Question

How did Mendel explain that it is possible that a trait is inherited but not expressed in an organism?

Answer 1

Some traits that are inherited may not express themselves. Such hidden traits are known as recessive traits. Mendel explained this phenomenon with the help of monohybrid cross. In a monohybrid cross performed by Mendel, tall plant was crossed with a dwarf plant which produced all tall plants in F₁ progeny. 
However, when these F₁ tall plants were crossed with each other, 'dwarf' trait, which was not observed in the F₁ generation, reappered in the F₂ progeny.

By this, it is concluded that dwarfness traits of parent pea plant were not lost.  They were suppressed in the F1 generation by the tallness traits and reappeared in the F2 generation. So, we can say that a trait that is inherited may not be expressed in an organism.