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The chromosome number is… (Class 12 Biology 2023, Q30)

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BiologyCBSE Class 12 · Board exam 2023, set 57/5/1 · 4 marks · Case-based

CBSE Class 12 Biology 2023: question 30

Set in the CBSE Class 12 board exam of 2023, paper 57/5/1, section D, question 30, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 4 marks · Case-basedCompetency-basedPrinciples of Inheritance and Variation

The chromosome number is fixed for all normal organisms leading to species specification whereas any abnormality in the chromosome number of an organism results into abnormal individuals. For example, in humans 46 is the fixed number of chromosomes both in male and female. In male it is ‘44 + XY’ and in female it is ‘44 + XX’. Thus the human male is heterogametic, in other words produces two different types of gametes one with ‘22 + X’ chromosomes and the other with ‘22 + Y’ chromosomes respectively. Human female, on the other hand is homogametic i.e. produces only one type of gamete with ‘22 + X’ chromosomes only.

Sometimes an error may occur during meiosis of cell cycle, where the sister chromatids fail to segregate called nondisjunction, leading to the production of abnormal gametes with altered chromosome number. On fertilisation such gametes develop into abnormal individuals.

  1. (a)

    State what is aneuploidy.

    1 mark
  2. (b)

    If during spermatogenesis, the chromatids of sex chromosomes fail to segregate during meiosis, write only the different types of gametes with altered chromosome number that could possibly be produced.

    1 mark
  3. (c)

    A normal human sperm (22 + Y) fertilises an ovum with karyotype ‘22 + XX’. Name the disorder the offspring thus produced would suffer from and write any two symptoms of the disorder.

    2 marks

  4. OR

    (c)

    Name a best known and most common autosomal aneuploid abnormality in human and write any two symptoms.

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/5/1.

Hints

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

Hint 1

Aneuploidy differs from polyploidy: it involves individual chromosomes, not whole sets.

Hint 2

If X and Y do not separate, one gamete gets both and the other gets neither.

Hint 3

Add the chromosomes of sperm and ovum to get the zygote's karyotype.

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) Aneuploidy is the gain or loss of one or more chromosomes. It results from failure of chromatids (or chromosomes) to segregate during cell division (nondisjunction).

    1 mark

  2. (b) If the sex chromosomes fail to separate during spermatogenesis, the abnormal gametes formed are 22 + XY and 22 + 0 (no sex chromosome).

    1 mark

  3. (c) 22 + Y (sperm) + 22 + XX (ovum) gives 44 + XXY. The disorder is Klinefelter's syndrome.

    1 mark

  4. Symptoms: the individual has overall masculine development but shows gynaecomastia (development of breasts), is tall, and is sterile.

    1 mark

  5. (c) OR: The commonest autosomal aneuploidy is Down's syndrome (trisomy of chromosome 21). Symptoms: short stature with a small round head, furrowed tongue and partially open mouth, broad palm with a characteristic palm crease, and physical, psychomotor and mental retardation.

Answer: (a) Gain or loss of chromosome(s) due to nondisjunction; (b) 22 + XY and 22 + 0; (c) Klinefelter's syndrome (44 + XXY): gynaecomastia, sterility [OR Down's syndrome: short stature with small round head, mental retardation]

Where the marks are

  • Correct definition of aneuploidy (1 mark)
  • Gametes 22 + XY and 22 + 0 (½ × 2 = 1 mark)
  • Names Klinefelter's syndrome (1 mark) with any two symptoms (½ × 2 = 1 mark)
  • OR: names Down's syndrome (1 mark) with any two symptoms (½ × 2 = 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 paper2023 board paper, set 57/5/1: 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.