A dihybrid cross between two heterozygotes produces a phenotypic ratio of 9:3:3:1.

Here's why:

* Dihybrid cross: This involves two traits, each with two alleles.

* Heterozygotes: Both parents have one dominant and one recessive allele for each trait.

* Phenotypic ratio: This refers to the ratio of different physical appearances in the offspring.

Example:

Let's say we're crossing two pea plants that are heterozygous for seed shape (round, R, vs. wrinkled, r) and seed color (yellow, Y, vs. green, y).

* Parent 1: RrYy

* Parent 2: RrYy

When you use a Punnett square to work out the possible combinations, you'll find:

* 9 offspring will have round yellow seeds (R-Y-)

* 3 offspring will have round green seeds (R-yy)

* 3 offspring will have wrinkled yellow seeds (rrY-)

* 1 offspring will have wrinkled green seeds (rryy)

This gives the classic 9:3:3:1 phenotypic ratio.