Dihybrid Cross Calculator

How to use this calculator

1. Choose both parental genotypes for the first locus, represented by A and a.
2. Choose both parental genotypes for the second locus, represented by B and b.
3. Read all nine genotype probabilities and the four phenotype groups.

Formula and method

For the assumptions behind these results, read the explanation below. You can also explore Punnett Square and Allele Frequency.

Combining two inheritance problems

A dihybrid cross tracks two loci at once. Each parent supplies one allele at each locus to an offspring. Solving the single-locus crosses first and multiplying their probabilities gives the same distribution as a sixteen-cell Punnett square when the loci assort independently.

The calculator lists all nine possible combined genotype classes, including classes with zero probability. The three classes at the first locus are AA, Aa, and aa; combining each with BB, Bb, and bb creates the complete list.

Genotype and phenotype are different

A genotype describes the allele combination. A phenotype describes the expressed trait under a specified model. With complete dominance, AA and Aa share the dominant phenotype, while aa has the recessive phenotype. Grouping the nine genotypes by their expression produces four phenotype categories.

The familiar 9:3:3:1 ratio applies to two parents heterozygous at both loci. Changing either parent changes the probabilities. For example, a double-recessive parent can transmit only the recessive allele at each locus.

When not to use independent multiplication

Genes linked closely on the same chromosome may not assort independently. Recombination frequency and the arrangement of parental alleles then matter, and this simple multiplication no longer describes the cross. Sex linkage, incomplete dominance, epistasis, and viability differences also require a different model.

Each offspring represents another probabilistic outcome. Four offspring are not required to show every phenotype, and a small family can differ substantially from the expected proportions. The values describe theoretical chances per offspring under the stated assumptions.