Dihybrid Cross Calculator
Cross two double-heterozygous parents (AaBb × AaBb) to build the full 16-combination dihybrid Punnett square, showing every genotype and the classic 9:3:3:1 phenotype ratio. Two 3D diagrams show the four gamete types each parent produces, and charts break down the genotype and phenotype distributions.
Reviewed by the ToolNestr Editorial Team — July 2026
Classic double-heterozygous cross: AaBb × AaBb.
Four gamete types
1. Parent 1's four gamete types
AB, Ab, aB, ab — each produced in exactly equal proportions due to independent assortment.
2. Parent 2's four gamete types
Identical set of four gamete types — combining both parents' options produces all 16 grid outcomes.
Dihybrid cross charts
How it works
The core idea in one line: when two genes assort completely independently of each other, the probability of any combined genotype is simply the product of each gene's individual probability — which is exactly why combining two separate 3:1 ratios multiplies out into the distinctive 9:3:3:1 pattern.
AaBb × AaBb → 9:3:3:1
phenotype ratio — both dominant : A dominant/B recessive : A recessive/B dominant : both recessive
4 gametes × 4 gametes = 16 combinations
AB, Ab, aB, ab from each parent
Because each gene segregates independently (the law of independent assortment), a double heterozygote produces all four possible gamete combinations (AB, Ab, aB, ab) in exactly equal proportions. Crossing two such parents means combining 4 gamete types from one parent with 4 from the other, producing 16 equally likely offspring genotype combinations. Sorting those 16 outcomes by phenotype — since dominance masks the difference between homozygous dominant and heterozygous — produces the classic 9 (both dominant) : 3 (one dominant, one recessive) : 3 (the reverse) : 1 (both recessive) ratio.
Worked example 1 — pea shape and color (round/yellow dominant)
Given: Cross two pea plants heterozygous for both seed shape (Aa: round dominant) and seed color (Bb: yellow dominant).
This exact 9:3:3:1 ratio, observed by Mendel in his original dihybrid experiments, was the key evidence for independent assortment of separate genes.
Worked example 2 — counting one specific phenotype category
Given: From the same cross, how many of the 16 combinations show the double-recessive phenotype (wrinkled AND green)?
Multiplying the individual probabilities for each gene (since they assort independently) gives the same answer as counting cells directly in the 16-cell grid.
The 9:3:3:1 dihybrid phenotype ratio broken down
Each phenotype category corresponds to a specific combination of dominant/recessive traits for the two genes.
| Phenotype | Count out of 16 | Fraction |
|---|---|---|
| Both dominant (A_B_) ★ | 9 | 9/16 (56.25%) |
| A dominant, b recessive (A_bb) | 3 | 3/16 (18.75%) |
| a recessive, B dominant (aaB_) | 3 | 3/16 (18.75%) |
| Both recessive (aabb) | 1 | 1/16 (6.25%) |
★ Reference row (worked example 1). The 9:3:3:1 pattern is the signature result of two independently assorting genes, each showing simple dominance.
Where dihybrid crosses actually matter
🌱 Mendel's dihybrid pea experiments
Mendel's original two-gene crosses in pea plants, yielding the 9:3:3:1 ratio, provided the key evidence for what became the law of independent assortment.
🧬 Testing for gene linkage
When a real dihybrid cross produces a ratio significantly different from 9:3:3:1, it often signals that the two genes are physically linked on the same chromosome rather than assorting independently.
🐁 Model organism genetics research
Researchers studying model organisms like fruit flies and mice use dihybrid cross logic to predict and interpret the inheritance of two traits simultaneously in breeding experiments.
🌾 Multi-trait crop breeding
Plant breeders use dihybrid (and more complex multi-gene) cross predictions to combine two desirable traits, like disease resistance and yield, into a single new variety.
Common misconceptions
"A dihybrid cross always produces exactly 9:3:3:1 in real life."
The 9:3:3:1 ratio is a probability prediction assuming independent assortment and a large sample size — a small real cross may deviate from this ratio just by chance, and linked genes will produce a systematically different ratio altogether.
"The two genes in a dihybrid cross must be on the same chromosome to interact."
For the classic 9:3:3:1 ratio to hold, the genes actually need to be on different chromosomes (or very far apart on the same one) so they assort independently — genes close together on the same chromosome tend to be inherited as a linked unit instead.
"16 offspring will show exactly 9, 3, 3, and 1 individuals of each phenotype."
16 is the total number of equally likely gamete combinations in the underlying probability model, not a guaranteed sample size — the ratio describes long-run proportions, most reliably seen with much larger numbers of actual offspring.
"Dihybrid crosses only apply to plants."
The same independent-assortment logic applies to any diploid organism with two genes on separate chromosomes — animals, fungi, and humans all follow the identical mathematical pattern for unlinked gene pairs.
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, Independent Assortment.
- • Campbell & Reece, Biology — Chapter 14, Mendel and the Gene Idea.
Assumes both genes are unlinked (independent assortment) and show simple complete dominance. Results are probability ratios, not guaranteed exact counts.
How to use this calculator
Confirm genotypes
This calculator models the classic AaBb × AaBb double-heterozygous cross.
View the 16-cell grid
Every possible offspring genotype combination is generated automatically.
Read the phenotype breakdown
The 9:3:3:1 ratio is tallied by phenotype category.
Related tools
Frequently asked questions
What is a dihybrid cross?
A dihybrid cross tracks the inheritance of two genes at once, typically between two parents that are heterozygous for both genes (AaBb × AaBb). It reveals whether the two genes assort independently of each other.
Why does AaBb × AaBb give a 9:3:3:1 ratio?
Each parent produces 4 equally likely gamete types (AB, Ab, aB, ab) via independent assortment. Combining 4 gamete types from each parent gives 16 equally likely offspring combinations, which sort into phenotype groups in a 9:3:3:1 ratio (both dominant : one dominant one recessive : the reverse : both recessive).
What is the law of independent assortment?
The law of independent assortment states that genes for different traits (located on different chromosomes, or far apart on the same one) are inherited independently of each other — the allele a gamete receives for one gene doesn't affect which allele it receives for another gene.
Does independent assortment always hold?
No — genes located close together on the same chromosome tend to be inherited together (linked genes), violating independent assortment. The dihybrid 9:3:3:1 ratio specifically assumes the two genes are unlinked.
How many gametes does a double heterozygote produce?
A double heterozygote (AaBb) produces 4 distinct gamete types in equal proportions: AB, Ab, aB, and ab — one for every combination of the two genes' alleles.