CBSE Class 10 Science 2022 · Paper 31(B)/6 · Question 11

Heredity 3 marks Medium

In one of his experiments with pea plants, Mendel observed that when tall pea plants (TT) were crossed with dwarf pea plants (tt), in the first generation (

F_{1}

) plants only tall plants appeared.

(a)What happens to the traits for dwarfness in this case ?
(b)When the $F_{1}$ generation plants were self-pollinated, he observed that in the plants of $F_{2}$ generation both tall plants and dwarf plants were present. Explain briefly why it happened.
(c)Mention the ratio obtained in such a cross.

Answer

Dwarfness is hidden in $F_{1}$ ( $T t$ ) because it is recessive; it reappears in one quarter of $F_{2}$ , giving tall : dwarf $= 3 : 1$ .

(a) The trait for dwarfness is recessive. It is not lost: every

F_{1}

plant is

T t

and carries the dwarf factor

t

, but because tallness (

T

) is dominant, the dwarf trait is not expressed and remains hidden in

F_{1}

. (b) The

F_{1}

plants (

T t

) form two kinds of gametes,

T

and

t

, in equal numbers, because the two factors separate during gamete formation. On self-pollination the gametes combine at random, giving

T T

T t

T t

and

tt

. The

tt

plants, one in four, have no factor for tallness, so the recessive dwarf trait shows again in

F_{2}

. (c) The

F_{2}

plants are in the ratio

3

tall

: 1

dwarf (phenotypic ratio); the genotypic ratio is

1 T T : 2 T t : 1 tt

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From the board paper

CBSE Class 10 Science 2022 · Paper 31(B)/6, Question 11 — all 15 questions with answers →

CBSE Class 10 Science 2022 · Paper 31(B)/6 · Question 11

Answer

From the board paper

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