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

• Mendel conducted the experiments using Pisum sativum or pea plant.
• He selected homozygous tall (TT) and dwarf (tt) pea plants.
• He crossed the tall pea plant with the dwarf pea plant.
  
• It was observed that the F${}_1$ generation are all tall plants.
• Thus, it was concluded that the gene causing tallness is dominant while the gene causing dwarfness is recessive.
• The trait expressing itself in the hybrid is the dominant one.
  
• This experiment proves Mendel’s first law of inheritance.
• It states that when a pair of contrasting factors is brought in a hybrid, one factor inhibits the appearance of the other, one which inhibits is the dominant one and which is inhibited is recessive.