In one of his experiments with pea plants Mendel observed that when a pure tall pea plant is crossed with a pure dwarf pea plant, in the first generation, $F_1$ only tall plants appear. (a) What happens to the traits of the dwarf plants in this case?(b) When the $F_1$ generation plants were selffertilised, he observed that in the plants of second generation, $F_2$ both tall plants and dwarf plants were present. Why it happened? Explain briefly.