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Differentiate between the… (Class 12 Biology 2025, Q31b)

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BiologyCBSE Class 12 · Board exam 2025, set 57/7/1 · 5 marks · Long answer

CBSE Class 12 Biology 2025: question 31(b)

Set in the CBSE Class 12 board exam of 2025, paper 57/7/1, section E, question 31(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(b) · 5 marks · Long answerPrinciples of Inheritance and Variation

Differentiate between the following :
(i) Polygenic Inheritance and Pleiotropy
(ii) Dominance, Codominance and Incomplete dominance

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/7/1.

Hints

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

Hint 1

(i) Compare how many genes are involved and how many traits are affected.

Hint 2

(ii) Think of what the heterozygote looks like in each case: pea height, ABO blood groups and snapdragon flower colour.

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. (i) Polygenic inheritance: a single trait is controlled by three or more genes. The phenotype shows the contribution of each allele, and the effects of the alleles are additive. Example: human skin colour, controlled by genes A, B and C.

    1 mark

  2. Pleiotropy: a single gene shows multiple phenotypic effects. Example: in phenylketonuria, a mutation in one gene causes mental retardation and reduced hair and skin pigmentation.

    1 mark

  3. (ii) Dominance: in a heterozygote, only the dominant allele is expressed, so the F₁ looks like the dominant parent. Example: tall (TT) × dwarf (tt) pea plants give all tall (Tt) F₁ plants.

    1 mark

  4. Codominance: in a heterozygote, both alleles are fully expressed together, so the F₁ shows the characters of both parents. Example: ABO blood groups, where Iᴬ Iᴮ gives the AB blood group.

    1 mark

  5. Incomplete dominance: in a heterozygote, neither allele is completely dominant, so the F₁ phenotype is intermediate between the two parents. Example: in snapdragon, red (RR) × white (rr) gives pink (Rr) F₁, and the F₂ ratio is 1 red : 2 pink : 1 white.

    1 mark

Answer: (i) Polygenic: many genes control one trait additively; Pleiotropy: one gene affects many traits. (ii) Dominance: only the dominant allele is expressed; Codominance: both alleles are expressed (AB blood group); Incomplete dominance: an intermediate phenotype (pink snapdragon).

Where the marks are

  • Correct explanation of polygenic inheritance (1 mark)
  • Correct explanation of pleiotropy (1 mark)
  • Dominance explained (1 mark)
  • Codominance explained (1 mark)
  • Incomplete dominance explained (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 paper2025 board paper, set 57/7/1: 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.