Two traits at once
A dihybrid cross studies the inheritance of two different characteristics at the same time, each controlled by a separate gene with dominant and recessive alleles. Mendel crossed pea plants that differed in seed shape (round R dominant, wrinkled r recessive) and seed colour (yellow Y dominant, green y recessive).
The F1 and F2 generations
Crossing pure-breeding round-yellow plants (RRYY) with pure-breeding wrinkled-green plants (rryy) gives an F1 generation that is all heterozygous RrYy and shows both dominant traits: round and yellow seeds. When two F1 plants (RrYy × RrYy) are crossed, each parent produces four kinds of gametes — RY, Ry, rY and ry — in equal numbers.
Example
A 4 × 4 Punnett square for RrYy × RrYy produces 16 combinations. The four phenotype classes appear in the ratio 9 round-yellow : 3 round-green : 3 wrinkled-yellow : 1 wrinkled-green, the classic 9:3:3:1 ratio.
The Law of Independent Assortment
Mendel's Law of Independent Assortment states that the alleles of one gene segregate independently of the alleles of another gene during gamete formation, provided the genes are on different chromosomes. This is why new combinations such as round-green and wrinkled-yellow appear in the F2 generation — combinations not seen in the parents. This reshuffling of alleles is one of the main sources of genetic variation among offspring. Note that a dihybrid cross is really two monohybrid crosses happening together, so each trait on its own still gives the familiar 3:1 ratio, and multiplying the two (3:1)×(3:1) produces the overall 9:3:3:1 pattern.
Key idea
Dihybrid cross of two double-heterozygotes (RrYy × RrYy) → phenotype ratio 9:3:3:1. Each single trait still follows the 3:1 ratio when considered on its own.
Remember
A double heterozygote produces 4 types of gametes. Independent assortment requires the two genes to be on different (non-homologous) chromosomes.