Assertion: An additional copy of chromosome number 21 causes Down's syndrome.
Reason: Aneuploidy occurs from an extra copy of chromosome.

Options

A. Assertion and reason both are correct statements the reason is the correct explanation of the assertion.
B. Assertion and reason both are correct statements but reason is not the correct explanation of the assertion.
C. Assertion is true but reason is false statement.
D. Assertion is false but reason is true statement.

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step 1: Understand the Assertion
The assertion states that "An additional copy of chromosome number 21 causes Down's syndrome."
- Down's syndrome, also known as Trisomy 21, is indeed caused by the presence of an extra copy of chromosome 21. Normally, humans have 46 chromosomes arranged in 23 pairs, and in Down's syndrome, there are three copies of chromosome 21 instead of the usual two.

### Step 2: Understand the Reason
The reason states that "Aneuploidy occurs from an extra copy of chromosome."
- Aneuploidy refers to an abnormal number of chromosomes in a cell, which can occur when there is an extra chromosome (like in Down's syndrome) or a missing chromosome. Thus, the statement about aneuploidy being related to an extra chromosome is correct.

### Step 3: Evaluate the Relationship Between Assertion and Reason
While both statements are true, we need to determine if the reason correctly explains the assertion.
- The assertion correctly identifies the cause of Down's syndrome as an additional chromosome 21. However, the reason does not specifically explain why this extra chromosome leads to Down's syndrome. The more precise explanation involves the process of nondisjunction during meiosis, which results in the extra chromosome.

### Conclusion
- The assertion is true.
- The reason is also true, but it does not provide the correct explanation for the assertion.

Thus, the correct answer is that both the assertion and reason are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
- Both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

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