Q19
1 markMCQSection A

Assertion (A): In reptiles, the temperature at which the fertilized eggs are kept decides the sex of the offsprings.

Reason (R): Sex is not genetically determined in some animals.

Heredity
Sex Determination

Options

(a)Both, Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
(b)Both, Assertion (A) and Reason (R) are true, and Reason (R) is not the correct explanation of Assertion (A).
(c)Assertion (A) is true, but Reason (R) is false.
(d)Assertion (A) is false, but Reason (R) is true.
Official Answer

The correct option is (a).

environmental cuestemperaturegenetically determinedreptilessex determination

Marking Scheme

  • 10.5 marks
  • 20.5 marks

Hint

Think about whether reptiles rely on sex chromosomes or environmental factors to determine sex, and how this relates to genetic determination.

Quick Oral Answer

In some reptiles, sex is determined by environmental factors like temperature rather than sex chromosomes. Therefore, sex is not genetically determined in these organisms, making the Reason the correct explanation for the Assertion.

Analysis & Explanation

The Assertion states that in some reptiles, the temperature at which fertilized eggs are kept decides the sex of the offspring. This is a scientifically proven fact (e.g., in turtles and lizards) and is directly mentioned in the NCERT text as an example of environmental sex determination.


The Reason states that sex is not genetically determined in some animals, which is also true (as environmental factors like temperature or social factors like in snails determine sex).


Since the lack of genetic sex determination in these animals is the underlying reason why environmental factors like temperature can determine their sex, the Reason is the correct explanation of the Assertion.


  • Why (a) is correct: Both statements are true, and the Reason correctly explains why temperature (an environmental cue) is able to determine sex in these reptiles (because they lack a genetic sex-determination mechanism).
  • Why (b) is incorrect: The Reason is not just true, but it directly explains the Assertion.
  • Why (c) and (d) are incorrect: Both Assertion and Reason are true statements.

Common Mistakes

  1. 1Students sometimes think that sex is genetically determined in all animals, forgetting that reptiles and snails use environmental cues or can change sex.

Interesting Facts

In some turtle species, higher incubation temperatures produce females, while lower temperatures produce males. In some crocodiles, the reverse is true!

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2025?

This question carries 1 mark in the CBSE Class 10 Science 2025 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 2025 paper?

This is a MCQ question from Section A in the CBSE Class 10 Science 2025 paper.