IB Biology D3.2 Inheritance: practice questions

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D3.2-03 · Paper 1A style · multiple choice · 1 mark

A carrier mother's son

A man with red-green colour blindness, a sex-linked recessive condition, has a partner with normal vision whose father had red-green colour blindness. They have a son.

What is the probability that their son has red-green colour blindness? [1]

1. 0 %
2. 25 %
3. 50 %
4. 100 %

Show the answer and the reasoning

Answer: C. C is correct. The woman inherited an X^b from her father but has normal vision, so she is X^BX^b. A son receives his X from his mother (and a Y from his father), so the chance he receives X^b is ½. The father's genotype does not matter for a son. A assumes sons cannot inherit the allele from a mother with normal vision. B is the probability that any child, of either sex, is a colour-blind son; the question tells you the child is a son. D wrongly assumes the father passes his X chromosome to his son.

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The 5 questions in D3.2

1. D3.2-01 Variation in cob lengthPaper 1A style · multiple choice · 1 mark
2. D3.2-02 Reading a pedigreePaper 1A style · multiple choice · 1 mark
3. D3.2-03 A carrier mother's sonPaper 1A style · multiple choice · 1 mark
4. D3.2-04 Speckled chickensPaper 2 style · short answer · 6 marks
5. D3.2-05 Testing a dihybrid ratioPaper 1B style · data-based · 9 marks · HL

More of D Continuity and change

• D1.1 DNA replication
• D1.2 Protein synthesis
• D1.3 Mutation and gene editing
• D2.1 Cell and nuclear division
• D2.2 Gene expression
• D2.3 Water potential
• D3.1 Reproduction
• D3.3 Homeostasis
• D4.1 Natural selection
• D4.2 Stability and change
• D4.3 Climate change

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Every Biology and Chemistry question here is original, written for practice in the style of the papers set since the 2025 syllabus, with invented data. Developed independently from the International Baccalaureate Organization, which has not endorsed it. Found a mistake? Email vytis@limeadmissions.com with the question id.