Question

(a) How do Mendel's experiments show that traits may be dominant or recessive?
(b) How do Mendel's experiments show that traits are inherited independently?

Answer 1

(a) Mendel crossed pure-bred tall pea plants with pure-bred dwarf pea plants and found that only tall pea plants were produced in the F₁ generation. From this, Mendel concluded that the F₁ generation showed the traits of only one of the parents: tallness. The trait of other parent plant, dwarfness, did not show in the progeny of the first generation. When tall pea plants of the first generation were crossed, then in F₂ generation, tall plants and dwarf plants were obtained in the ratio 3:1.
From these experiments, Mendel concluded that the trait for dwarfness of one of the parent pea plant had not been lost. It was merely concealed or suppressed in the first generation to reemerge in the second generation.

(b) Mendel chose two contrasting characters: shape and colour of seeds. The pea plants had round-yellow seeds and wrinkled green seeds. Mendel first crossed pure-bred pea plants with round-yellow seeds with pure-bred pea plants having wrinkled-green seeds and found that only round-yellow seeds were produced in the first generation. From this, he concluded that round shape and yellow colour of the seeds were dominant traits over the wrinkled shape and green colour of the seeds. When the F₁ generation pea plants with round-yellow seeds were cross-bred by self pollination, then four types of seeds with different combinations of shape and colour were obtained in the F₂ generation. Mendel observed that he had started with two combinations of characteristics in seeds and two new combinations of characteristics had appeared in the F₂ generation i.e. round-green and wrinkled-yellow. On the basis of this observation, Mendel concluded that though the two pairs of original characteristics​ combine in the F₁ generation, they separate and behave independently in subsequent generations.