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Punnett Square Calculator

Dihybrid cross — predict offspring genotype and phenotype probabilities for two independent traits.

Mother's Traits
Trait 1
Trait 2
Father's Traits
Trait 1
Trait 2
Trait 1 – Dominant allele
Recessive allele
Trait 2 – Dominant allele
Recessive allele
💡 Quick Summary

Predict offspring genotype and phenotype probabilities for a dihybrid (two-trait) cross. Select parental genotypes using radio buttons or custom input, define your allele symbols, and instantly see the 4×4 Punnett square, all 9 possible genotypes, ratios, and researcher-grade insights including parental gametes, heterozygosity metrics, chi-square goodness-of-fit, and expected offspring counts.

📋 How to Use
  1. Select or enter the Mother's Traits: choose a preset for Trait 1 (e.g. Aa) and Trait 2 (e.g. Bb), or type a full 4-character custom genotype (e.g. AaBb) in the custom input field.
  2. Repeat for the Father's Traits in the same way.
  3. Confirm or update the allele symbols: enter the dominant letter (e.g. A) and recessive letter (e.g. a) for each trait. They must each be unique single characters.
  4. The 4×4 Punnett square, all genotype chances, ratios, and researcher insights update automatically as you type.
  5. Scroll to the Researcher Insights sections to see parental gametes, heterozygosity metrics, and the chi-square analysis where you can enter your own observed offspring counts.
  6. Use Expected Offspring to enter a total count and calculate how many offspring of each phenotype class to expect.
  7. Click Download results as PDF to open a print-ready version of all results in a new window.
  8. Click Clear all changes to reset everything to the default AaBb × AaBb cross.
🧮 Formulas & Logic
Gamete generation
For parent [A₁A₂B₁B₂], four gametes are produced: A₁B₁, A₁B₂, A₂B₁, A₂B₂ — each at 25% frequency (independent assortment).
Genotypic probability
P(genotype) = cell count ÷ 16 × 100%
Phenotypic ratio
Standard AaBb × AaBb yields 9 A_B_ : 3 A_bb : 3 aaB_ : 1 aabb
Chi-square statistic
χ² = Σ (O − E)² ÷ E, where O = observed count, E = expected count from Punnett probabilities
Degrees of freedom
df = number of phenotype classes − 1
Heterozygosity (locus)
H = count of heterozygous genotypes at locus ÷ 16
📊 Result Interpretation
9 : 3 : 3 : 1 phenotypic ratio

The classic result of an AaBb × AaBb (dihybrid) cross. Arises when the two genes assort independently and dominance is complete at both loci.

1 : 1 : 1 : 1 ratio

Produced by a dihybrid × double recessive (AaBb × aabb) test cross. Confirms independent assortment and reveals the gamete frequencies of the dihybrid parent.

3 : 1 ratio in a dihybrid

When both parents share the same homozygous allele at one locus, that locus has no variation in offspring, collapsing the cross effectively to a monohybrid ratio.

All offspring identical

If both parents are homozygous at both loci (e.g. AABB × aabb), all offspring are identical dihybrids — this is an F₁ generation with 100% heterozygosity.

Chi-square p > 0.05

No statistically significant deviation from expected Mendelian ratios. Your data are consistent with the theoretical model.

Chi-square p ≤ 0.05

Statistically significant deviation. Possible causes: linked genes, selection, small sample size, or non-Mendelian inheritance.

Heterozygosity index 0%

All offspring at this locus are homozygous — either because one or both parents are homozygous.

Heterozygosity index 50%

Half of all offspring are heterozygous at this locus — typical of a Aa × AA or Aa × aa cross.

Heterozygosity index 100%

All offspring are heterozygous at this locus (e.g. AA × aa cross).

🔬 Applications
  • Plant and animal breeding — predicting phenotype frequencies in F₂ and back-cross generations
  • Genetic counselling — estimating probability of offspring inheriting combinations of two recessive disorders
  • Teaching Mendel's Law of Independent Assortment (Second Law)
  • Experimental design — calculating minimum sample sizes needed to detect deviations from expected ratios
  • Test-cross analysis — confirming the genotype of an organism expressing a dominant phenotype at both loci
  • Agricultural genetics — selecting for or against specific trait combinations in crop improvement
  • Conservation genetics — estimating heterozygosity to predict fitness and genetic diversity in managed populations
⚠️ Common Mistakes & Warnings
Assumes independent assortment

This calculator models two genes that segregate independently — located on different chromosomes or far apart on the same chromosome. Linked genes violate this assumption and will produce ratios that deviate from the 9:3:3:1 classic pattern.

Complete dominance only

The phenotype model used here assumes complete dominance: any organism carrying at least one dominant allele expresses the dominant phenotype. It does not model codominance, incomplete dominance, or epistasis.

Theoretical probabilities, not guarantees

The ratios shown are expected probabilities from random fertilisation. Actual offspring counts in small litters or samples will deviate due to chance. The chi-square section helps you assess whether observed deviations are within expected statistical variation.

Allele symbols must be unique

Each allele symbol (d1, r1, d2, r2) must be a single unique character. Using the same letter for dominant and recessive, or for both traits, will prevent calculation.

❓ Frequently Asked Questions

What is a dihybrid cross?
A dihybrid cross examines the simultaneous inheritance of two independent traits. Each parent contributes two pairs of alleles (one pair per trait), producing 4 possible gamete types and 16 possible offspring genotype combinations in the Punnett square.
Why does AaBb × AaBb give a 9:3:3:1 ratio?
Of 16 equally likely offspring combinations: 9 carry at least one dominant allele at both loci (A_B_), 3 carry dominant at locus 1 only (A_bb), 3 carry dominant at locus 2 only (aaB_), and 1 is homozygous recessive at both (aabb). The ratio is only exact in large samples.
What is a test cross and how does it appear here?
A test cross crosses an organism of unknown genotype with a fully homozygous recessive individual (aabb). Select aabb for the father and your unknown genotype for the mother. If the unknown is AaBb, you get a 1:1:1:1 offspring ratio, confirming the genotype and independent assortment.
What does the chi-square test tell me?
The chi-square goodness-of-fit test compares your experimentally observed offspring counts against the counts predicted by the Punnett square. A non-significant result (p > 0.05) means your data are consistent with Mendelian ratios. A significant result suggests possible linkage, selection, or a non-Mendelian mechanism.
What are parental gametes and why do they matter?
Gametes are the reproductive cells contributed by each parent. A dihybrid parent (AaBb) produces four gamete types (AB, Ab, aB, ab) at equal frequencies of 25% each, assuming independent assortment. If the genes were linked, gamete frequencies would deviate — this is the basis for mapping genes by recombination frequency.
What does "breeding true" mean in the Heterozygosity section?
An organism "breeds true" for both traits if it is homozygous at both loci (AABB or aabb). Self-crossing or mating two such organisms produces 100% identical offspring with no phenotypic variation.
How many offspring do I need for a reliable chi-square test?
A standard rule of thumb is that the expected count in each class should be ≥ 5. For a 9:3:3:1 cross with 4 classes, this means a minimum of ~80 offspring (smallest class = aabb at 1/16 × 80 = 5). More offspring improve statistical power substantially.

Dihybrid cross calculations are based on Mendel's Law of Segregation and Law of Independent Assortment. Assumes complete dominance and no genetic linkage between the two loci.