If mendel had crossed a true breeding dominant plant with a true breeding recessive plant, in which of the three generations is the recessive trait visible?

If Mendel crossed a true breeding dominant plant with a true breeding recessive plant the recessive trait is visible in the F3 generation.

Explanation:

In Mendel’s cross the selected parents are a tall pea plant and a short pea plant. Tall is the dominant trait and short is the recessive trait.  Both the parents are homozygous.

Letter T represents tall and t represents short. Hence the phenotype TT is homozygous dominant and tt is homozygous recessive.  

The genotype Tt represents tall since T is dominant over t. Mendel’s cross can be represented as  

F1 generation      Tt     Tt   Tt   Tt

F2 generation     TT   Tt  Tt tt  

b brown mice / white sand dunes