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In guinea pigs, black… (Class 12 Biology SQP 2025-26, Q24)

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BiologyCBSE Class 12 · Sample paper, session 2025-26 · 3 marks · Short answer

CBSE Class 12 Biology sample paper 2025-26: question 24

From the official CBSE sample question paper for 2025-26, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerPrinciples of Inheritance and Variation

In guinea pigs, black coat colour (G) dominates over white (g) and brown eyes (B) dominate over blue (b). The alleles for coat colour and eye colour are not linked. What will be the probability of the offspring having blue eyes and a white coat if both parents are heterozygous for eye and coat colour? Find the probability using a Punnett square

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2025-26.

Hints

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

Hint 1

Write the genotype of a parent heterozygous for both genes and list its gametes.

Hint 2

Blue and white are both recessive, so the offspring must be double homozygous recessive.

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. Both parents are heterozygous for both traits: BbGg × BbGg. Since the genes are not linked, they assort independently and each parent forms four gametes: BG, Bg, bG, bg.

    1 mark

  2. Punnett square:

    ♂ \ ♀BGBgbGbg
    BGBBGGBBGgBbGGBbGg
    BgBBGgBBggBbGgBbgg
    bGBbGGBbGgbbGGbbGg
    bgBbGgBbggbbGgbbgg

    Phenotypic ratio = 9 brown eyes, black coat : 3 brown eyes, white coat : 3 blue eyes, black coat : 1 blue eyes, white coat.

    1 mark

  3. Blue eyes and white coat needs genotype bbgg, which appears in only 1 of 16 boxes.
    Probability .

    1 mark

Answer: Probability of blue-eyed, white-coated offspring (bbgg) = 1/16 = 6.25%.

Where the marks are

  • Writes the cross BbGg × BbGg with the four gametes (1 mark)
  • Correct 4×4 Punnett square (1 mark)
  • Probability of bbgg = 1/16 (6.25%) (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 2025-26: 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.