Name the genetic disorder caused due to the presence of extra copy of chromosome no. 21.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Question**: The question asks for the name of a genetic disorder that is caused by the presence of an extra copy of chromosome number 21.

2. **Identifying the Genetic Disorder**: The genetic disorder associated with an extra copy of chromosome 21 is known as Down syndrome.

3. **Understanding the Terminology**: Down syndrome is also referred to as "trisomy 21" because it involves having three copies of chromosome 21 instead of the usual two.

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