Which mutation is seen in Down’s syndrome –
**Core Concept**
Down's syndrome is a genetic disorder caused by an abnormality in the number of chromosomes in an individual. It is characterized by intellectual disability, delayed speech, and physical characteristics such as a flat face and short neck. The condition is most commonly caused by the presence of an extra copy of chromosome 21.
**Why the Correct Answer is Right**
The mutation seen in Down's syndrome is a trisomy of chromosome 21, which means that there is an extra copy of the chromosome. This is due to a nondisjunction event during meiosis, where the pair of chromosomes does not separate properly, resulting in 24 chromosomes in each gamete instead of the usual 23. When a gamete with 24 chromosomes is fertilized by a normal gamete with 23 chromosomes, the resulting zygote has 47 chromosomes, leading to the development of Down's syndrome.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the correct number of chromosomes involved in Down's syndrome.
* **Option B:** This option is incorrect because it refers to a different genetic disorder, Turner syndrome, which is characterized by a missing X chromosome in females.
* **Option C:** This option is incorrect because it is a type of genetic mutation that involves a deletion of genetic material, whereas Down's syndrome is caused by an extra copy of chromosome 21.
**Clinical Pearl / High-Yield Fact**
Down's syndrome is the most common chromosomal disorder, affecting approximately 1 in 700 births worldwide. It is also known as trisomy 21.
**Correct Answer: C. Trisomy 21.**