Mendel conducted experiments by crossing tall pea plants (TT) with short pea plants (tt). In the F1 generation, all progeny plants were tall. This indicated that the tall trait expressed itself while the short trait remained hidden.

When Mendel self-pollinated the F1 plants, the F2 generation showed both tall and short plants. The genetic combinations in F2 generation were TT, Tt and tt in a 1:2:1 ratio. Interestingly, both TT and Tt plants appeared tall, while only tt plants appeared short.

This experiment demonstrated that a single copy of 'T' is enough to make the plant tall, while both copies have to be 't' for the plant to be short. This proved that traits like 'T' are dominant traits because they express themselves even when present in a single copy.

On the other hand, traits like 't' are called recessive traits because they require both copies to be present for expression. The short trait that disappeared in F1 generation reappeared in F2 generation, confirming its recessive nature.

Thus, Mendel's experiments established that traits may be dominant or recessive based on whether they express themselves with one copy or require two copies of the gene.