In his first experiment, Mendel studied the inheritance of one trait, seed colour, in pea plants. He crossed pure-breeding plants with green seeds and pure-breeding plants with yellow seeds. The F1 generation (first filial generation) produced only yellow-seeded plants. This result suggested that the allele for yellow seeds was dominant over the allele for green seeds.

Mendel then allowed the F1 plants to self-fertilize. The F2 generation produced both yellow-seeded and green-seeded plants in a ratio of approximately 3:1. This result suggested that the F1 plants were heterozygous for seed colour, meaning that they carried both the dominant allele for yellow seeds and the recessive allele for green seeds.

Mendel's first experiment provided evidence for the particulate nature of inheritance and the dominance of certain alleles over others. It also laid the foundation for his subsequent experiments on the inheritance of multiple traits and the development of his theory of heredity.