Chromosome involved in Down’s Syndrome is –

**Core Concept**
Down's Syndrome, also known as Trisomy 21, is a genetic disorder caused by the presence of an extra copy of chromosome 21. This extra genetic material alters the development of the brain and body, leading to the characteristic features of the condition.

**Why the Correct Answer is Right**
The correct answer is due to the fact that Down's Syndrome is caused by an extra copy of chromosome 21, resulting in a total of 47 chromosomes instead of the usual 46. This extra chromosome is the result of a nondisjunction event during meiosis, where the homologous pairs of chromosomes fail to separate properly. The presence of an extra copy of chromosome 21 leads to the overexpression of genes on this chromosome, which disrupts normal cellular function and leads to the characteristic features of Down's Syndrome.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because Down's Syndrome is not caused by a deletion of chromosome 21, but rather an extra copy.
* **Option B:** This option is incorrect because Down's Syndrome is not caused by a translocation of chromosome 21 to another chromosome, although this can occur in some cases.
* **Option C:** This option is incorrect because Down's Syndrome is not caused by a mutation in a specific gene on chromosome 21, but rather the extra copy of the entire chromosome.

**Clinical Pearl / High-Yield Fact**
It's worth noting that Down's Syndrome is the most common chromosomal disorder, and it's estimated to occur in about 1 in every 700 births.

**Correct Answer: C. 21**