Here's why:

* Dihybrid cross: A dihybrid cross involves two traits, each with two alleles. This means you are tracking two genes simultaneously.

* 9:3:3:1 ratio: This specific ratio indicates that the two genes are independently assorting. In other words, the alleles for one trait are not linked to the alleles for the other trait.

Let's break down the ratio:

* 9: Represents the offspring that inherit the dominant allele for both traits.

* 3: Represents the offspring that inherit the dominant allele for one trait and the recessive allele for the other.

* 3: Represents the offspring that inherit the recessive allele for one trait and the dominant allele for the other.

* 1: Represents the offspring that inherit the recessive allele for both traits.

Example:

Imagine a dihybrid cross involving flower color (purple vs. white) and seed shape (round vs. wrinkled). The 9:3:3:1 ratio would indicate:

* 9: Purple flowers, round seeds

* 3: Purple flowers, wrinkled seeds

* 3: White flowers, round seeds

* 1: White flowers, wrinkled seeds

Important Note: The 9:3:3:1 phenotypic ratio is only observed in the F2 generation (the offspring of the F1 generation) of a dihybrid cross.