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In mice, the traits… (Class 12 Biology SQP 2026-27, Q24)

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BiologyCBSE Class 12 · Sample paper, session 2026-27 · 3 marks · Short answer

CBSE Class 12 Biology sample paper 2026-27: question 24

From the official CBSE sample question paper for 2026-27, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerCompetency-basedPrinciples of Inheritance and Variation

In mice, the traits for running ability and hair colour assort independently. Running (R) is dominant over walking in circles (r).
Black hair (B) is dominant over brown hair (b). A heterozygous running, brown-haired female mouse was artificially inseminated using sperm from a heterozygous running, heterozygous black male.

A. Write the genotypes of both parents and draw a Punnett square showing all possible offspring combinations. (1)
B. Using the Punnett square, state the phenotypic ratio for: Running–Black, Running–Brown, Walking–Black, Walking–Brown. (1)
C. If 64 offspring were produced, calculate how many would be expected to show the Running–Black phenotype. (1)

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

Write the genotypes first: brown is recessive, so the female must be bb.

Hint 2

The female forms only two kinds of gamete; the male forms four.

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. Female (heterozygous running, brown) = Rrbb, with gametes Rb and rb. Male (heterozygous running, heterozygous black) = RrBb, with gametes RB, Rb, rB and rb.

    ♀ \ ♂RBRbrBrb
    RbRRBbRRbbRrBbRrbb
    rbRrBbRrbbrrBbrrbb
    1 mark

  2. B. Out of 8 boxes:

    • Running–Black (RRBb, RrBb ×2) = 3/8
    • Running–Brown (RRbb, Rrbb ×2) = 3/8
    • Walking–Black (rrBb) = 1/8
    • Walking–Brown (rrbb) = 1/8

    Phenotypic ratio = 3 : 3 : 1 : 1 (written as 6 : 6 : 2 : 2 on a 4×4 square).

    1 mark

  3. C. Running–Black offspring = .

    1 mark

Answer: Parents Rrbb × RrBb; ratio 3:3:1:1 (Running-Black : Running-Brown : Walking-Black : Walking-Brown); 24 Running–Black offspring.

Where the marks are

  • Correct parental genotypes, gametes and Punnett square (1 mark)
  • Phenotypic ratio 3:3:1:1 (1 mark)
  • 64 × 3/8 = 24 (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Principles of Inheritance and Variation, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2026-27: 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.