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 appeared.
(i) What happens to the traits of the dwarf plants in this case?
(ii) When the `F_(1)` generation plants were self-fertilised, he observed that in the plants of second generation, `F_(2)` both tall plants and dwarf plants were present. Why it happened? Explain briefly.

Explanation

Through his monohybrid cross, Mendel showed that the traits may be dominant or recessive. He crossed true-breeding tall (TT) and dwarf (tt) pea plants.. All the Fl plants obtained were tall. Not a single dwarf plant was produced.
This shows that:
a] The tall trait will be expressed in both homozygous and heterozygous condition, while the dwarf trait gets expressed only in homozygous condition.
Hence in the above experiment, the dwarf trait is masked by the tall trait in heterozygous condition.
b] Mendel self-pollinated the `F_1` plants to obtain the next generation of offsprings (`F_2` generation). To his surprise, he observed that not all the `F_2` generation plants were tall. 
One-fourth of the F, plants were short while the rest were tall.
  25% of the plants inherited dwarf trait in homozygous condition. Hence the trait got  xpressed in 25% of the population.