The answer is cross-pollination, by using parents that had different traits.
Gregor Mendel researched pea plants and he established the principles of heredity. To explain his experiments and discoveries, he invented terms dominant and recessive in reference to traits. In his experiments, he used cross pollination which includes the crossing of parents with different traits. For example, he crossed a plant with green peas and a plant with yellow peas (two different traits) and studied how these traits are inherited to the next generation.
