Down’s syndrome occurs due to nondisjunction of which chromosome?
**Core Concept**
Down's syndrome is a genetic disorder caused by an abnormality in the number of chromosomes, specifically due to **nondisjunction** of a particular chromosome. This results in an extra copy of the chromosome, leading to **trisomy**. The underlying principle involves **meiotic errors** during gametogenesis.
**Why the Correct Answer is Right**
The correct answer is related to **chromosome 21**, as Down's syndrome is characterized by **trisomy 21**. This occurs when there is a failure of chromosome 21 to separate properly during **meiosis**, resulting in an extra copy of the chromosome. The **nondisjunction** of chromosome 21 leads to the characteristic features of Down's syndrome.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not specify the correct chromosome involved in Down's syndrome.
**Option B:** Incorrect as it is not related to the specific chromosome responsible for Down's syndrome.
**Option C:** Incorrect because, although it is a chromosome, it is not the one associated with Down's syndrome.
**Why Each Wrong Option is Incorrect (continued)**
**Option D:** This option is not provided, but typically, any option not specifying chromosome 21 would be incorrect.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that **trisomy 21** accounts for the majority of Down's syndrome cases, and the risk increases with **advanced maternal age**.
**Correct Answer:** Correct Answer: C. Chromosome 21.