Mendel counted 556 pea seeds in the first experiment of his dihybrid cross. He crossed plants differing in seed shape and seed color, then examined the resulting F2 seeds.
The 556 seeds fell into four visible combinations: 315 round and yellow, 101 wrinkled and yellow, 108 round and green, and 32 wrinkled and green. These totals sum to 556 and approximate the expected 9:3:3:1 ratio for a dihybrid cross involving independently assorting traits.
The experiment was important because it extended Mendel’s earlier work beyond one characteristic at a time. The two traits were seed shape—round versus wrinkled—and seed color—yellow versus green. Mendel represented the contrasting hereditary factors with letter pairs and followed the plants into later generations to determine how the combinations behaved.
A common mix-up is to confuse 556 with one of the four category counts, especially 315, the largest group. The figure 556 is the total number of F2 seeds in this particular first dihybrid experiment, not the number of plants in every stage of Mendel’s broader research.