(a) Mendel crossed tall pea plants with dwarf pea plants.
The F1 generation contained all tall plants. When F1 generation underwent selfing, the trait that was unexpressed in F1 (dwarf) was observed in some F2 progeny. Thus, both traits, tall and dwarf, were expressed in F2 generation in the ratio 3 : 1.
Mendel proposed that something was being passed unchanged from generation to generation. He called these things factors (presently called genes). Factors contain and carry hereditary information.
He also observed that traits might not show up in an individual but were passed to the next generation.