## **Core Concept**
The Barr body, also known as the sex chromatin body, is a characteristic feature of cells in female mammals. It represents an inactivated **X chromosome** that is typically found in female cells, which have two X chromosomes. The formation of the Barr body is a result of **X-chromosome inactivation**, a process that ensures dosage compensation between males (XY) and females (XX) for genes located on the X chromosome.

## **Why the Correct Answer is Right**
The Barr body appears in female fetuses due to X-chromosome inactivation. This process usually begins around the **16th to 20th day of embryonic development**, before the primitive germ cells have completed their first meiotic division. As a result, by around **6 weeks** of gestation, most cells in a female fetus have one of their two X chromosomes inactivated, leading to the formation of a Barr body. This inactivation is a random process, which explains why females are mosaic for X-linked traits.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because the Barr body formation occurs later than the early stages of embryonic development.
- **Option B:** This option suggests a timeframe that is too early for the consistent appearance of Barr bodies in a significant number of cells.
- **Option D:** This option suggests a much later timeframe than when Barr bodies typically begin to appear.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the presence of Barr bodies is used as a diagnostic tool to determine the **sex chromatin pattern** in genetic analyses. Typically, **one Barr body is found in the cells of females (46,XX)**, while **no Barr bodies are found in the cells of males (46,XY)**. The presence of more than one Barr body suggests an abnormal number of X chromosomes, such as in **47,XXX** or **47,XXY** (Klinefelter syndrome) individuals.

## **Correct Answer:** . 6 weeks