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Answer the following… (Class 12 Biology 2026, Q32b)

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BiologyCBSE Class 12 · Board exam 2026, set 57/5/1 · 5 marks · Long answer

CBSE Class 12 Biology 2026: question 32(b)

Set in the CBSE Class 12 board exam of 2026, paper 57/5/1, section E, question 32(b), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(b) · 5 marks · Long answerPrinciples of Inheritance and Variation

Answer the following questions : 2+3=5
(i) How does a chromosomal disorder differ from a Mendelian disorder ? Write one example for each.
(ii) Name the phenomenon that leads to situations like 'XO' abnormality in humans. Also name this genetic disorder. How are individuals with an XO chromosomal abnormality affected ? Write its symptoms as well as karyotype.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/5/1.

Hints

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

Hint 1

Think about whether the defect lies in one gene or in a whole chromosome.

Hint 2

What do we call a change in chromosome number caused by gaining or losing a single chromosome?

Hint 3

XO means one sex chromosome is missing in a female. Count the autosomes plus the sex chromosomes.

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. (i) Difference: A Mendelian disorder is caused by a mutation or alteration in a single gene and is passed on in the Mendelian pattern, so it can be traced through a pedigree. A chromosomal disorder is caused by the absence, excess or abnormal arrangement of one or more whole chromosomes (for example, through failure of chromatids to separate during cell division). It does not follow Mendelian inheritance.

    1 mark

  2. Example of a Mendelian disorder: haemophilia, sickle-cell anaemia, colour blindness, phenylketonuria, thalassemia or cystic fibrosis (any one).

    ½ mark

  3. Example of a chromosomal disorder: Down's syndrome (trisomy 21), Klinefelter's syndrome (XXY) or Turner's syndrome (XO) (any one).

    ½ mark

  4. (ii) Phenomenon: Aneuploidy, specifically monosomy. It arises from non-disjunction during gamete formation, which gives a gamete with no sex chromosome.

    ½ mark

  5. Disorder: Turner's syndrome.

    ½ mark

  6. How affected individuals are affected (symptoms): They are females who are sterile because their ovaries are rudimentary. They show lack of (underdeveloped) secondary sexual characters and short stature.

    1½ marks

  7. Karyotype: 44 + XO, a total of 45 chromosomes (one X chromosome is missing).

    ½ mark

Answer: (i) Mendelian disorder: single-gene mutation, follows Mendelian inheritance (e.g. haemophilia). Chromosomal disorder: gain or loss of whole chromosome(s), not Mendelian (e.g. Down's syndrome). (ii) Aneuploidy (monosomy); Turner's syndrome; sterile females with rudimentary ovaries, no secondary sexual characters and short stature; karyotype 44 + XO (45).

Where the marks are

  • Correct difference: single-gene alteration with Mendelian inheritance versus change in chromosome number or structure (1 mark)
  • One correct example of a Mendelian disorder (½ mark)
  • One correct example of a chromosomal disorder (½ mark)
  • Names the phenomenon as aneuploidy/monosomy (½ mark)
  • Names the disorder as Turner's syndrome (½ mark)
  • Any three symptoms: sterility, rudimentary ovaries, lack of secondary sexual characters, short stature (½ × 3 = 1½ marks)
  • Karyotype 44 + XO, i.e. 45 chromosomes (½ 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 paper2026 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.