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In human beings, there are 23… (Class 10 Science 2025, Q38)

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ScienceCBSE Class 10 · Board exam 2025, set 31/5/1 · 4 marks · Case-based

CBSE Class 10 Science 2025: question 38

Set in the CBSE Class 10 board exam of 2025, paper 31/5/1, section E, question 38, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q38 · 4 marks · Case-basedCompetency-basedHeredity

In human beings, there are 23 pairs of chromosomes. Out of these 23 pairs of chromosomes (i.e. 46 chromosomes), 22 pairs of chromosomes are called autosomes and one pair of chromosomes, i.e. two chromosomes are called sex chromosomes. The sex chromosomes are of two types – 'X' chromosomes and 'Y' chromosomes. The sex of a child (i.e. progeny), is decided at the time of fertilisation. In other words, at the time of zygote formation the sex chromosomes inherited from the parents of a child decide whether the new born will be a boy or a girl.

  1. (a)

    What are chromosomes?

    1 mark
  2. (b)

    Why is the pair of sex chromosomes in human males called mismatched pair?

    1 mark
  3. (c)

    (A) Show with the help of a flow chart that the statistical probability of getting a boy or a girl is 50 : 50.

    2 marks

  4. OR

    (c)

    (B) Mention two examples of animals where sex is not determined genetically like human beings. Describe in brief the method of sex determination in these animals.

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 31/5/1.

Hints

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

Hint 1

Recall what genes are made of and where they are located.

Hint 2

Compare the sizes of the X and Y chromosomes.

Hint 3

For the flow chart, list the gametes each parent can produce, then combine them.

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) Chromosomes are thread-like structures in the nucleus, made of DNA and proteins. They carry genes, the information for inheriting traits from parents to offspring.

    1 mark

  2. (b) In males the sex-chromosome pair is XY. The X chromosome is of normal size but the Y chromosome is much shorter. The two partners are unequal, so the pair is called mismatched.

    1 mark

  3. (c)(A) Flow chart: Father (XY) → sperms carry either X or Y (50% each). Mother (XX) → all eggs carry X. Then: X egg + X sperm → XX (girl); X egg + Y sperm → XY (boy).

    1 mark

  4. Each type of sperm is equally likely to fertilise the egg, so the probability of a boy = probability of a girl = , i.e. 50 : 50. The sex of the child depends on the chromosome inherited from the father.

    1 mark

  5. (c)(B) Example 1, some reptiles (e.g. turtles, lizards): The temperature at which the fertilised eggs are incubated decides whether the offspring is male or female.

    1 mark

  6. Example 2, snails: An individual snail can change its sex during its life, which shows that its sex is not fixed genetically.

    1 mark

Answer: (a) Thread-like DNA–protein structures in the nucleus carrying genes; (b) the Y chromosome is much shorter than the X; (c)(A) XY × XX gives XX : XY = 1 : 1 (50 : 50); (c)(B) reptiles (incubation temperature decides sex) and snails (can change sex).

Where the marks are

  • Defines chromosomes as DNA-containing structures carrying genes/hereditary information (1 mark)
  • States that the Y chromosome is short while the X is normal-sized (1 mark)
  • (c)(A) Correct flow chart showing XX and XY parents, their gametes and the offspring (1 mark)
  • (c)(A) Concludes that there is an equal (50 : 50) chance of a boy or a girl (1 mark)
  • (c)(B) Reptiles: incubation temperature determines sex (1 mark)
  • (c)(B) Snails: individuals can change sex (1 mark)

Revise the chapter

Practise this chapterBoard-style questions from Heredity, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 31/5/1: 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.