Punnett Square Calculator
Enter two parent genotypes for a single gene (e.g. Aa × Aa) to build the full 2×2 Punnett square, showing every possible offspring genotype and the resulting genotype and phenotype ratios. Two 3D diagrams compare a homozygous and heterozygous cross, and charts show the classic 1:2:1 genotype and 3:1 phenotype ratios.
Reviewed by the ToolNestr Editorial Team — July 2026
Homozygous vs heterozygous parents
1. Homozygous parent (AA)
Both allele spheres are the same color — this parent can only pass on one type of allele.
2. Heterozygous parent (Aa)
One dominant allele (indigo) and one recessive allele (amber) — this parent can pass on either one.
Punnett square ratio charts
How it works
The core idea in one line: each parent contributes exactly one of its two alleles to each offspring, entirely by chance, so combining both parents' possible contributions into a grid reveals every genetically possible outcome and its exact probability.
Aa × Aa → 1 AA : 2 Aa : 1 aa
the classic heterozygous cross — genotype ratio 1:2:1
1 AA : 2 Aa : 1 aa → 3 : 1
phenotype ratio — dominant (AA, Aa) : recessive (aa)
The law of segregation states that each parent's two alleles separate during gamete formation, so each gamete carries only one allele for the gene. Listing one parent's possible gametes across the top of a grid and the other parent's down the side, then filling in every cell with the combination of the corresponding row and column, produces every possible offspring genotype with equal probability per cell. Tallying how many cells produce each phenotype (grouping genotypes that look the same due to dominance) gives the final predicted phenotype ratio.
Worked example 1 — a classic heterozygous cross (Aa × Aa)
Given: Both pea plant parents are heterozygous for seed shape (Aa), where A (round) is dominant over a (wrinkled).
This exact 3:1 ratio is what Gregor Mendel observed in his original pea plant experiments, forming the foundation of classical genetics.
Worked example 2 — a testcross (Aa × aa)
Given: One parent is heterozygous (Aa) and the other is homozygous recessive (aa) — a classic "testcross" used to determine an unknown genotype.
A 1:1 phenotype ratio from a testcross confirms the tested parent was heterozygous (Aa) — if it had been homozygous dominant (AA), all offspring would show the dominant trait instead.
Common monohybrid cross outcomes
The phenotype ratio depends entirely on which genotypes are being crossed.
| Cross | Genotype ratio | Phenotype ratio |
|---|---|---|
| AA × AA | 4 AA | All dominant |
| Aa × Aa ★ | 1 AA : 2 Aa : 1 aa | 3 : 1 |
| Aa × aa | 2 Aa : 2 aa | 1 : 1 |
| aa × aa | 4 aa | All recessive |
★ Reference row (worked example 1). This is the cross Mendel used to establish the 3:1 phenotype ratio as a hallmark of simple dominant/recessive inheritance.
Where Punnett squares actually matter
🌱 Mendel's original pea plant experiments
Gregor Mendel's 19th-century pea plant crosses, analyzed using exactly this logic, established the foundational laws of inheritance that underlie all of modern genetics.
🐕 Animal breeding programs
Breeders use Punnett square logic to predict the likelihood of offspring inheriting desired (or undesired) traits before making breeding decisions.
🧬 Genetic counseling
Genetic counselors use Punnett squares to help prospective parents understand the probability of passing on a recessive genetic condition, especially when both parents are known carriers.
🌾 Crop breeding
Agricultural scientists predict trait inheritance in crop breeding programs — such as disease resistance or yield traits — using the same monohybrid cross logic.
Common misconceptions
"A Punnett square guarantees exactly a 3:1 ratio in any 4 offspring."
A Punnett square shows probabilities, not guarantees — with only 4 actual offspring, you might see 4:0, 2:2, or any other split by chance. The predicted ratio becomes increasingly accurate only as the number of offspring grows large.
"The dominant allele is always the more common one in a population."
An allele's dominance describes how it behaves when paired with a different allele in an individual — it has no bearing on how common that allele is in the overall population, which depends on separate population genetics factors.
"Heterozygous individuals show a blend of both traits."
For simple complete dominance, a heterozygote (Aa) shows only the dominant phenotype, not a blend — blending only occurs in incomplete dominance, a different inheritance pattern covered by a separate calculator.
"A Punnett square can predict any trait, no matter how complex."
The basic Punnett square models simple, single-gene traits with clean dominant/recessive inheritance. Many real traits (height, skin color, many diseases) are polygenic or influenced by environment, requiring far more complex models.
Formula sources & further reading
The formulas here are standard, traceable to:
- • OpenStax, Biology 2e — Chapter 12, "Mendel's Experiments and Heredity" (free, peer-reviewed). openstax.org
- • Klug, Cummings & Spencer, Concepts of Genetics — Chapter 3, Mendelian Genetics.
- • Campbell & Reece, Biology — Chapter 14, Mendel and the Gene Idea.
Assumes simple complete dominance and independent assortment for a single gene. Results are exact probability ratios, not guaranteed outcomes.
How to use this calculator
Enter both parent genotypes
Type each parent's allele pair for the gene, using uppercase for dominant and lowercase for recessive.
View the cross grid
The full Punnett square fills in with every possible offspring genotype combination.
Read the ratios
Genotype and phenotype ratios are tallied and displayed automatically.
Related tools
Frequently asked questions
What is a Punnett square?
A Punnett square is a grid used to predict the possible genotypes of offspring from a genetic cross. Each parent's alleles are placed along one edge, and the grid cells show every possible allele combination the offspring could inherit.
What is the difference between genotype and phenotype ratio?
Genotype ratio counts the actual allele combinations (like 1 AA : 2 Aa : 1 aa). Phenotype ratio groups those by observable trait — since AA and Aa both show the dominant trait, the classic 1:2:1 genotype ratio becomes a 3:1 phenotype ratio (dominant:recessive).
Why does Aa × Aa give a 3:1 phenotype ratio?
Each parent contributes one of two alleles (A or a) with equal probability. Combining two parents' alleles independently gives four equally likely combinations: AA, Aa, aA, aa — three of which (AA, Aa, aA) show the dominant phenotype, and one (aa) shows the recessive phenotype.
What if both parents are homozygous?
A homozygous × homozygous cross (like AA × aa) produces offspring that are all identical — 100% Aa in this case — since each parent can only contribute one type of allele.
Does a Punnett square predict exact numbers or just probabilities?
A Punnett square shows probabilities/ratios, not guaranteed exact counts. A real cross with only 4 offspring might not show exactly a 3:1 split — the predicted ratio becomes more accurate as the number of offspring increases, following the law of large numbers.