How do Mendel's experiment show that traits are inherited independently?

Mendel crossed two pea plants differing in contrasting traits of two characters i.e a dihybrid cross.

He crossed a pea plant having yellow coloured and rounded seeds with another pea plant having green coloured and wrinkled seed. 

The F₁ generation has all round and yellow seeds.

The F₂ generation, all the characters are inherited independently. (round yellow, round green, wrinkled yellow, wrinkled green).