(a) Why is the F1 progeny always of tall plants when a tall plant is crossed with a short pea plant? (b) How is F2 progeny obtained by self-pollination of F1 progeny different from F1 progeny? Give reason for this observation. (c) State a conclusion that can be drawn on the basis of this observation. (AI 2020)

(a) Only tall plants are obtained in the F1 offspring when a tall plant (TT) crosses with a short tea plant (tt). This is due to the fact that only one of two contrasting features is present in the offspring of the first generation. The characteristic that shows in the F1 generation is therefore dominant, whereas the one that does not show is recessive. All the plants are tall since the character TT for the tall plant is dominant.

(b) On selfing F₁ progeny

The F₂ generation, we obtained both tall and dwarf plants. The appearance of suppressed recessive traits in individuals of the F₂ generation in the Mendelian cross indicates that characters of recessive traits are not lost. When the F₁ generation plants were allowed to self-fertilise both the parental trait were expressed in definite proportion in the F₂ generation.

(C) As seen above, there is only one dominant allele expressed. The law of dominance is responsible for this. Two different alleles stay together and do not mix up in a heterozygous person. They break apart during gamete formation, ensuring that every gamete receives just one allele and is always pure. This is called the “law of purity of gametes”.