Dihybrid Cross Punnett Square Calculator
Two-locus inheritance, gamete combinations & offspring probabilities.
Mendelian model: independent assortment and complete dominance at both loci.
Parent Genotypes & Trait Setup
Offspring & Inheritance Summary
Mendelian exampleAaBb × AaBb · independent assortment
Interactive Punnett Square
Parent 1 gametes across · Parent 2 gametes down
All AaBb outcomes combined: 4/16 (25%)
Genotype Probability Breakdown
9 unique genotypes · total probability 100%Independent assortment, complete dominance and equal gamete viability. Linkage, epistasis and sex-linked inheritance are outside this model.
Genetics Model Checks
- 1Confirm both parental genotypes
- 2Combine one allele from each locus
- 3Compare probabilities, not exact counts
Formula & Worked Example
AB × ab: ¼ × ¼ = 1/16 → AaBb
A_B_: ¾ × ¾ = 9/16 = 56.25%
A_bb: ¾ × ¼ = 3/16 = 18.75%
aaB_: ¼ × ¾ = 3/16 = 18.75%
aabb: ¼ × ¼ = 1/16 = 6.25%
Frequently Asked Questions
Why is the classic ratio 9 : 3 : 3 : 1?
Each independently assorting locus has a 3:1 dominant/recessive phenotype ratio under complete dominance. Multiplying the probabilities gives 9/16, 3/16, 3/16 and 1/16.
What changes in a test cross?
AaBb × aabb gives four equally likely phenotype classes under the model, producing 1:1:1:1.
Do all crosses need a 4 × 4 grid?
No. Each parent produces up to four unique gametes. Homozygous parents can produce fewer types; the table adapts to the unique gametes and their probabilities.
Does 9 : 3 : 3 : 1 predict exact offspring counts?
No. It is the expected probability ratio. Actual small samples may differ by chance.
More genetics questions
What is a dihybrid cross?
It tracks the inheritance of two loci. The classic example crosses two double heterozygotes, but the calculator also handles homozygous and test-cross combinations.
Which genotype notation is accepted?
Enter two allele pairs, such as AaBb, AABb or aabb. Within each pair the letter must identify the same locus, and both parents must list the same two distinct loci in the same order. Other letters, such as RrYy, are accepted.
Why do homozygous parents have fewer gametes?
Two identical alleles at a locus give the same allele in every gamete. Duplicate gamete combinations are merged with their probabilities; the grid is 1×1, 1×2, 2×2 or up to 4×4 as needed.
What is the difference between a cell and a genotype probability?
A grid cell is one pair of unique parental gametes. Its probability is the product of their probabilities. Several cells can create the same genotype; the genotype summary adds those cells.
What do underscores mean in A_B_?
An underscore stands for either allele at that locus. A_B_ means at least one dominant A and one dominant B. The four phenotype classes are kept in a consistent order, including zero-probability classes.
Can I use linked genes or epistasis?
Not with this model. It assumes independent assortment, complete dominance at both loci, equal allele segregation, random fertilization and no differences in viability. Linked genes need haplotype phase and recombination data.
What does Pea Traits change?
It adds illustrative trait names: dominant round versus recessive wrinkled for the first locus, and dominant yellow versus recessive green for the second. It does not change genotype probabilities. The decorative illustration is not a measured biological sample.
Can I swap parents or copy results?
Yes. Swapping parents preserves the offspring probabilities and transposes the gamete axes. Copy includes parental genotypes, gametes, phenotype ratios, genotype probabilities and the model assumptions.
Sources & References
Nature Education · ScitableDihybrid crosses & independent assortment
NCBI / NLM · PubMedMendelian inheritance & phenotype ratiosReference information is not institutional endorsement.
Calculate two-locus gametes, offspring genotype probabilities and complete-dominance phenotype ratios with an interactive Punnett square.