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All human chromosomes are not… (Class 10 Science 2023, Q38)

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ScienceCBSE Class 10 · Board exam 2023, set 31/6/1 · 4 marks · Case-based

CBSE Class 10 Science 2023: question 38

Set in the CBSE Class 10 board exam of 2023, paper 31/6/1, 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

All human chromosomes are not paired. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes. But men have a mismatched pair in which one is normal sized while the other is a short one.

  1. (a)

    In humans, how many chromosomes are present in a Zygote and in each gamete?

    0 marks
  2. (b)

    A few reptiles rely entirely on environmental cues for sex determination. Comment.

    0 marks
  3. (c)

    "The sex of a child is a matter of chance and none of the parents are considered to be responsible for it." Justify it through flow chart only.

    0 marks
  4. (c)

    Why do all the gametes formed in human females have an X chromosome?

    0 marks

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 31/6/1.

Hints

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

Hint 1

Gametes carry half the chromosome number of body cells.

Hint 2

For reptiles, think of an environmental factor that acts on the eggs after they are laid.

Hint 3

Write the sex chromosomes of the mother and father, list the gametes each can make, and 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) A human zygote has 46 chromosomes (23 pairs). Each gamete (sperm or egg) has 23 chromosomes, which is half the number.

    1 mark

  2. (b) In some reptiles sex is not decided genetically. The temperature at which the fertilised eggs are incubated decides whether the young developing in them will be male or female. For example, in turtles a high incubation temperature produces females, while in lizards it produces males.

    1 mark

  3. (c) [Option A] Flow chart (in words):
    Father (XY) → two kinds of sperm: 50% carry X, 50% carry Y.
    Mother (XX) → only one kind of egg: all carry X.
    X-sperm + X-egg → XX zygote → girl.
    Y-sperm + X-egg → XY zygote → boy.

    1 mark

  4. Which type of sperm fertilises the egg is purely a matter of chance, so each child has a 50 : 50 probability of being a boy or a girl. Neither parent can be held responsible for the sex of the child.

    1 mark

  5. (c) [Option B] A human female has a perfect pair of sex chromosomes, XX. During gamete formation the chromosome pairs separate, so each egg receives one chromosome of each pair. Since both sex chromosomes are X, every egg gets an X chromosome.

    (Option B carries the same 2 marks as Option A.)

Answer: (a) Zygote: 46 chromosomes (23 pairs); gamete: 23. (b) The incubation temperature of the eggs decides the sex in some reptiles. (c) A: XY father gives X or Y sperm, XX mother gives only X eggs; XX gives a girl, XY gives a boy, by chance (50%). (c) B: females are XX, so every egg gets one X.

Where the marks are

  • (a) 46 chromosomes in the zygote and 23 in each gamete (½ + ½ marks)
  • (b) Temperature of the fertilised eggs decides the sex (1 mark)
  • (c) Correct flow chart showing parental XX and XY, the gametes, and the XX/XY offspring with the role of chance (2 marks)
  • OR (c) Female sex chromosomes are XX, so each gamete receives one X (2 marks)

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 31/6/1: 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.