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A pregnant human… (Class 12 Biology SQP 2023-24, Q24)

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BiologyCBSE Class 12 · Sample paper, session 2023-24 · 3 marks · Short answer

CBSE Class 12 Biology sample paper 2023-24: question 24

From the official CBSE sample question paper for 2023-24, section C, question 24, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q24 · 3 marks · Short answerCompetency-basedHuman Reproduction

A pregnant human female was advised to undergo MTP. It was diagnosed that the fetus she was carrying had developed from a zygote having 45 chromosomes with only one X chromosome.

  1. a)

    What is this condition called and how does it arise?

    0 marks
  2. b)

    Why was she advised to undergo MTP?

    0 marks

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Which chromosomal disorder has the karyotype 45, XO?

Hint 2

What error during meiosis leads to a gamete missing a chromosome?

Hint 3

Think about how missing one X affects ovaries and sexual development.

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 condition is Turner's syndrome (karyotype 45, XO).

    ½ mark

  2. It is an aneuploidy of the sex chromosomes in which one X chromosome is missing. It arises from non-disjunction of sex chromosomes during gametogenesis, so a gamete lacking a sex chromosome fuses with a normal X-bearing gamete.

    ½ mark

  3. b) MTP was advised because the child would be a sterile female with serious defects:

    • Rudimentary ovaries, so she would be infertile
    • Lack of secondary sexual characters (poorly developed breasts) and delayed or absent menstruation
    2 marks

Answer: a) Turner's syndrome (45, XO), an aneuploidy caused by non-disjunction of the sex chromosomes. b) The child would have rudimentary ovaries (sterility), lack secondary sexual characters and have absent/delayed menstruation.

Where the marks are

  • Names Turner's syndrome (0.5 mark)
  • Explains absence of one X / aneuploidy due to non-disjunction (0.5 mark)
  • Any two symptoms such as rudimentary ovaries, lack of secondary sexual characters, infertility (1 + 1 marks)

Revise the chapter

  • Chapter: Human Reproduction (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Human Reproduction, one at a time, marked against the answer key and marking points.Back to the full paperSample paper 2023-24: 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.