**Core Concept**
In humans, sex chromosomes determine the sex of an individual. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). A Barr body, also known as an X chromosome inactivation body, is a structure formed when one of the two X chromosomes in a female cell is inactivated to avoid a doubling of gene expression.

**Why the Correct Answer is Right**
The presence of a Barr body in female cells is a result of X chromosome inactivation, which occurs to compensate for the presence of two X chromosomes. This process involves the inactivation of one of the X chromosomes, forming a Barr body. The inactivated X chromosome is tightly coiled and heterochromatinized, making it visible as a distinct body under a microscope. The correct answer refers to a condition where Barr body is not present, indicating that the individual has only one X chromosome.

**Why Each Wrong Option is Incorrect**
**Option A:** Turner syndrome is a condition where an individual has only one X chromosome (X0), but the presence of a Barr body is not the defining feature of this condition.
**Option B:** Klinefelter syndrome is a condition where an individual has an extra X chromosome (XXY), and in these cells, two Barr bodies can be present.
**Option C:** Mosaicism can occur when there is a mixture of cells with different numbers of X chromosomes, but the absence of a Barr body is not a defining characteristic of this condition.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the presence of a Barr body can be used as an indicator of sex chromosome abnormalities.

**Correct Answer:** D. A female with only one X chromosome (Turner syndrome) would not have a Barr body.