Describe Gregor Mendel's experiment with pea plants that led to the formulation of the Law of Dominance. What specific observations supported this law?

Mendel's experiment for the Law of Dominance involved crossing pure-breeding tall pea plants with pure-breeding dwarf pea plants (P generation). 

He observed that all the offspring in the first filial (F1) generation were tall. When he self-pollinated these F1 tall plants, the second filial (F2) generation produced both tall and dwarf plants in a 3:1 ratio. 

The specific observation that supported the Law of Dominance was that in the F1 generation, only one of the parental traits (tallness) was expressed, while the other trait (dwarfness) remained hidden or masked. This indicated that one allele (for tallness) was dominant over the other (for dwarfness).