What is the expected phenotypic ratio from a dihybrid cross?

In a dihybrid cross, two traits are considered, each controlled by different alleles, typically with complete dominance. When two individuals that are heterozygous for both traits (e.g., AaBb x AaBb) are crossed, the outcome can be determined using a Punnett square.

When you set up the square for this cross, you will end up with 16 total combinations of alleles. The expected phenotypic ratio from this cross is a proportion of the different combinations of traits expressed. In this case, the resulting phenotypes will consist of:

• 9 individuals expressing both dominant traits (A_B_)

• 3 individuals expressing the dominant trait for the first character and the recessive for the second (A_bb)

• 3 individuals expressing the recessive trait for the first character and the dominant for the second (aaB_)

• 1 individual expressing both recessive traits (aabb)

When you tally these contributions, you arrive at a ratio of 9:3:3:1, which represents the expected distribution of phenotypes in the offspring of a dihybrid cross. This ratio is foundational in genetics and represents the independent assortment of alleles, confirming the principle that different traits can segreg