The dominant stem length in Mendel’s pea plants was tall.
Mendel compared tall pea plants with dwarf plants, using plant height as one of his clearest inherited characteristics. When he crossed true-breeding tall and dwarf lines, all plants in the first-generation offspring grew tall. Tall was therefore the dominant phenotype, while dwarf was recessive.
The dwarf characteristic had not disappeared from the hybrids. It was still carried in their inherited factors and became visible again when the hybrids produced a second generation. The familiar Mendelian pattern predicted approximately three tall plants for every one dwarf plant in that generation.
Mendel’s terminology described what could be observed, not a general rule that tallness is always biologically dominant. Dominance depends on the particular alleles and trait being studied. Modern genetic work has linked the classic dwarf pea phenotype to changes affecting gibberellin production, a plant-hormone pathway involved in stem growth.
Stem length was one of seven contrasting pea traits in Mendel’s 19th-century experiments, alongside flower color, seed shape, pod color, and flower position.