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In some families, either… (Class 10 Science 2023, Q38)

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ScienceCBSE Class 10 · Board exam 2023, set 31/5/1 · 4 marks · Case-based

CBSE Class 10 Science 2023: question 38

Set in the CBSE Class 10 board exam of 2023, 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 some families, either rural or urban, females are tortured for giving birth to a female child. They do not seem to understand the scientific reason behind the birth of a boy or a girl. In fact the mother is not responsible for the sex of the child and it has been genetically proved that the sex of a newborn is determined by what the child inherits from the father.

  1. (a)

    State the basis on which the sex of a newborn baby is determined in humans.

    1 mark
  2. (b)

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

    1 mark
  3. (c)

    How is the original number of chromosomes present in the parents restored in the progeny ?

    2 marks
  4. (c)

    Explain by giving two examples of the organisms in which the sex is not genetically determined.

    2 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/5/1.

Hints

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

Hint 1

The mother always contributes the same sex chromosome. Which one?

Hint 2

Compare the sizes of the two sex chromosomes in a male.

Hint 3

For (c), think about what happens to the chromosome number when gametes are made, and what happens when they fuse.

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 mother always gives an X chromosome, while the father's sperm carries either X or Y. If a sperm carrying X fuses with the ovum (X), the child is XX, a girl.

    ½ mark

  2. If a sperm carrying Y fuses with the ovum (X), the child is XY, a boy. So the sex of the baby depends on the chromosome inherited from the father.

    ½ mark

  3. (b) In males, one sex chromosome is a normal-sized X and the other is a much shorter Y. The two members of the pair are unequal, so it is called a mismatched pair.

    1 mark

  4. (c) During gamete formation, the chromosome number is reduced to half, so each germ cell carries only one set of chromosomes.

    1 mark

  5. When the male and female gametes fuse at fertilisation, the zygote gets one set from each parent. This restores the original (diploid) number of chromosomes in the progeny.

    1 mark

  6. (c) OR, Example 1: Reptiles (e.g. some turtles). The temperature at which the fertilised eggs are incubated decides whether the young developing in them will be male or female.

    1 mark

  7. Example 2: Snails. An individual can change its sex during its lifetime, so sex is not genetically fixed.

    1 mark

Answer: (a) Father's sperm carrying X gives a girl (XX); sperm carrying Y gives a boy (XY). (b) X is normal-sized and Y is short. (c) Gametes carry half the chromosomes, and fertilisation restores the full number. OR: reptiles (incubation temperature decides sex) and snails (can change sex).

Where the marks are

  • X-bearing sperm + ovum gives a girl (½ mark)
  • Y-bearing sperm + ovum gives a boy (½ mark)
  • Explains that X is normal-sized and Y is short, so the pair is unequal (1 mark)
  • Chromosome number is halved during gamete formation (1 mark)
  • Fusion of gametes restores the original number (1 mark)
  • OR: reptiles, with the role of incubation temperature (½ + ½ mark)
  • OR: snails, which can change sex during their lifetime (½ + ½ 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 paper2023 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.