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Study the given pedigree… (Class 12 Biology 2023, Q23)

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BiologyCBSE Class 12 · Board exam 2023, set 57/1/1 · 3 marks · Short answer

CBSE Class 12 Biology 2023: question 23

Set in the CBSE Class 12 board exam of 2023, paper 57/1/1, section C, question 23, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q23 · 3 marks · Short answerCompetency-basedPrinciples of Inheritance and Variation

Study the given pedigree chart and answer the questions that follow.
(a) Is the trait given in the chart dominant or recessive ? Give reason in support of your answer.
(b) Is this trait autosomal or sex-linked ? Give reason in support of your answer.
(c) Write the possible genotypes of the children numbers '1' and '3' of the second generation.

Figure in the paper: Pedigree: Gen I unaffected parents (female circle, male square). Gen II children: 1 (unaffected male, married to an affected female shown as filled circle), 2 (affected male, filled square), 3 (unaffected female, married to an unaffected male). Gen III: child 1 (unaffected female, of Gen II-1 and affected female), 2 (affected male), 3 (unaffected female), 4 (unaffected female), the last three children of Gen II-3. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/1/1.

Hints

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

Hint 1

Look at generation I: can two unaffected parents have an affected child if the trait is dominant?

Hint 2

Check whether the trait appears in both sexes.

Hint 3

An unaffected mother of an affected son must carry the allele.

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) The trait is recessive.

    ½ mark

  2. Reason: both parents in generation I are unaffected, yet their son (II-2) is affected. So the parents must be carriers of a hidden (recessive) allele.

    ½ mark

  3. (b) The trait is autosomal.

    ½ mark

  4. Reason: both males and females are affected (an affected female in generation II, affected males in generations II and III), so both sexes have an equal chance of showing it.

    (Also accepted by the scheme: sex-linked (X-linked recessive), because more males than females are affected and the affected son in generation III is born to an unaffected carrier mother.)

    ½ mark

  5. (c) If autosomal: Child '1' (unaffected male) is AA or Aa.

    ½ mark

  6. Child '3' (unaffected female with an affected son) must be Aa (carrier).

    (If X-linked: Child '1' is ; Child '3' is , a carrier.)

    ½ mark

Answer: (a) Recessive; (b) autosomal (sex-linked also accepted); (c) Child 1 – AA/Aa (or XY), Child 3 – Aa (or XʰX carrier)

Where the marks are

  • Recessive (½ mark) with reason: unaffected parents have an affected child (½ mark)
  • Autosomal with reason: both sexes affected, or sex-linked with reason: more males affected (½ + ½ mark)
  • Genotype of child 1 (½ mark) and child 3 (½ mark), consistent with (b)

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 paper2023 board paper, set 57/1/1: 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.