Dihybrid cross calculator.
Work out a two-gene cross. Choose each parent genotype for two independent genes and the calculator returns the offspring phenotype ratio, like the classic 9:3:3:1.
Parent genotypes
LiveUppercase A and B are the dominant alleles, lowercase a and b the recessive. The two genes assort independently, so the result is the product of the two single-gene crosses.
Phenotype ratio
9 : 3 : 3 : 1
A_B_ : A_bb : aaB_ : aabb
A two-gene cross assuming independent assortment and simple dominance. Calculations run in your browser; nothing you enter is stored.
How it works
Two genes at once
A dihybrid cross follows two genes through a single mating. Because the genes assort independently (Mendel’s second law), each gene behaves like its own monohybrid cross, and the combined result is simply the product of the two.
Crossing AaBb with AaBb gives a 3:1 dominant-to-recessive split for each gene, and multiplying those together produces the famous 9:3:3:1 phenotype ratio over 16 equally likely combinations.
Reference
Common dihybrid crosses.
The offspring phenotype ratio, out of 16, for each pairing of two-gene parent genotypes.
| Cross | Phenotype ratio |
|---|---|
| AaBb x AaBb | 9 : 3 : 3 : 1 |
| AaBb x Aabb | 6 : 6 : 2 : 2 |
| AaBb x aaBb | 6 : 2 : 6 : 2 |
| AaBb x aabb | 4 : 4 : 4 : 4 |
| AABB x aabb | 16 : 0 : 0 : 0 |
The full guide
The complete guide to dihybrid crosses.
What a dihybrid cross is, where 9:3:3:1 comes from, and how to read the result.
What is a dihybrid cross?
A dihybrid cross tracks the inheritance of two different genes at the same time, for example seed shape and seed colour in Mendel’s peas. Each parent carries two alleles per gene, and passes one of each to every offspring.
Because the two genes are on different chromosomes, or far apart, they are inherited independently, which is what lets us treat each gene separately and then combine the odds.
Where 9:3:3:1 comes from
Cross two double heterozygotes, AaBb by AaBb, and each gene on its own gives a 3:1 phenotype ratio. Multiplying the two 3:1 splits gives four classes in the proportions 9:3:3:1: nine show both dominant traits, three show the first dominant and second recessive, three the reverse, and one shows both recessive traits.
Those sixteen boxes are the dihybrid Punnett square, and every cell is equally likely.
Reading the ratio
The four numbers are the counts of A_B_, A_bb, aaB_ and aabb out of sixteen offspring. The underscore means either allele, so A_ covers both AA and Aa. Convert any count to a percentage by dividing by sixteen.
These are expected proportions across many offspring, not a guarantee for any single one.
When it does not apply
The 9:3:3:1 ratio assumes independent assortment and simple dominance. Linked genes, codominance, incomplete dominance, epistasis or sex linkage change the outcome and need a more detailed model.
For a single gene use the Punnett square calculator; for three genes step up to the trihybrid cross.
The formula
Two genes,
one ratio.
Independent assortment means a dihybrid cross is the product of two single-gene crosses, giving 9:3:3:1 from AaBb by AaBb.
Trihybrid cross ›# Dihybrid (two genes)
independent assortment
ratio = geneA_cross × geneB_cross
# AaBb x AaBb
9 : 3 : 3 : 1 (out of 16)Questions
Dihybrid questions.
How do I calculate a dihybrid cross?
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Treat each gene as its own cross, then multiply. Choose both parent genotypes for the two genes above and the calculator combines them into the offspring phenotype ratio.
Why is the dihybrid ratio 9:3:3:1?
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Because crossing AaBb by AaBb gives a 3:1 split for each gene, and multiplying the two 3:1 ratios gives nine both-dominant, three and three mixed, and one both-recessive, out of sixteen.
What does A_B_ mean?
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The underscore stands for either allele, so A_ covers AA and Aa. A_B_ therefore means anything showing both dominant traits.
Does this assume independent assortment?
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Yes. The 9:3:3:1 model assumes the two genes are inherited independently and show simple dominance. Linked genes need a different approach.
Is this dihybrid cross calculator free?
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Yes. It is completely free with no sign-up, and every calculation runs locally in your browser, so nothing you enter is stored or sent anywhere.
About the developer
Jean Borg
Jean builds and maintains every calculator on freecalculators.pro from Malta, with a focus on tools that are fast, free and show their working. The dihybrid cross calculator covers two-gene crosses with independent assortment, for education.