Dihybrid crosses involve two genes with two differing alleles for each gene. In Mendel's genetic experiments on pea plants, he studied the inheritance of two traits simultaneously. He also studied how these traits were inherited through two generations, finding the ratios of the resulting offspring.

The most notorious pair of traits which Mendel studied simultaneously was height and flower colour. He did this by taking pure-breeding strains of pea plants and crossed them.

• 1 tall plant with purple flowers

• 1 short plant with white flowers
  

From this, he found that the offspring of a cross between these parents always produced plants which were tall and purple flowered; this was the F1, or first filial generation.

Members of the F1 generation were then self-pollinated, and in the F2 generation, there were four different phenotypes;

• Tall plants with purple flowers

• Tall plants with white flowers

• Short plants with purple flowers

• Short plants with white flowers

The genotype/phenotype ratio which can be derived from this is 9:3:3:1. Refer to the table below, where;

• T represents the allele for tallness

• t represents the allele for shortness

• P represents the allele for purple flowers

• p represents the allele for short flowers