Inbred, true-breeding lines were available. Easily observable traits were available such as flower color, seed color, seed shape, pod color, pod shape, flower position, and plant height.
Peas are normally self-fertilizing, but artificial cross fertilizations are readily done.
F1 plants are true hybrids. Hybrid F1 plants were next self-fertilized. The segregation of traits in the next generation was followed. Definition of some terms. The Language of Genetics.