Skip to content

A man whose father was… (Class 12 Biology 2025, Q1)

questionbuddy.in/q/cbse-12-biology-2025-57-1-1-q1-man-whose-father-colour-blind

BiologyCBSE Class 12 · Board exam 2025, set 57/1/1 · 1 mark · MCQ

CBSE Class 12 Biology 2025: question 1

Set in the CBSE Class 12 board exam of 2025, paper 57/1/1, section A, question 1, for 1 mark. Try it first; the hints and our step-by-step solution are below.

Q1 · 1 mark · MCQPrinciples of Inheritance and Variation

A man whose father was colour-blind marries a woman who had a colour-blind mother and normal father. What percentage of male children of this couple will be colour-blind ?

  1. (A)25%
  2. (B)0%
  3. (C)50%
  4. (D)75%

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

Hints

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

Hint 1

Ask where a son's X chromosome comes from.

Hint 2

Work out the genotype of the woman from her parents.

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. The man has a single X. It came from his mother, not from his colour-blind father, so he is normal ( is ruled out): . The woman got from her colour-blind mother and from her normal father, so she is a carrier: .

  2. Sons receive their X only from the mother: (colour-blind) and (normal). So 50% of the sons are colour-blind, option (C).

    1 mark

Answer: 50%, option (C)

Where the marks are

  • Identifies the mother as a carrier and gives 50% of sons affected (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 57/1/1: 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.