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In a family, the father, the… (Class 12 Biology 2024, Q26)

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BiologyCBSE Class 12 · Board exam 2024, set 57/1/1 · 3 marks · Short answer

CBSE Class 12 Biology 2024: question 26

Set in the CBSE Class 12 board exam of 2024, paper 57/1/1, section C, question 26, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q26 · 3 marks · Short answerCompetency-basedPrinciples of Inheritance and Variation

In a family, the father, the daughter and the son are colour blind, whereas the mother has normal vision. Do you think the son and the daughter have inherited the disease from their father ? Work out a cross to justify your answer.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/1/1.

Hints

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

Hint 1

Colour blindness is carried on the X chromosome. Which chromosome does a son get from his father?

Hint 2

A normal mother with a colour-blind child must be carrying something. Write her genotype and cross it with the father's.

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. No. The son has inherited colour blindness from his mother, while the daughter has inherited it from both mother and father.

    ½ mark

  2. Colour blindness is an X-linked recessive trait. A son receives his X chromosome only from his mother and his Y from his father, so his defective X () must come from the mother. A colour-blind daughter must be : one from the father and one from the mother. A normal-visioned mother with a colour-blind child must therefore be a carrier.

    ½ mark

  3. Parents: Mother (normal, carrier) × Father (colour blind).

    1 mark

  4. Gametes: mother gives or ; father gives or .

    colour-blind daughter colour-blind son
    carrier daughter normal son

    The colour-blind son () received from the mother, and the colour-blind daughter () received from each parent.

    1 mark

Answer: No – the son inherited it from the mother (carrier ); the daughter inherited it from both parents. Cross: .

Where the marks are

  • Answers No (½ mark)
  • Son from mother; daughter from both parents (½ mark)
  • Mother carrier, father (½ + ½ marks)
  • Correct cross/Punnett square showing offspring (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 paper2024 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.