Important Note : The classic 9:3:3:1 result applies only when the two genes assort independently and each trait follows simple complete dominance.
Use this Dihybrid Cross (Punnett Square) Calculator to solve a two-trait Punnett square quickly. It helps you generate gametes, build the full 4×4 square, and see genotype and phenotype probabilities for classic Mendelian dihybrid crosses.
Reviewed by: AjaxCalculators Editorial Team
Last updated: April 24, 2026
Method source: Standard Mendelian dihybrid cross logic using independent assortment and complete dominance at two loci
Editorial standards: AjaxCalculators Editorial Policy
What This Dihybrid Cross (Punnett Square) Calculator Calculates
This calculator helps you solve a dihybrid cross involving two independent traits.
It shows:
- Parent genotypes at two loci
- Possible gametes from each parent
- The full 4×4 Punnett square
- Phenotype probabilities
- Genotype probabilities
The live tool is built for the standard case of complete dominance, using an A/a locus and a B/b locus.
How the Dihybrid Cross (Punnett Square) Calculator Works
A dihybrid cross tracks inheritance for two genes at the same time.
For each parent, the calculator first determines all possible gametes. For example, a parent with genotype AaBb can produce four gamete types:
AB, Ab, aB, ab
Those gametes are placed across the top and side of a 4×4 Punnett square. Each box combines one gamete from each parent to create an offspring genotype.
After all 16 boxes are filled, the calculator counts:
- how many offspring fit each genotype
- how many offspring fit each phenotype
Why the Classic 9:3:3:1 Ratio Happens
In the special case of:
AaBb × AaBb
each parent produces four gametes with equal probability under independent assortment. That creates 16 equally likely genotype combinations.
When complete dominance applies at both loci, the classic phenotype ratio becomes:
- 9 with both dominant phenotypes: A_B_
- 3 with dominant A and recessive b: A_bb
- 3 with recessive a and dominant B: aaB_
- 1 with both recessive phenotypes: aabb
This is the standard 9:3:3:1 dihybrid phenotypic ratio taught in introductory genetics.
Assumptions and Important Notes
- This calculator is built for two loci that assort independently.
- It assumes complete dominance at each locus.
- It also assumes there is no gene interaction that changes the phenotype pattern.
- The classic 9:3:3:1 phenotype ratio applies only under those standard Mendelian assumptions.
- If the genes are linked, incompletely dominant, codominant, or affected by epistasis, the result may differ from the classic ratio.
- The square stays 4×4, but some gamete headers may repeat when a parent is not heterozygous at one or both loci.
Worked Example
Suppose both parents are:
AaBb × AaBb
Step 1: List the gametes for each parent
Each parent can make:
AB, Ab, aB, ab
Step 2: Build the 4×4 Punnett square
Place one parent’s four gametes across the top and the other parent’s four gametes down the side.
Step 3: Fill the 16 offspring boxes
Each box combines alleles from one top gamete and one side gamete.
Step 4: Count phenotypes
Under complete dominance and independent assortment, the phenotype counts become:
- 9 A_B_
- 3 A_bb
- 3 aaB_
- 1 aabb
Step 5: Convert to percentages
Because there are 16 equally likely cells, one cell represents 6.25% in this special case.
How to Use This Dihybrid Cross (Punnett Square) Calculator
- Select the mother’s genotype at the A locus.
- Select the mother’s genotype at the B locus.
- Select the father’s genotype at the A locus.
- Select the father’s genotype at the B locus.
- Review the generated gametes, the 4×4 Punnett square, and the phenotype/genotype probabilities.
How to Interpret the Result
Phenotype probabilities show the visible trait combinations expected under the calculator’s assumptions.
Genotype probabilities show the exact allele combinations among the offspring.
If a parent is not heterozygous at both loci, the gamete list may contain repeated headers. That does not mean the tool is wrong. It reflects the fact that not all four gamete slots are distinct in that case.
Practical Uses of a Dihybrid Cross (Punnett Square) Calculator
- solve two-trait Punnett squares faster
- check genotype and phenotype ratios for homework
- understand how independent assortment creates multiple gamete combinations
- compare Mendelian expectations across different parental genotypes
- visualize why AaBb × AaBb produces the classic 9:3:3:1 phenotype ratio
References
- OpenStax Biology 2e – laws of inheritance and the classic 9:3:3:1 dihybrid ratio
- LibreTexts – dihybrid crosses, product rule, and the assumptions behind the 9:3:3:1 ratio
- Khan Academy – law of independent assortment and 4×4 Punnett squares
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Disclaimer: This calculator is for educational use only. It is designed for standard Mendelian dihybrid crosses with independent assortment and complete dominance, so real biological inheritance patterns may differ when genes are linked or when other inheritance mechanisms are involved.