Assertion (A) : Human female has a perfect pair of sex chromosome.

Reason (R) : Sex chromosome contributed by the human male in the zygote decides the sex of a child.

Heredity

Sex Determination

Options

(A)Both (A) and (R) are true and (R) is the correct explanation of (A).

(B)Both (A) and (R) are true, but (R) is not the correct explanation of (A).

(C)(A) is true, but (R) is false.

(D)(A) is false, but (R) is true.

Official Answer

The correct option is B.

perfect pairsex chromosomesXXXYfather determines sexindependent statements

Marking Scheme

11 mark for selecting the correct option B.

Hint

Determine if both statements are individually true. Then check if the mechanism of sex determination by the father's sperm explains why females have two identical $X$ chromosomes.

Quick Oral Answer

In humans, females have two identical sex chromosomes ( $XX$ ), making it a perfect pair. Males have a mismatched pair ( $XY$ ). Since females only produce gametes with $X$ chromosomes, the sex of the child is determined solely by whether the fertilizing sperm carries an $X$ or a $Y$ chromosome from the father.

Analysis & Explanation

The Assertion (A) states that a human female has a perfect pair of sex chromosomes. According to the NCERT text, 'Women have a perfect pair of sex chromosomes, both called X.' Thus, Assertion (A) is true. The Reason (R) states that the sex chromosome contributed by the human male in the zygote decides the sex of a child. According to the NCERT text, 'the sex of the children will be determined by what they inherit from their father. A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him will be a boy.' Thus, Reason (R) is also true. However, the fact that the father's sex chromosome determines the sex of the child does not explain why females have a perfect pair of sex chromosomes ( $XX$ ). The two statements are independent biological facts. Therefore, both (A) and (R) are true, but (R) is not the correct explanation of (A).

Common Mistakes

1Students often choose option A thinking that because both statements are about sex chromosomes and sex determination, one must explain the other.
2Some students mistakenly believe that females have mismatched chromosomes.

Interesting Facts

In some reptiles, sex is determined by environmental temperature during egg incubation, whereas in humans, it is strictly genetically determined by the sex chromosome carried by the sperm.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2024?

This question carries 1 mark in the CBSE Class 10 Science 2024 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Heredity" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Sex Determination" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2024 paper?

This is a MCQ question from Section A in the CBSE Class 10 Science 2024 paper.