When a tall pea plant was crossed with a dwarf pea plant, the $F_{1} $ generation plants were all tall. When $F_{1} $ plants were selfed, the $F_{2} $ generation plants were not all tall; they were tall and dwarf plants.
This shows that the $F_{1} $ plants had inherited both the parental traits but did not express dwarfness/recessive in the presence of the trait for tallness/dominant.