Dihybrid Cross Ratio

A dihybrid cross tracks inheritance of two different genes at the same time, usually when each gene has a dominant and recessive allele and the genes assort independently. If both parents are heterozygous for both traits, each can make four types of gametes in equal proportions, and combining those gametes gives offspring phenotypes in the ratio 9 dominant-dominant : 3 dominant-recessive : 3 recessive-dominant : 1 recessive-recessive. This ratio comes from the product rule for independent probabilities, so the two single-trait 3:1 patterns combine into 9:3:3:1.

Use this when solving genetics problems that involve two traits at once, such as seed shape and seed color, and the genes are assumed to assort independently. It is especially useful for predicting phenotypes from a cross like AaBb × AaBb.

• Using 9:3:3:1 when the genes are linked or do not assort independently
• Confusing genotype ratios with phenotype ratios
• Forgetting that the 1 part refers to the double recessive phenotype, not just one recessive trait