CBSE Class 10 Science 2022 · Paper 31/2/2 · Question 15

(a) Tallness is dominant over shortness. Each F1 plant receives a factor for tallness from the tall parent and one for shortness from the short parent ($\text{Tt}$), but only the dominant trait, tallness, is expressed; the two factors do not blend. So all the F1 plants are tall, not of medium height. (b) Shortness (short plant height) is the recessive trait of the tall and short pair; in the round and wrinkled pair of seed shape that part (c) and its alternative use, wrinkled seed is the recessive trait. (c)(i) When the F1 plants (tall with round seeds, $\text{TtRr}$) self-pollinate, the F2 shows two new combinations not seen in either parent: tall plants with wrinkled seeds and short plants with round seeds, $3$ of every $16$ plants each, that is, in the ratio $3:3$ ($1:1$) between them. The complete F2 ratio is $9$ tall round : $3$ tall wrinkled : $3$ short round : $1$ short wrinkled. OR (c)(ii) (i) Tall with round seeds: $\frac{9}{16}\times 1600=900$ plants; (ii) short with wrinkled seeds: $\frac{1}{16}\times 1600=100$ plants. Conclusion: the traits for plant height and for seed shape are inherited independently of each other, which is why new combinations appear in the F2; tall and round are the dominant traits.