Dihybrid Cross Calculator - Punnett Square
Cross two genes at once and get the full 4x4 Punnett square, genotype ratio, and phenotype ratio.
🌺 What is the Dihybrid Cross Calculator?
The dihybrid cross calculator builds the full 4x4 (16-cell) Punnett square used to predict offspring genotypes and phenotypes when two genes are inherited at the same time, each with two alleles showing simple complete dominance. This extends the single-gene monohybrid cross to two independently assorting genes, following Gregor Mendel's law of independent assortment, the foundation of classical genetics.
Genetics students use this calculator to check dihybrid cross homework problems, especially the classic 9:3:3:1 ratio produced by crossing two double heterozygotes (AaBb x AaBb). Instructors use it to quickly generate the correct grid and ratios for any of the 81 possible parent genotype combinations when preparing practice problems. It also helps learners see exactly how gamete formation (each parent producing up to 4 distinct gamete types) leads to the 16 equally likely offspring combinations.
A common point of confusion is expecting every dihybrid cross to produce 9:3:3:1. That ratio only appears when both parents are fully heterozygous at both genes, crossing other genotype combinations, like a test cross (AaBb x aabb) or a cross involving a homozygous parent at one gene, produces different, still-predictable ratios such as 1:1:1:1 or 3:1.
This calculator is useful because manually working out a 16-cell grid by hand is slow and error-prone, especially when parents are not both fully heterozygous. It automatically derives each parent's correct gamete types, fills every cell, and reduces both the nine-category genotype ratio and the four-category phenotype ratio to their simplest whole-number form.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Classic Double Heterozygous Cross (AaBb × AaBb)
Parent 1 = AaBb, Parent 2 = AaBb
Example 2 — Dihybrid Test Cross (AaBb × aabb)
Parent 1 = AaBb, Parent 2 = aabb
Example 3 — One Gene Homozygous (AABb × AaBb)
Parent 1 = AABb, Parent 2 = AaBb
❓ Frequently Asked Questions
🔗 Related Calculators
What is a dihybrid cross?
A dihybrid cross tracks the inheritance of two genes at once, each with two alleles, instead of just one gene as in a monohybrid cross. Crossing two double heterozygotes (AaBb x AaBb) produces a 4x4, 16-cell Punnett square and the classic 9:3:3:1 phenotype ratio when both genes assort independently.
Why does AaBb x AaBb give a 9:3:3:1 ratio?
Each parent produces 4 equally likely gamete types (AB, Ab, aB, ab), giving 16 equally likely offspring combinations. Of these, 9 show both dominant traits (A_B_), 3 show dominant gene 1 and recessive gene 2 (A_bb), 3 show recessive gene 1 and dominant gene 2 (aaB_), and 1 shows both recessive traits (aabb), a 9:3:3:1 ratio.
How many gametes does a dihybrid parent produce?
A fully heterozygous dihybrid parent (AaBb) produces 4 distinct gamete types (AB, Ab, aB, ab) in equal proportion. A parent homozygous at one gene, like AABb, still produces 4 gametes total, but only 2 distinct types (AB, Ab), each represented twice.
What does the A_B_ notation mean in the phenotype ratio?
The underscore represents 'either allele' at that position, since it does not change the phenotype. A_B_ means the dominant phenotype for both genes regardless of whether the underlying genotype is AABB, AABb, AaBB, or AaBb, all four genotypes look identical because A and B are each dominant.
What is Mendel's law of independent assortment?
The law of independent assortment states that alleles for different genes separate independently of one another during gamete formation, this holds true for genes on different chromosomes or far apart on the same chromosome. It is why a dihybrid cross's gamete types occur in the equal 1:1:1:1 proportions this calculator assumes.
What phenotype ratio results from a dihybrid test cross (AaBb x aabb)?
Crossing a double heterozygote with a double homozygous recessive individual gives a 1:1:1:1 phenotype ratio across all four possible phenotype combinations (A_B_, A_bb, aaB_, aabb). This test cross is the classic method for confirming that two genes assort independently, rather than being genetically linked.
What happens if one parent is homozygous at one gene, like AABb x AaBb?
The homozygous gene (AA) only contributes an A allele to every gamete, so all offspring inherit at least one dominant A allele and show the dominant phenotype for gene 1. The result reduces to a simple 3:1 ratio for gene 2 alone (3 A_B_ : 1 A_bb), since gene 1's phenotype no longer varies.
How is a dihybrid Punnett square different from two separate monohybrid squares?
A dihybrid Punnett square combines both genes into a single 4x4 grid of 16 cells, tracking every combination of both genes simultaneously in one offspring. Two separate monohybrid squares would only tell you the ratio for each gene independently, not how the two genes combine in the same individual.
Can this calculator handle any combination of parent genotypes, not just AaBb x AaBb?
Yes. Select any of the three genotypes (homozygous dominant, heterozygous, or homozygous recessive) independently for gene 1 and gene 2 in both parents, all 81 possible parent genotype combinations are supported and produce the correct 16-cell grid and ratios.
Why do genes need to be on different chromosomes for this ratio to hold?
Genes located close together on the same chromosome tend to be inherited together (genetic linkage) rather than assorting independently, which skews the offspring ratios away from 9:3:3:1. This calculator assumes independent assortment, the standard textbook dihybrid cross scenario, and does not model linkage or recombination frequency.
What is the difference between genotype ratio and phenotype ratio in a dihybrid cross?
The genotype ratio (up to nine categories like AABB, AABb, AaBb, and so on) counts every distinct two-gene allele combination separately. The phenotype ratio (up to four categories: A_B_, A_bb, aaB_, aabb) groups genotypes that look identical because of dominance, this is why the genotype ratio has more categories than the simpler phenotype ratio.