AIIMS 2005 Botany Sex Determination Assertion Reason Question

To analyze the given question, we will break it down into the assertion and the reason, and then evaluate their truthfulness and relationship.

Assertion

In humans, the gamete contributed by the male determines whether the child produced will be male or female.

This assertion is true. In humans, sex determination is influenced by the type of sperm that fertilizes the egg. Males have XY sex chromosomes, meaning they can produce two types of gametes: X (which will contribute to a female offspring if fertilizing an X-bearing egg) and Y (which will contribute to a male offspring if fertilizing an X-bearing egg). Females have XX sex chromosomes and can only produce X gametes. Therefore, it is indeed the male's gamete (either X or Y) that determines the sex of the child.

Reason

Sex in humans is a polygenic trait depending upon a cumulative effect of some genes on X-chromosome and some on Y-chromosome.

This reason is false. Sex determination in humans is not polygenic but is primarily based on the presence of specific sex chromosomes. Specifically, the presence of a Y chromosome (and the SRY gene on it) leads to male development, while its absence (resulting in two X chromosomes) leads to female development. While there are other genes that may influence sexual characteristics or traits (like secondary sexual characteristics), the fundamental process of sex determination does not rely on a cumulative genetic effect but rather on a specific single gene on the Y chromosome (SRY).

Conclusion

With the above evaluations:

• The assertion is true.
• The reason is false.

Thus, the correct choice is C: "If Assertion is true statement but Reason is false."

Clarification of Other Options

• Option A: Incorrect because while both statements are not true, the reason does not correctly explain the assertion.
• Option B: Incorrect for the same reason as A; the reason does not support the assertion.
• Option D: Incorrect because the assertion is true.

Key Concepts

1. Sex Determination: In humans, sex is determined by the combination of sex chromosomes contributed by the parents. The basic rule is:

   • XX = Female
   • XY = Male

2. Chromosomal Basis: The presence of the Y chromosome (specifically the SRY gene) triggers male development. If the SRY gene is absent (i.e., the individual has two X chromosomes), female development occurs.

Summary

The assertion correctly identifies the male gamete's role in determining the child's sex, while the reasoning provided mistakenly describes sex as a polygenic trait. Thus, the answer is C, as the assertion holds true while the reasoning does not support it.