In order to trace the inheritance of traits Mendel crossed pea plants having one contrasting character or a pair of contrasting characters. When he crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds, he observed that no plants with wrinkled and green seeds were obtained in the F_1 generation. When the F_1 generation pea plants were cross-bred by self-pollination, the F_2 generation had seeds with different combinations of shape and colour also. (a) Write any two pairs of contrasting characteristics of pea plant used by Mendel other than those mentioned above. (b) Differentiate between dominant and recessive traits. (c) State the ratio of the combinations observed in the seeds of F_2 generation (in the above case). What do you interpret from this result ? (c) Given below is a cross between a pure violet flowered pea plant (V) and a pure white flowered pea plant (v). Diagrammatically explain what type of progeny is obtained in F_1 generation and F_2 generation : Pure violet flowered plant × Pure white flowered plant. (V V) (v v)

ScienceCBSE Class 10 · Board exam 2023, set 31/1/1 · 4 marks · Case-based

CBSE Class 10 Science 2023: question 38

Set in the CBSE Class 10 board exam of 2023, paper 31/1/1, section E, question 38, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q38 · 4 marks · Case-basedCompetency-basedHeredity

In order to trace the inheritance of traits Mendel crossed pea plants having one contrasting character or a pair of contrasting characters. When he crossed pea plants having round and yellow seeds with pea plants having wrinkled and green seeds, he observed that no plants with wrinkled and green seeds were obtained in the $F_{1}$ generation. When the $F_{1}$ generation pea plants were cross-bred by self-pollination, the $F_{2}$ generation had seeds with different combinations of shape and colour also.

(a)
Write any two pairs of contrasting characteristics of pea plant used by Mendel other than those mentioned above.
1 mark
(b)
Differentiate between dominant and recessive traits.
1 mark
(c)
State the ratio of the combinations observed in the seeds of $F_{2}$ generation (in the above case). What do you interpret from this result ?
2 marks
OR
(c)
Given below is a cross between a pure violet flowered pea plant (V) and a pure white flowered pea plant (v). Diagrammatically explain what type of progeny is obtained in $F_{1}$ generation and $F_{2}$ generation :
Pure violet flowered plant × Pure white flowered plant.
(V V) (v v)
2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Recall Mendel's seven pairs of characters in pea plants, such as plant height and flower colour.

Hint 2

Think about which trait shows in a hybrid that carries one gene of each kind.

Hint 3

For two traits together, Mendel got four seed types in F₂. For one trait, draw a Punnet square for Vv × Vv.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

(a) Two other pairs of contrasting characters Mendel used: tall vs dwarf plants (height of stem)
½ mark
and violet (purple) vs white flowers (flower colour). Inflated vs constricted pods or green vs yellow pods are also accepted.
½ mark
(b) Dominant trait: a trait that is expressed even when only one copy of its gene (factor) is present, e.g. Tt shows tallness.
½ mark
Recessive trait: a trait that is suppressed when a dominant gene is present. It is expressed only when both copies are recessive, e.g. tt is dwarf.
½ mark
(c)(a) In the $F_{2}$ generation the seeds appear as round-yellow : round-green : wrinkled-yellow : wrinkled-green in the ratio 9 : 3 : 3 : 1.
1 mark
Interpretation: new combinations (round-green and wrinkled-yellow) appear. This shows that the genes for seed shape and seed colour are inherited independently of each other (law of independent assortment).
1 mark
(c)(b) [alternative choice, 2 marks if attempted instead of (c)(a)] Parents: VV (pure violet) × vv (pure white). Gametes: V and v (½ mark).
$F_{1}$ : all Vv, all violet flowered, because V is dominant (½ mark).
Self-pollination of $F_{1}$ : Vv × Vv. Gametes V, v from each parent (½ mark).
$F_{2}$ (Punnet square): VV, Vv, Vv, vv. 3 violet : 1 white phenotypically, and genotype ratio 1 VV : 2 Vv : 1 vv (½ mark).

Answer: (a) Tall/dwarf plants, violet/white flowers (or any other valid pairs); (b) dominant trait shows even with one copy, recessive trait shows only when both copies are recessive; (c)(a) 9 : 3 : 3 : 1, meaning the traits are inherited independently; (c)(b) F₁ all Vv violet; F₂ VV : Vv : vv = 1 : 2 : 1, i.e. 3 violet : 1 white

Where the marks are

Any two valid contrasting pairs (½ + ½ marks)
Meaning of dominant trait (½ mark)
Meaning of recessive trait (½ mark)
Ratio 9 : 3 : 3 : 1 (1 mark)
Interpretation: traits inherited independently (1 mark)
OR (c): correct cross showing parents/gametes, F₁ Vv violet, selfing, F₂ 3 violet : 1 white (½ × 4 = 2 marks)

Revise the chapter

Chapter: Heredity (notes, formulas, where marks are lost and every past question)
Topic: Mendel's Experiments and Monohybrid Cross

Practise this chapterBoard-style questions from Heredity, one at a time, marked against the answer key and marking points.Back to the full paper2023 board paper, set 31/1/1: every question, by section.

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CBSE Class 10 Science 2023: question 38

Hints

Step-by-step solution

Revise the chapter

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