Generate a 4×4 Punnett square for a dihybrid cross (e.g. AaBb × AaBb) and calculate genotype and phenotype ratios, including the classic 9:3:3:1 result.
Last updated: March 2026
A dihybrid cross is a genetic cross between two organisms that examines the inheritance of two different traits simultaneously. Each trait is controlled by a different gene with two alleles (one dominant and one recessive). This type of cross was first studied by Gregor Mendel in his famous pea plant experiments.
The dihybrid cross generates a 4×4 Punnett square (16 possible offspring combinations) because each parent can produce four different types of gametes through independent assortment during meiosis. The classic phenotypic ratio for a cross between two heterozygotes (AaBb × AaBb) is 9:3:3:1.
This concept is fundamental to understanding Mendel's Law of Independent Assortment, which states that alleles for different traits are distributed to gametes independently of one another. Dihybrid crosses are essential in genetics, agriculture, and breeding programs.
Follow these steps to complete a dihybrid cross:
Standard genetic notation:
Valid inputs must:
Classic Mendel's Pea Plant Cross (AaBb × AaBb):
The 9:3:3:1 ratio is the classic phenotypic ratio for a dihybrid cross between two heterozygotes (AaBb × AaBb). It means 9/16 offspring show both dominant traits, 3/16 show trait A dominant and B recessive, 3/16 show A recessive and B dominant, and 1/16 show both recessive traits.
Independent assortment is Mendel's Second Law, stating that alleles for different traits segregate independently during gamete formation. When both parents are heterozygous for both traits (AaBb), each produces 4 different gamete types with equal probability (25% each). Homozygous loci produce duplicate gametes with higher frequency.
The 9:3:3:1 ratio only occurs when both parents are heterozygous for both traits (AaBb × AaBb). Different parental genotypes produce different ratios. Also, linked genes (on the same chromosome) don't assort independently and deviate from this ratio.
The underscore (_) represents 'any allele.' So 'A_' means the organism has at least one dominant A allele (could be AA or Aa), which is enough to express the dominant phenotype. Similarly, 'aa' means both alleles must be recessive.
For a cross between two heterozygotes (AaBb × AaBb), the phenotypic ratio is 9:3:3:1 under independent assortment.
A dihybrid cross produces 16 total offspring combinations in the Punnett square. For AaBb × AaBb specifically, there are 9 unique genotypes, which collapse into 4 phenotypes. Different parental genotypes produce different numbers of unique genotypes and phenotypes.
This calculator assumes complete dominance (one allele completely masks another). For incomplete dominance or codominance, you'd need different phenotype categories. The genotype ratios remain the same, but phenotype interpretation changes.
Linked genes (located close together on the same chromosome) don't follow independent assortment. They tend to be inherited together, reducing recombinant gamete types and deviating from expected ratios. This calculator assumes unlinked genes on different chromosomes.
Homozygous parents (e.g., AA) produce identical gametes (all A), appearing as duplicates in the 4×4 grid. This is pedagogically correct—it shows that each meiotic division has equal 1/4 probability, even when outcomes are identical. The 'Unique Gametes' section shows the distinct types and their actual frequencies.
Dihybrid crosses are essential in agriculture and animal breeding to predict offspring traits when selecting for multiple characteristics simultaneously. Breeders use these ratios to understand the probability of desired trait combinations appearing in the next generation.
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