1 AA : 2 Aa : 1 aa genotype ratio and a 3 : 1 dominant-to-recessive phenotype ratio when dominance is complete.
AaBb × AaBb gives a 9 : 3 : 3 : 1 phenotype ratio for two independently assorting genes with complete dominance, from 16 boxes.
Genotype is the pair of alleles an organism carries, such as Aa. Phenotype is the trait you can observe, such as purple flowers. Different genotypes can produce the same phenotype.
In incomplete dominance the heterozygote has an intermediate phenotype. Red (RR) × white (rr) snapdragons give pink (Rr) offspring, and a cross of two pinks gives 1 red : 2 pink : 1 white.
In codominance both alleles are fully expressed in the heterozygote. ABO blood type AB is the classic example, where both A and B antigens appear on red blood cells.
Yes, if both parents carry the O allele (genotype AO). Each child then has a 1 in 4 chance of type O.
Males have one X chromosome, so a single recessive allele produces the trait. A carrier mother (XᴬXᵃ) and unaffected father (XᴬY) have a 50% chance that each son is affected and a 50% chance that each daughter is a carrier.
A trihybrid cross of two triple heterozygotes (AaBbCc × AaBbCc) has 8 gametes per parent and 64 boxes. Its phenotype ratio is 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1.
No. It gives probabilities for each pregnancy, not guaranteed counts. A 3 : 1 ratio means each offspring has a 75% chance of the dominant trait, but small families often differ from the ratio.
Simple Punnett squares assume genes assort independently and follow Mendelian dominance. They do not account for linked genes, polygenic traits such as height, epistasis or environmental effects.
Sources
- NIH National Human Genome Research Institute: Punnett Square
- OpenStax Biology 2e, Chapter 12: Mendel's Experiments and Heredity
- NCBI Bookshelf: Blood Groups and Red Cell Antigens (ABO)
- MedlinePlus Genetics: What are the different ways a genetic condition can be inherited?
- Nature Education Scitable: Mendelian Genetics
This Punnett square calculator is an educational tool based on simple Mendelian inheritance. It is not a genetic test or medical advice; for family health questions, speak with a genetic counselor.