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What is the probability… (Class 10 Science SQP 2023-24, Q30)

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ScienceCBSE Class 10 · Sample paper, session 2023-24 · 3 marks · Short answer

CBSE Class 10 Science sample paper 2023-24: question 30

From the official CBSE sample question paper for 2023-24, section C, question 30, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 3 marks · Short answerHeredity

What is the probability of a girl or a boy being born in a family? Justify your answer.

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

Think about the sex chromosomes of a woman and of a man.

Hint 2

Which parent can give two different kinds of sex chromosome to the child?

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. The probability of a girl or a boy being born is equal: 50% each (1 in 2).

    1 mark

  2. Humans have 23 pairs of chromosomes: 22 pairs of autosomes and 1 pair of sex chromosomes. Women have a perfect pair of sex chromosomes: XX.

    ½ mark

  3. Men have a mismatched pair: one normal-sized X and one short Y, i.e. XY. So half the sperms carry X and half carry Y.

    ½ mark

  4. Every egg from the mother carries an X chromosome, so the child always gets X from the mother.

    ½ mark

  5. If the child gets X from the father, it is XX, a girl. If it gets Y from the father, it is XY, a boy. Both kinds of sperm are equally likely to fertilise the egg, so the chance is 50 : 50.

    ½ mark

Answer: 50% chance of a girl and 50% chance of a boy; sex is decided by whether the father's sperm carries X or Y.

Where the marks are

  • States equal (50%) probability of a girl and a boy (1 mark)
  • Females have XX sex chromosomes (0.5 mark)
  • Males have XY sex chromosomes (0.5 mark)
  • Mother always passes on an X chromosome (0.5 mark)
  • X from father gives a girl, Y from father gives a boy (0.5 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 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.