The dominant seed shape in Gregor Mendel's pea plants was round. Mendel compared round seeds with wrinkled seeds, and roundness appeared in the first generation when true-breeding plants with contrasting seed shapes were crossed.
The result helped reveal what Mendel called dominant and recessive traits. When hybrid plants produced a second generation, the visible seed shapes generally appeared in an approximate 3:1 ratio: about three round seeds for every one wrinkled seed. Round was the dominant phenotype, while wrinkled was recessive.
The distinction concerns seed appearance, not the shape of the entire pea pod. Mendel separately studied pod shape, flower color, plant height, seed color, and flower position. His 1866 paper described these inheritance patterns without using the modern term gene; he referred instead to hereditary factors. The significance of the experiment was that a hidden trait could disappear in one generation and reappear predictably in the next.