How do Mendel's experiments show that traits may be dominant or recessive?

Explanation

When two pure bred parents with different traits for a character is crossed, they always give rise to a Fl hybrid in which phenotypically the dominant trait is expressed. . When this hybrid is self fertilised, 75% of the F2 generation will have a trait which is dominant and a mere 25% will have the recessive trait.
  Eg: Crossing of Homozygous tall plants with short plants give heterozygous tall plants (hybrid) in the Fl generation.

This shows that tallness is a dominant trait in pea plants. When these hybrid plants are self fertilised, they give rise to tall and short plants in the phenotypic ratio of 3:1. So trait for shortness is considered recessive.