A mother has a first child of “Trisomy-21” (Down’s syndrome). What is the percentage of occurrence of reccurence in 2nd child?

**Core Concept**
The question is based on the genetic principles of **Trisomy-21**, also known as **Down's syndrome**, which is a chromosomal disorder. The risk of recurrence in subsequent pregnancies is a critical aspect of **genetic counseling**.

**Why the Correct Answer is Right**
The risk of having another child with **Down's syndrome** is higher in families who have already had a child with the condition. This increased risk is due to the possibility of **Robertsonian translocations** or other genetic factors that may contribute to **nondisjunction** during meiosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the actual recurrence risk is not this low for a condition like **Down's syndrome**.
**Option B:** This option might seem plausible but does not accurately reflect the established recurrence risk for **Trisomy-21**.
**Option D:** This option overestimates the recurrence risk, which is not supported by genetic epidemiology data on **Down's syndrome**.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that the risk of **Down's syndrome** increases with maternal age, and genetic counseling should include discussions about this risk.

**Correct Answer:** C. 1%