Mendel studied seven different traits in pea plants.
The seven traits were seed shape, seed color, flower color, flower position, pod shape, pod color, and plant height. Each trait had contrasting forms—for example, round versus wrinkled seeds, yellow versus green seeds, and tall versus dwarf plants.
Mendel selected pea plants because they were easy to grow, had clearly distinguishable varieties, and could self-pollinate or be cross-pollinated under controlled conditions. He followed the traits across generations, allowing him to compare large numbers of offspring rather than relying on isolated examples.
The number seven refers to seven trait categories, each with two contrasting forms. It does not mean that Mendel observed only seven plants or seven total characteristics in every experiment. He cultivated and tested thousands of plants, using the repeated patterns to formulate principles later called Mendelian inheritance.
The seven-trait set is also why diagrams of Mendel’s experiments often show seven pairs, such as tall/dwarf stems and inflated/constricted pods.