CBSE Class 10 Science 2017 · Paper 531/1 · Question 11

Mendel crossed pure-breeding tall pea plants with pure-breeding short ones. All the F₁ plants were tall; none was short or of medium height. When he let the F₁ plants self-pollinate, the F₂ generation had both tall and short plants, in the ratio of about 3 tall : 1 short. Interpretation: since short plants reappeared in F₂, the F₁ plants must have inherited both the tallness and the shortness factors, but only tallness was expressed in them. So each plant carries two copies of the factor (gene) for a trait, and the F₁ plants were Tt. The trait expressed when the two copies differ, tallness, is dominant; the trait that stays hidden in F₁ and shows only when both copies are t (tt), shortness, is recessive. TT and Tt plants are tall, and only tt plants are short.