The scientist Gregor Mendel investigated differences in the length of the stem in the pea plant, Pisum sativum. In 1866, he published the results of his investigation into this trait (characteristic). [Figure 2.1] Mendel observed that the pea plants he grew either had tall stems or dwarf (short) stems. In his investigation, Mendel carried out crosses using pea plants with these two phenotypes. From the results of these crosses, Mendel demonstrated that tall stems were dominant to dwarf stems in pea plants. It is now known that the stem length trait in pea plants is controlled by one gene that has two alleles: • a dominant allele, Le • a recessive allele, le.