Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : The sex of a human offspring is determined by the presence or absence of a Y chromosome in the sperm.
Reason (R) : Sperm cells from males contain either an X or a Y chromosome, while egg cells from females contain only an X chromosome.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

In humans, males are the heterogametic sex (XY) and determine the sex of the offspring.
In contrast, in some other species (such as birds), females are the heterogametic sex (ZW) and determine the offspring's sex.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
This question focuses on the biological mechanism of genetic sex determination in humans, which relies on the inheritance of sex chromosomes (X and Y).

Step 2: Detailed Explanation:
Let us analyze the assertion and reason individually:
Assertion (A):
The biological sex of a human offspring is determined by whether the fertilizing sperm carries an X or a Y chromosome.
If a sperm carrying a Y chromosome fertilizes the egg, the resulting zygote will have XY chromosomes, leading to male development (driven by the SRY gene on the Y chromosome).
If a sperm carrying an X chromosome fertilizes the egg, the zygote will have XX chromosomes, leading to female development.
Thus, the presence or absence of the Y chromosome determines the offspring's genetic sex.
Therefore, Assertion (A) is correct.
Reason (R):
Human females have homogametic sex chromosomes (XX), meaning all mature egg cells (ova) carry a single X chromosome.
Human males have heterogametic sex chromosomes (XY).
During meiosis, the sex chromosomes segregate so that half of the sperm cells carry an X chromosome, and the other half carry a Y chromosome.
Therefore, Reason (R) is correct.
Conclusion:
Because females can only contribute an X chromosome, the genetic sex of the offspring depends entirely on the sex chromosome carried by the father's sperm.
This biological fact directly explains why the assertion is true.
Thus, (R) is the correct explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are correct, and (R) is the correct explanation of (A).
This matches Option (A).