Law of independent assortment.

The P plants of this dihybrid cross are true-breeding: one with yellow-round seeds and the other with green-wrinkled seeds.

The F1 dihybrids are heterozygous for both characters.

Self-pollination of the F1 yields a phenotypic ratio of 9:3:3:1 in the F2, NOT the 3:1 typical of a monohybrid cross.

This id due to the independent assortment of the 2 traits during meiosis, assuming they are on different chromosomes.

One way to ensure you have all combinations of gametes for a dihybrid cross is the FOIL method.