Punnett Square Calculator (Dihybrid Cross)

What Is a Dihybrid Cross? A dihybrid cross examines inheritance of TWO traits simultaneously. Gregor Mendel performed dihybrid crosses on peas in 1866 in what is now the Czech Republic, establishing the Law of Independent Assortment. He crossed plants differing in seed color (yellow or green) and seed shape (round or wrinkled), two traits that sit on different chromosomes. The result was a classic 9:3:3:1 phenotype ratio in the F2 generation.

Mendel’s Laws Law of Segregation: Each organism carries two alleles for each trait. During gamete formation, alleles separate so each gamete carries only one. Law of Independent Assortment: Alleles of different genes assort independently into gametes (assuming genes are on different chromosomes).

Genotype Notation Dominant alleles are written in uppercase (A), recessive in lowercase (a). Homozygous dominant: AA. Heterozygous: Aa. Homozygous recessive: aa. For two traits: AABB, AaBb, aabb, AaBB, etc. A dihybrid (AaBb) parent produces four gamete types, AB, Ab, aB and ab, each with probability 1/4.

The 16-Square Punnett Grid A dihybrid cross requires a 4×4 grid = 16 boxes. Each row is a gamete from Parent 1, each column a gamete from Parent 2. The 9:3:3:1 ratio (for AaBb × AaBb) means: 9/16 dominant for both traits (A_B_) 3/16 dominant for trait 1, recessive for trait 2 (A_bb) 3/16 recessive for trait 1, dominant for trait 2 (aaB_) 1/16 recessive for both traits (aabb)

Phenotype vs Genotype Genotype: the actual allele combination (e.g. AaBb). Phenotype: the observable expression, which depends on the dominance relationships. With simple dominance: A_ (AA or Aa) gives the same phenotype as AA. In a 9:3:3:1 cross there are 9 genotypic classes but only 4 phenotypic classes.

Applications in Genetics Plant breeding: predicting offspring traits for crop improvement. Animal husbandry: coat color, horn presence, and other visible traits. Medical genetics: predicting disease risk when both parents are carriers. Forensics: using genetic markers to determine paternity or relatedness.

Reading the grid

The grid this calculator draws is the Punnett square itself. Parent 1’s gametes run down the left edge, Parent 2’s across the top, and each cell is the genotype an offspring gets from that pair of gametes. Every cell carries equal probability, which is the whole reason the square works: counting boxes gives you the ratio directly, without any algebra.

For AaBb × AaBb that is a 4×4 grid, sixteen boxes, and counting the phenotype classes gives 9:3:3:1. For a cross where one parent is homozygous at a locus, that parent makes fewer gamete types and the grid is smaller. AABb × AaBb produces a 2×4 grid of eight boxes, and the answer is 3:1, not 9:3:3:1. Fewer boxes is not an error, it just means one parent had less to vary.

The testcross option is the special case worth knowing. Crossing anything against aabb, the double recessive, makes the grid read out the unknown parent’s gametes directly, because the recessive parent contributes nothing that can mask them. That is why breeders use it to work out whether a dominant-looking plant is AA or Aa.

Limits of the Model This calculator assumes simple dominance (no codominance, incomplete dominance, or epistasis). It assumes the two genes are on different chromosomes (independent assortment). Linked genes (on the same chromosome) violate the 9:3:3:1 ratio. Sex-linked traits require modified analysis.