Down's syndrome is caused due to

Question

Accepted Answer

Down's syndrome is a genetic disorder caused by the presence of an extra full or partial copy of chromosome 21. Let's break down why Option 2 is correct and the others are incorrect.

**Explanation for Option 2 (trisomy of 21st chromosome) being correct:**

*   **Down's Syndrome Definition:** Down's syndrome, also known as Trisomy 21, is a chromosomal disorder caused by an error in cell division that results in an extra full or partial copy of chromosome 21.
*   **Trisomy Explained:** "Trisomy" refers to the presence of three copies of a particular chromosome instead of the usual two copies in diploid cells. In the case of Down's syndrome, individuals have three copies of chromosome 21.
*   **Genetic Basis:** Normally, humans have 46 chromosomes, arranged in 23 pairs. One pair consists of sex chromosomes (XX for females, XY for males), and the other 22 pairs are autosomes. In Down's syndrome, there are three copies of the 21st chromosome, leading to a total of 47 chromosomes (47, XX, +21 or 47, XY, +21). This extra genetic material from chromosome 21 is responsible for the characteristic physical features and developmental delays associated with Down's syndrome.

**Explanation for why other options are incorrect:**

*   **Option 1: monosomy of 21st chromosome**
    *   **Monosomy Explained:** "Monosomy" refers to the presence of only one copy of a particular chromosome instead of the usual two.
    *   **Why it's incorrect:** Monosomy of an autosome (like chromosome 21) is generally lethal and results in spontaneous abortion early in development. It is not associated with Down's syndrome. Down's syndrome is caused by an *extra* chromosome, not a missing one.

*   **Option 3: an extra chromosome in male**
    *   **Why it's incorrect:** While Down's syndrome does involve an extra chromosome, this option is too general and incomplete. Down's syndrome is not exclusive to males; it affects both males and females equally. More importantly, the *specific* chromosome involved (chromosome 21) and the *type* of abnormality (trisomy) are crucial for defining Down's syndrome. Other conditions, like Klinefelter syndrome (XXY), involve an extra sex chromosome in males but are distinct from Down's syndrome.

*   **Option 4: an extra chromosome in female**
    *   **Why it's incorrect:** Similar to Option 3, this option is too general and incomplete. Down's syndrome affects both females and males. The key is the presence of an extra copy of the *21st chromosome*, not just any extra chromosome in a female. Other conditions, like Triple X syndrome (XXX), involve an extra sex chromosome in females but are distinct from Down's syndrome.

Therefore, the most precise and correct answer is that Down's syndrome is caused due to **trisomy of 21st chromosome**.