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The gene combination of… (Class 10 Science 2025, Q30)

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ScienceCBSE Class 10 · Board exam 2025, set 31/6/1 · 3 marks · Short answer

CBSE Class 10 Science 2025: question 30

Set in the CBSE Class 10 board exam of 2025, paper 31/6/1, section C, question 30, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 3 marks · Short answerHeredity

The gene combination of purple flowered pea plants is denoted as (WW) and that of white flowered pea plants as (ww), when these two plants are crossed generation is obtained.

  1. (a)

    List two observations made by Mendel in generation plants.

    0 marks
  2. (b)

    Give the (i) percentage of white flowered plants and (ii) ratio of the gene combinations WW, Ww and ww in generation.

    0 marks
  3. (c)

    Write one difference between dominant and recessive trait.

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/6/1.

Hints

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

Hint 1

Crossing WW with ww gives only one type of F1 plant. Which colour shows?

Hint 2

Make a checkerboard for Ww × Ww and count each combination.

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.

  1. (a) Observation 1: all the plants were purple flowered.

    ½ mark

  2. (a) Observation 2: no white or intermediate (mixed) coloured flowers appeared. Only the dominant parental trait was expressed.

    ½ mark

  3. (b)(i) On selfing (Ww × Ww), the offspring are WW, Ww, Ww, ww. Only ww is white, so white flowered plants = 1 out of 4 = 25%.

    ½ mark

  4. (b)(ii) WW : Ww : ww = 1 : 2 : 1

    ½ mark

  5. (c) A dominant trait expresses itself even in the presence of its contrasting trait (e.g. in Ww). A recessive trait remains unexpressed in the presence of the dominant form and appears only when both copies are recessive (ww).

    1 mark

Answer: (a) All F1 plants purple; no white/mixed flowers; (b)(i) 25%; (ii) 1 : 2 : 1; (c) dominant expresses in presence of contrasting trait, recessive does not.

Where the marks are

  • Any two correct F1 observations (½ × 2 = 1 mark)
  • 25% white flowered plants (½ mark)
  • Ratio 1 : 2 : 1 (½ mark)
  • One correct difference between dominant and recessive trait (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Heredity, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/6/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.