Punnett Square Calculator (Dihybrid Cross)
Calculate Mendelian genetics outcomes for dihybrid crosses.
Enter alleles for two traits and get phenotype ratios, genotype frequencies, and offspring probabilities.
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/green) and seed shape (round/wrinkled) — two unlinked traits. 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, 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 — depends on 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.
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.