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 $F_1$ progeny.However, when these $F_1$ tall plants were crossed with each other, both tall and dwarf pea plants were obtained in $F_2$ generation. Reappearance of the dwarf pea plants in $F_2$ generation proves that the dwarf trait was inherited but not expressed in $F_1$ generation. So, dwarfness traits of plants (pea plant) were not lost but are suppressed in the $F_1$ generation and thus reappeares in $F_2$ generation.