Compute genotype and phenotype ratios for a two‑trait (dihybrid) cross. Enter parental genotypes, visualize gametes and offspring combinations in an interactive Punnett square, and explore Mendelian inheritance patterns.
A dihybrid cross is a genetic cross between two individuals that are both heterozygous for two different genes (or traits). In classical Mendelian genetics, the classic dihybrid cross is AaBb × AaBb, which yields the famous 9:3:3:1 phenotypic ratio. This ratio reflects the independent assortment of alleles at two loci, a cornerstone of Mendelian inheritance first demonstrated by Gregor Mendel in his pea‑plant experiments.
For a dihybrid cross AaBb × AaBb, the expected phenotypic ratio is:
9 : 3 : 3 : 1
(A_B_ : A_bb : aaB_ : aabb)
Mendel's Law of Independent Assortment states that alleles of different genes assort independently of one another during gamete formation. This means that the allele a parent passes on for one trait does not influence the allele passed on for another trait. For a dihybrid cross, each parent produces four types of gametes in equal proportions (AB, Ab, aB, ab). When these gametes combine randomly, the resulting offspring exhibit the 9:3:3:1 phenotypic ratio. This law holds true for genes located on different chromosomes or far apart on the same chromosome (no linkage).
The calculator parses the genotype strings you provide (e.g., AaBb) into two loci, each with two alleles. It then generates all possible gametes by taking one allele from each locus. For a heterozygous dihybrid (AaBb), the gametes are AB, Ab, aB, ab. The tool then constructs a Punnett square by combining every gamete from Parent 1 with every gamete from Parent 2, yielding all possible offspring genotypes. From these, it computes:
The tool also handles cases where one or both parents are homozygous at one or both loci, producing fewer gamete types and simpler ratios.
The following data are verified and match standard Mendelian expectations.
| Cross | Gametes (P1 × P2) | Phenotype Ratio | Genotype Ratio (simplified) |
|---|---|---|---|
| AaBb × AaBb | AB,Ab,aB,ab × AB,Ab,aB,ab | 9:3:3:1 | 1:2:1:2:4:2:1:2:1 |
| AaBb × aabb (testcross) | AB,Ab,aB,ab × ab | 1:1:1:1 | 1:1:1:1 |
| AAbb × aaBB | Ab × aB | All AaBb (1:0:0:0) | All AaBb (1) |
| AaBB × Aabb | AB,aB × Ab,ab | 3:1:0:0 (A_B_ : A_bb) | 1:1:1:1 (AABb, AaBb, AaBb, aaBb) |
| AaBb × aaBb | AB,Ab,aB,ab × aB,ab | 3:3:1:1 | varies |
A plant breeder wishes to develop a new variety of tomato that is both disease‑resistant (dominant allele R) and high‑yielding (dominant allele Y). They cross two heterozygous plants: RrYy × RrYy. Using our calculator, they can predict that among the offspring, 9/16 will exhibit both desirable traits (R_Y_), 3/16 will be disease‑resistant but low‑yielding (R_yy), 3/16 will be susceptible but high‑yielding (rrY_), and 1/16 will have neither trait (rryy). This allows the breeder to estimate the number of plants to grow in order to obtain a desired number of double‑dominant individuals.
Gregor Mendel (1822–1884) performed his groundbreaking experiments on pea plants (Pisum sativum) in the garden of the St. Thomas Abbey in Brno, Austria (now Czech Republic). He studied seven contrasting traits, including seed shape (round vs. wrinkled) and seed color (yellow vs. green). In his dihybrid crosses, Mendel observed that the two traits segregated independently, leading to the 9:3:3:1 ratio in the F2 generation. This was a radical departure from the blending inheritance theory of his time and laid the foundation for modern genetics. Mendel's work was largely ignored until 1900, when it was independently rediscovered by Hugo de Vries, Carl Correns, and Erich von Tschermak.