how do mendels experiments shoe that a trait could be dominant or recessive?

Mendel selected true breeding tall (TT) and dwarf (tt) pea plants.When a tall pea plant is crossed with a short (dwarf) pea plant, all the F1 hybrids are tall. (i.e., in this case, the gene causing tallness is dominant while the gene causing dwarfness is recessive.).The trait expressing itself in the hybrid is the dominant one.

(Mendel’s first law of inheritance states that when a pair of contrasting factors is brought in a hybrid, one factor inhibits the appearance of the other. The one which inhibits is the dominant one and which is inhibited is recessive.)