Skip to content

How can a single gene… (Class 12 Biology SQP 2026-27, Q25)

questionbuddy.in/q/cbse-12-biology-2026-27-sqp-q25-how-can-single-gene-defect

BiologyCBSE Class 12 · Sample paper, session 2026-27 · 3 marks · Short answer

CBSE Class 12 Biology sample paper 2026-27: question 25

From the official CBSE sample question paper for 2026-27, section C, question 25, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q25 · 3 marks · Short answerPrinciples of Inheritance and Variation

How can a single gene defect caused by the mutation result in Phenylketonuria (PKU)? Explain the molecular mechanism underlying pleiotropy.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2026-27.

Hints

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

Hint 1

PKU involves a defect in metabolising one amino acid.

Hint 2

Think about what happens when one enzyme in a pathway is missing, and how many traits that could affect.

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. PKU is an autosomal recessive inborn error of metabolism caused by a mutation in a single gene. The affected person lacks the enzyme (phenylalanine hydroxylase) that converts the amino acid phenylalanine into tyrosine.

    1 mark

  2. As a result, phenylalanine accumulates and is converted into phenylpyruvic acid and other derivatives. These build up in the brain, causing mental retardation, and are also excreted in the urine because of poor absorption by the kidneys.

    1 mark

  3. Molecular basis of pleiotropy: a single gene product (an enzyme) acts in a metabolic pathway. When it is defective, many dependent reactions are affected, so one gene shows multiple phenotypic effects. PKU itself is an example, causing mental retardation and reduced hair and skin pigmentation.

    1 mark

Answer: A mutant gene fails to make the enzyme that converts phenylalanine to tyrosine. Phenylalanine and phenylpyruvic acid accumulate in the brain and cause mental retardation. Pleiotropy arises because one gene product affects metabolic pathways in different ways, giving multiple phenotypic effects.

Where the marks are

  • Mutation causes lack of the enzyme converting phenylalanine to tyrosine (1 mark)
  • Accumulation of phenylalanine/phenylpyruvic acid in the brain causes mental retardation (1 mark)
  • Pleiotropy: a single gene affects metabolic pathways, giving multiple phenotypic expressions (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 paperSample paper 2026-27: 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.